Which kind of tube valve is the best?

Question: What kind of valve is the best one, a Presta valve (6mm) or a Schrader valve (8mm).
Which has the best reliability, which one is easiest to fix a flat on?
Answer: Presta's main benefit is more about how the valve functions, in regards to getting pumped up to higher pressures.  Because the stem of the valve needs pressure in the pump head to get high enough before it pushes the valve stem in, and start flowing air into the tire, when you try to get to 120 to 160 lbs of pressure it works better than a Schrader valve where the valve stem is depressed the whole time, back pressure can leak out.
Otherwise I would say tomato - tomahto (potato - potaato?)
Comment: I would add that Schrader valves are better where it's important that you be able to use a gas station pump.
Comment: I have read this a few places and I am not sure why people are saying that you need air pressure to depress a Schrader valve. There might be some pumps that do this, but all the pumps I have ever owned use a small rod to push the core down, absolutely independent of air pressure.
Comment: @neilfein I use presta, and picked up a valve adapter for under a buck. keep it in my panier in case I need to use gas station pumps (or other non-presta pumps). works great. weighs maybe a gram.
Comment: @BillyNair We are agreed.  Car tire style (Schrader) need something to depress the valve. The thinner racing tire style (Presta) use air pressure to depress it on each pump.
Comment: @neilfein: If you have presta valves, you should definitely carry an adapter so that you can use normal pumps. If not, you aren't properly equipped!
Comment: Almost all modern bike pumps have some sort of check valve to avoid back pressure, and many have dual purpose heads for presta/schrader use.
Answer:
smaller hole in the rim is good (presta +)
schrader has piece that inserts into the valve (screws in) with a spring mechanism to seal it - these parts sometimes fail (schrader -)
presta uses tire pressure to seal it (presta +)
rocks can lodge in uncapped schrader and cause leaks (schrader -)
presta does not need a valve cap to keep rocks out (presta +)
presta has a locknut to prevent valve movement (presta +)
You can prep a presta valve tire for installation without a pump - using your mouth (presta +)

Presta is better but is insignificant for most people. Use the tube that fits your rim.
Comment: Too many broken presta core bolts. No good! (presta -)
Comment: to be fair "the Presta core bolt will sometimes get bent and make it difficult or impossible to open the valve (Presta-)"

And... I use my mouth to blow up a schrader all the time!! just push the core in with your canine...
Comment: Schrader valves are compatible with car tire pumps (schrader +)
Comment: Continental makes a schrader tube with a lock nut on the valve stem - it works great. I wish this would become standard practice!
Comment: You are fell in love with presta. But there are the same amount of pros for schradder and cons for presta as well.
Answer: My conclusion after many years of using not two, but three types of valves is that the best is the one that results most practical for you, acording to type of riding, type of pumping methods available and of course the type of bike/tire/rims you are using. Neither valve type is absolutely better than other, but one of them may result better for your particular circumstances.
For example when I was younger, the modern mountain bike was not yet popular in my country, so the most available tube had dunlop valves, also, the classical stick pump was available in the only bike store, in many hardware stores and even some supermarkets and department stores that sold sporting equipment. Back in that time, it was crazy to have a schrader valve, because you could only buy cheap and bad quality foot pumps, and the gas stations here where too careless to mantain their pumps in working order.
Nowadays, I have schraders in my DH bike, presta in my XC bikes and dunlops in my wife's XC bike. Besides that, I have many presta-to-schrader adaptors in my toolbox, patch kit, in my car, bacpack, etc. So I use a schrader pump head most of the time. This gives me huge flexibility, I can pump with whatever I have in hand! That is, In the car I have a small "compressor", an electrical pump that runs on 12v from the car's battery. At home I use a cheap but decent floor pump that has schrader head from a hardware store. Here the advantage is that it cost a fifth of a "proper biker's floor pump". And my Mini-stick pumps are set to schrader mode. Yes, I preffer to screw in the adaptor than to reverse the inners of the pump head.
I still have some dunlop tubes because here you can buy up to four or five dunlop tubes for the price of one schrader or presta. I use schraders on my DH bike because of the high volume. Schrader provides a wider air passage, so they are less restrictive and that allows for faster pumping with a hand of floor pump. Obvioulsy a DH tire does not require high pressure. And the prestas are used because higher quality tubes or certain specific sizes are only available in presta/shcrader.
After about 20 years of riding BMX (as a kid), Road, XC, DH, and even BSOs, i have never noticed a significative difference that forces me to choose one over another, and I find equal difficulty inflating them or repairing flats in any tube type.
The only possible advantage I see en prestas is that when you need to deflate them, you unscrew the valve closure and press it with yout finger, you don't need a thin object like with schraders. (such an object may already be in your emergency tool set, for example a 3mm hex from a multitool does the trick). The other advantage may be the locking nut that avoids the valve receeding into the rim when pressing the pump head against it.
Presta may be mandatory for narrow rims, like the ones in road bikes. Dunlop and Schrader both require the same diameter for the rim hole.
Of the three valve types, only dunlop actually requires air pressure to both open the valve while pumping and to seal it, and in practice it is the only one that closes itself while the pump is being filled with air. The typical stick pump for dunlop valves would not have a check valve on the output.
Both, Schrader and Presta specific pump heads press the inner pin or the closing nut respectively to open the valve. (I'm not saying prestas cannot be opened by air pressure from the pump, but most pump designs press on the closing nut). For that reason, modern pumps do have a check valve or an equivalent mechanism.
After that being said, whether it is easier to pump to a certain pressure depends more on the pump design, including the pump head. Some manufacturers simply have a better presta design than a shcrader design, but the opposite may be true for another make/model of pump.
That is, given that you have a good pump, it is equally as hard/easy to inflate a tube, and you will always find people that swears by othe or the other type, but the choice is yours. If you actually have the option, choose the one that is compatible with the best affordable pump that you have or can acquire.
Comment: +1 for mentioning Dunlop valves, which are the preferred valve of BSO in Japan, including the beloved Mama-bikes (Mamachari).
Answer: The best one is the one that fits your rim, as all rims are drilled a specific diameter for either Presta or Schrader.
Presta adds a very small amount of ease-of-use when it comes to letting out pressure with it's 'quick-release' valve, but the difference is trivial.
Answer: A Schrader valve can be pumped up with a car pump, often it is easer to find someone with a car pump than a bike pump when needed.  (You can also use a bike pump when needed)
Also a cheap car foot pump is a lot better than most bike pumps.
Answer: Many riders and mechanics recommend discarding the lock nut on the Presta valve because it causes more problems than it cures. I got rid of the lock nuts on the 4 bikes I have that use the Presta valve and have experienced no problems after doing so.
Do You Need Those Presta Valve Nuts?
As to valves fitting rims...you can drill a 3/8 inch hole in a Presta valve rim and use Schrader vale tubes in it. You just have to make sure you find a tube that matches the recommendations for tire pressure of the tire. Also make sure you "clean up" the hole so that no rough edges or burrs are present.
Comment: Welcome to Bicycles SE. This post doesn't really do anything to answer the OP's question and would probably be better placed as a comment.
Comment: Drilling a hole in a narrow rim is a bad idea from several standpoints.  You weaken the rim and you create a situation where the tire cannot properly settle onto the rim because the thick area around the valve prevents it.
Answer: A schrader valve require air pressure to open, A presta does not.
The biggest difference will seen when airing up a tire past 100+lbs, A shcrader will take more pumps to get from 100 to 110, because at that high of a pressure, more air is required to open the valve, whereas on a presta all the air goes into airing up the tire.
Comment: This is actually the opposite of the truth.  A schrader valve requires the pin to be pushed mechanically (it's normally held closed by a small spring).  Therefore the valve is open the entire time that the pump is attached.  The reason that the schrader takes longer to pump from 100 to 110 is because you are losing air between strokes of the pump whereas the presta valve is only open when the pressure in the pump exceeds the pressure in the tire, so you are not losing air between strokes.

Which kind of tube valve is the best?


Waves in water always circular

Question: I have had a question since childhood.  Why do we always get circular waves (ripples) in water even when we throw irregularly shaped object in it?
Comment: Even for a long stick?
Comment: I suppose that irregularly shaped object is small compared with the ripples?
Answer: Actually the ripples are not circular at all. See photo below.

For example, a long stick will generate a straight water front on from its sides and circular waves from its edges. Something similar to a rectangle where the two short sides are replaced by semi-circles.
As the waves spread, the straight front will retain its length, whereas the circular sides will grow in bigger and bigger circles, hence the impression that on a large body of water the waves end up being circular - they are not, but very close.
The reason that an irregular object generates "circular" ripples is therefore this: as the waves propagate, the irregularities are maintained but spread across a larger and larger circular wave front.
A very good example of this phenomenon is the Cosmic Microwave Background (CMB) where electromagnetic waves from the Big Bang are measured after having spread for 13.7 billion years. Although the CMB is really, really smooth - because of the "circular ripple" effect, if you like, we can still measure small irregularities, which we think are due to the "irregular shape" of the Big Bang at a certain time.
Comment: The usual name for "as the waves propagate, the irregularities are maintained but spread across a larger and larger circular wave front" is Huygen's principle http://en.wikipedia.org/wiki/Huygens%27_principle
Comment: Correct, I've added your link to the answer. Thank you.
Answer: I see this question already has an accepted answer, but I'll add a few general notes for completeness.
We begin with the question of how a free surface, i.e. the interface between two fluids, responds to a pressure (ie normal stress) disturbance.
This is the Cauchy-Poisson problem. Cauchy famously solved this problem, originally a prize question posed by the French Academy of Sciences, in 1815 at the age of 26. Poisson, one of the judges, added to this in his 1816 paper, and Cauchy published his work in his 1827 memoir, with an additional several hundred pages of notes.
Now, we consider an infinitely deep, infinite basin of water with a quiescent surface at $z=0$, above which there is air, which for simplicity we assume has pressure $P_\textrm{atm}=0$.  We assume the only restoring force here is gravity, since we are thinking ahead to modeling the coarse grain features of the fluid response. Note, this scenario is different than the image shown by @Sklivvz, in which capillary effects are present. These effects are very interesting, and at the end of this post I will some remarks about this phenomenon.
We begin by assuming the flow is irrotational, which means that there exists a velocity potential $\phi$, where $\nabla \phi = \textbf{u}$, with $\textbf{u}$ the fluid velocity, such that $\nabla^2\phi =0$ everywhere in the water. The conditions at the free surface $z=\eta$ are
\begin{equation}
 \eta_t+\nabla \phi \cdot \nabla \eta= \phi_z,\tag i
\end{equation}
i.e. the kinematic boundary condition, constraining fluid particles on the surface to remain on the surface, and
\begin{equation}
 \phi_t+\frac{1}{2}(\nabla \phi)^2 +gz = 0,\tag{ii}
\end{equation}
which is the dynamic boundary condition, ensuring continuity of pressure across the interface.
Finally, we have the condition that there is no flow at the bottom, i.e.
\begin{equation}
\phi_z \to 0 \quad  as \quad z\to -\infty\tag{iii}
\end{equation}
Now, although the governing equation is linear (it's Laplace's equation), the boundary conditions are nonlinear and are evaluated at one of the variables that we are solving for, namely $\eta$. To make progress then, we assume the velocities and surface heights are weak/small, so that we can linearize these equations and evaluate them at $z=0$. The boundary conditions become
\begin{equation}
 \eta_t = \phi_z,\tag{i'}
\end{equation}
and
\begin{equation}
\phi_t +g\eta = 0 ,\tag{ii'}
\end{equation}
where again, these are evaluated at $z=0$.  Now, for a particular wave with wavenumber $k$, the solution to Laplace's equation, with the condition (iii), can be found by assuming a separation of variables, which implies
\begin{equation}
\phi = \zeta(x,y,t) e^{kz}.
\end{equation}
This means that Laplace's equation becomes
\begin{equation}
\nabla^2 \phi = \phi_{xx}+\phi_{yy}+k\phi_z = 0,
\end{equation}
which, open substitution of $(i')$ and $(ii')$ becomes
\begin{equation}
\phi_{tt} - \frac{g}{k}\nabla^2_H \phi =0,
\end{equation}
where $\nabla_H \equiv \partial_{xx}+\partial_{yy}$ and we recognize $\frac{g}{k}$ as the square of the deep water phase velocity, so that the above is a 2d wave equation. Note, these waves are dispersive, i.e. the phase velocity inversely depends on the wavenumber. Long waves travel faster then short waves.
Next, we assume the solutions are oscillatory in time (which can formally be shown to be the case when we assume that $\zeta$ is separable in space and time), with frequency $\omega(k)=\sqrt{gk}$, i.e. $\zeta = \psi(x,y)e^{-i\omega t}$, so that our governing equation becomes
\begin{equation}
\nabla^2_H \psi + k^2 \psi=0,
\end{equation}
which we recognize as Helmholtz's equation. (The $k$ in the dispersion relation $\omega(k)$ for propagation along the surface is equal to $k$ in $e^{kz}$ because of the Laplace equation.)
Now, let's start with the case where we assume our solution will have a circular symmetry, and build more interesting cases from there. We transform our governing equation into polar coordinates $(r,\theta)$ to find
\begin{equation}
\psi_{rr} +\frac{1}{r}\psi_r +k^2 \psi = 0.
\end{equation}
This is a bessel equation with solution $\psi = J_o(kr)$, where $J_o$ is a zero order Bessel function of the first kind.
This analysis was done for one wavenumber $k$, but our operators are all linear so in general the solution will be a linear superposition of these waves, i.e.
\begin{equation}
\eta(r,\theta,t) =  \int_0^{\infty}f(k) J_o(kr)e^{-i\omega(k)t} \ dk,
\end{equation}
where $f(k)$ represent the mode coefficients, which are determined by the initial conditions.
For example, if we consider the response of the fluid to a point disturbance, we have
\begin{equation}
\eta(r,\theta,0) =\delta(r),
\end{equation}
where $\delta$ is the dirac delta function. We find $f(k)$ by the Hankel-transform, which tells us
\begin{equation}
f(k)=  \int_0^{\infty}  \delta(r)  J_o(kr) \ dr =1.
\end{equation}
Therefore, the governing equation is
\begin{equation}
\eta(r,\theta,t) = \int_0^{\infty} J_p(kr)e^{-i\omega(k)t} \ dk.
\end{equation}
These integrals (related to Hankel-Transforms) are notoriously difficult to solve and progress is usually only made under asymptotic conditions, when the method of stationary phase can be applicable. For instance, under the assumption $gt^2/r \ll 1$, we find (for details see Lamb, 1932, section 239)
\begin{equation}
\eta(r,\theta,t) \sim \frac{\sqrt{g}t}{r^{\frac{5}{2}}}\left(\cos gt^2/4r - \sin gt^2/4r\right).
\end{equation}
A sample solution is shown below.

So, we have finally developed some of the machinery necessary to talk about your question! Assuming that dropping an object into the water generates waves that are linear (this is clearly debatable for certain objects, as well as the time/length scales you're concerned with, but it is a point I will not address here), we can use superposition to model an object as a series of point excitations.
For instance, what happens when we drop a stick into the water? Well, if we model this as a super position of a bunch of point sources along the $y=0$ axis, we find the solution looks like
\begin{equation}
\eta(x,y,t) =\sqrt{g}t^2 \lim_{N\to \infty}\frac{1}{N}\sum_{n=1}^N \frac{1}{((x-x_n)^2+y^2)^{\frac{5}{2}}} \left(\cos \frac{gt^2}{4\sqrt{(x-x_n)^2+y^2}} - \sin  \frac{gt^2}{4\sqrt{(x-x_n)^2+y^2}}\right),
\end{equation}
where $x_n = n/N$, for instance. Locally, this disturbance will not produce symmetric rings, and indeed will have regions that have very minimal curvature. However, for $x\gg x_n$, this clearly will take the form of the symmetric rings given in the first example. A sample of this type of disturbance is shown below.

We can see here that the wave front is "flatter" along the y-axis.
So to conclude, a lot has to do with the scales over which you are making your observation. But it is shown, at least qualitatively, that non-symmetric patterns can be formed for axially asymmetric forcing.
I might have rambled a bit, but I hope this helps!

Nick

Some other notes:
-This is a crude way to model swell, with a storm over the ocean acting as a normal pressure disturbance. Indeed, one can make asymptotic estimates of the wavelength as a function of distance from the disturbance, which has been corroborated for swell. Also, deep water wave dispersion implies, as surfers empirically know, when a swell arrives, the longer period waves show up first.
-If we consider a point pressure disturbance on a uniformly moving stream, one gets the famous Kelvin ship wake pattern.

Capillary effects are very interesting in these problems. However, how to model all of the combined effects related to capillarity is nontrivial. For instance, one cannot model capillary waves without including viscous dissipation (these waves have high wavenumber, and viscous damping goes as $k^2$, hence, it's more important for these waves), as well as boundary layer effects, since capillary waves can be a strong source of vorticity. This vorticity can lead to wave front curvature, and further asymmetries which are nontrivial to model. This is an active area of research.

References:
Lamb (1932) Hydrodynamics
Whitham (1974) Linear and nonlinear waves
Comment: +1 A handy reference for waves in water. I'd never actually appreciated that the nonlinearity people talk about in water waves is in the boundary conditions. Is this right? I guess I kind of assumed it came from the convective term in the full Navier-Stokes equations. So if you did this analysis in a finite depth, sloping bottom tank (i.e. say numerically with the full nonlinearity of the boundary conditions taken account of), do you get growing waves as they reach the "beach" that behave somewhat like real ones?
Comment: You can rewrite the advective term in the N-S equations as a divergent term and a rotational term so that for irrotational flow the complexity of this term, i.e. the rotational component, drops out. This is why potential flow is so developed, and also boring. Indeed, for irrotational water waves, it is the boundary conditions (which dictate all of the dynamics in the interior) that lead to the complexity. (I'll answer your question in my next comment).
Comment: @NickP I took the liberty to correct a typo in ii') and change $\eta$ to $\phi$ in the wave-equation. This is consistent with Eq.(4) http://en.wikipedia.org/wiki/Airy_wave_theory But I see a problem with your solution of the Helmholtz equation: you obtain standing waves only. A propagating wave, entering the Green's function for Helmholtz equation, should be a Hankel function, no just the Bessel function of the first kind. Also, I cannot agree with your decomposition of a point source into Bessel functions - $f(k)$ should be $k$, not 1.
Comment: The description for shallow water is analogous, except b.c. iii) changes to $\phi_z\to 0$ at $z=-h$, where $h$ is the depth of the water. To see how waves behave as they approach a beach, one would simply consider conservation of energy. For simplicity, we assume the flow is time independent, so that we have $\partial (c a^2)\partial x = 0$ which means $c a^2 = const$, where $c$ is the shallow water phase velocity $c=\sqrt{gh}$ and $a$ is the amplitude of the incident wave. This means that $a \sim h^{-\frac{1}{4}}$ so that as $h\to 0$, $a$ gets large and at some point "breaks".
Comment: Thanks: I really like your plots. Are you going to upload the "Manipulate" command and definitions to the [Wolfram Demonstrations Project](http://demonstrations.wolfram.com/)? I suggest this would be a good want to show. Another with a sloping bottom would be awesome!
Comment: Hi @Slaviks, thanks for spending the time to go through this question. I'll address your points as I get to them. First, thanks for the typo correction. Next, I don't understand why you want to write the wave equation in terms of the velocity potential instead of the surface elevation. They are, of course, equivalent descriptions in linear theory, as the mapping between the two is simple, but I think the later is easier for people to understand. Unless you can convince me that there is some benefit to this way of thinking, I'll change those edits back to what they were.
Comment: @Slaviks I have checked that I am likely incorrect with my inverse Hankel transform of the delta function, which is likely due to my incorrect definition of the Hankel transform. Also, your point about my restriction to just the Bessel function of the first kind solution is also likely justified. For now, I point the reader to this: http://www.ap.ide.titech.ac.jp/~takada/waves/text2.pdf, but I will update this solution later tonight. Now, it's time to surf.
Comment: @NickP My rationale for not jumping from $\phi$ to $\eta$ too fast is, firstly, that it requires explanation, and secondly, that the initial conditions for $\eta$ ($\eta= \delta(r)$, $\eta_t= 0$) are not identical to those of $\phi$. This need to be taken into account performing the Hankel transforms.
Answer: waves always travel with a constant speed. For waves in water to travel at a constant speed they need to be circular. And hence the ripples in water are always circular.
Comment: This has the same problem as Santosh Linkha's answer. It only explains waves coming from a point perturbation. See my comment below his answer.
Answer: I don't think that water ripples are circular in nature because of the size of the object is small compared to ripples because you can even create perfect ripple with kitchen knife.
It is circular in nature because the transmission of disturbance from a source event takes place in a wave fashion in a 2-D medium. And intensity of wave is distributed equally in all directions of medium.
As you know wave motion all particles in ripple execute a harmonic motion, they remain in phase i.e. particles at equal distance from center remain at equal height, there by creating a circle.
Comment: This only explains why waves that come from a single point perturbation are circular. But for an irregular object the waves coming from different point will interfere and will create a pattern that's _not_ circular. It will become asymptotically circular when the waves get much bigger than the original perturbation.
Answer: (the same question was posted on Quora and this is a copy of my answer there. Most answers there really boil down to, "It's circular. Just look at it! You get circles! That's why it's circular.")
I'd phrase it like this: The primary idea of most wave equations is that disturbances will only propagate at speed $c$. So we can look at the ideal propagated disturbance as opposed to worrying about real world water waves. (this is for the best anyways, because you could apply it to explosions or electromagnetic waves w/ no loss of generality)
So, we have some shape $S$, which is a set of points in the plane (let's take it to be the filled-in shape with its boundary). We want to look at how the set $D(t)$ changes over time, where $D(t)$ is the set of points which has a distance from some point of $S$ less than or equal to $ct$. (so $D(t)$ expands outwards in a "zone of causality")
Theorem. $\mbox{Area}(D(t))$ ~ $\pi (c t)^2$. That is:
Theorem. The area of $D(t)$ is asymptotic to $\pi (c t)^2$. That is:
Theorem. $\lim_{t\to\infty} \frac{D(t)}{\pi (c t)^2}=1$. That is:
Theorem. The zone is basically a circle. (OK, this isn't equivalent to the statement above, but the proof below can show it.)
Proof. Pick any point of the plane to be the center of your circle. Whenever I say "the circle" from now on, it means the circle with that center. At $t=0$, make one circle that encompasses the shape you start with completely. Then, if the circle has radius $r_0$, wait until time  $t=\frac{2 r_0}{c}$. Now the "zone of causality" contains the circle you started with. At any time $t$, then, the border of the zone of causality will be sandwiched between the circle of radius $ct+r_0$ and the circle of radius $ct-r_0$ (which has radius $r_0$ at the time in question). Then $\pi (ct+r_0)^2\ge \mbox{Area}(D(t)) \ge \pi (ct-r_0)^2$ for all $t> \frac{2 r_0}{c}$. So $ (1+\frac{r_0}{ct})^2\ge \frac{ \mbox{Area}(D(t))}{\pi (ct)^2} \ge (1-\frac{r_0}{ct})^2$. As $t\to\infty$ the left and right go to one, so the middle must too. $\square$
The statements of the form "this contains that" can be made completely rigorous using inequalities like the triangle inequality. This says nothing about the border of the zone of causality.
Answer: Equation
$$η(r,θ,t)∼\frac{\sqrt g t}{r^{5/2}}(\cos gt^2/4r−\sin gt^2/4r)$$ is not physically consistent
may be
$$η(r,θ,t)∼\frac{\sqrt g t}{r^{1/2}}(\cos gt^2/4r−\sin gt^2/4r) ?$$
Comment: may be

η(r,θ,t)∼√g t/r^-1/2(cosgt2/4r−singt2/4r) ?
Comment: We do have [MathJax](https://physics.stackexchange.com/help/notation) here, I've edited this in for you but please try to keep this in mind going forwards.
Comment: Though this seems to be more of a comment on [this post](https://physics.stackexchange.com/a/90820/25301) than an answer.

Waves in water always circular


What is the liquid in a Radon U-tube Manometer?

Question: What is the liquid in a radon U-tube Manometer?  I'm assuming that it's not water because of evaporation.  Mine is clear in color, but I've been reading that it's normally blue or red.  Is it a particular type of oil?  Can it be refilled/replaced?  Some bubbles have appeared recently in my gauge that were not there last summer.  Can they be purged out?  It doesn't seem to be happening naturally with the vacuum produced by the fan and it makes reading the gauge very difficult.
Answer: It is oil of a specific density to correspond to the scale. I believe red is one density and blue another, at least for Dwyer manometers. I'm fairly sure replacement oil is available. If you don't care about the accurate reading and only that the fan is working, you could just use plain mineral oil, but you should post a notice next to the gauge that the oil is non-standard and what the last known good reading of the new oil was. I wouldn't use vegetable oil as it may get rancid.
Answer: From what I remember, the oil is typically vegetable oil with a food coloring added to it. I'm not too sure it can be replaced but I would call your local Radon specialist and ask over the phone. No need for someone to come out and make you pay to ask. If they won't answer the question over the phone, call someone else. They can't want your business too badly if they won't answer a simple question.
Comment: Thanks for the info.  I'll check it out and hopefully be able to post the answer back here.
Answer: When my radon mitigation system was installed I was told the blue liquid would fade to clear in a few years.  The company said it's not a problem since you can still read the meter by looking a bit closer, and definitely not worth a service call as long as you can verify the fan is working.
Comment: I think this is helpful but doesn't directly answer the question (what is the liquid)
Comment: Should it have been made a comment on the original question? I was not sure how to do that.
Comment: You need a bit of reputation in order to comment. I don't think this is a "bad" answer as it does provide some good info (that color isn't necessary that relevant), but the other answers provide a direct answer.


Is it faster to have four times 2 GB or two times 4 GB of RAM with a dual-channel mainboard?

Question: I'm building a Core i7-860 system which supports dual-channel memory (up to 16 GB in four DIMMs), my target is 8 GB. From the performance perspective, is it better to have two times 4 GB or four times 2 GB?
Answer: Four sticks would cause more strain on the memory controller and motherboard chipset. It would take slightly longer for the CPU to add and retrieve data from four sticks as opposed to two.
For this reason, 2 x 4 GB would be faster then 4 x 2 GB.
EDIT - there is a much better technical explanation on The Hyphenated Site to enforce my answer, although it does mention lower sized sticks:

It's better to use two 2 GB modules --
  not for any appreciable speed
  difference (although there may be a
  small advantage -- more in a bit) --
  but for a more reliable memory
  subsystem.
Most desktop systems use unbuffered
  RAM modules -- this results in very
  large loads on the address and data
  buses when you have more than two
  modules installed, and can
  significantly degrade the signalling
  on these buses. The memory subsystem
  "sees' one load per memory chip -- so
  with two modules installed, that's up to
  32 loads (with double-sided modules)
  ... and with four modules installed
  that's as many as 64 electrical loads
  on the bus. Some systems
  automatically adjust for this higher
  load by either increasing the voltage
  a small amount; reducing the clock
  frequency of the memory (thus slightly
  slowing it down); or by adding a cycle
  to the SPD's latency setting (again,
  slightly slowing it down).   These
  adjustments help keep the memory
  subsystem reliable -- but mean that 4
  x 1 GB modules would be slightly slower
  than 2 x 2 GB modules on these systems.
  But regardless of whether there's any
  timing differences, the memory will
  definitely be more reliable with only
  two modules.

Credit to garycase at The Hyphenated Site for this answer.
Comment: Intuitively, I would think that what Joe Taylor says might be true (4x2GB leaving more room for concurrent work) but I may be completely wrong.

But the difference will be probably small anyway, I'll probably go for 2x4GB as this will allow an upgrade to 16GB in the future if needed.
Comment: It might make more sense to link to the corsair presentation garycase cites instead of EE directly since without paying you have to jump through a google hoop to see EE content.
Comment: are any of those user application benchmarks, or are they all synthetic tests?  If the former I'd like a link to them.
Comment: I removed my *comment* above as I have completely failed at finding the article I was reading at work earlier. My answer remains exactly the same, although I have additionally backed it up with a much better technical explanation than mine.
Comment: It would be nice to know *how much* slower four memory modules would be. I know that a precise figure will be hard to come by, but are we talking about 0.1%? 1%? 10%?
Answer: 2x4GB will be slightly faster in benchmarks because the motherboard has to relax timings slightly to handle two DIMMs per channel but in real world use you'll never notice a difference.  It also can matter if you're overclocking significantly where the second set of DIMMs will limit your ability to push the memory clocks quite as high.
If costs are about the same, 2x4 GB will give you room to expand to 16 GB of memory in the future if you need it.  If costs are significantly higher and you don't anticipate needing more RAM, there's no reason not to save money and use 4x2GB instead.
Comment: The price of 2x4GB is slightly higher than 4x2GB, however, I also value the possibility to upgrade to 16GB in the future so I'll probably go for that.
Answer: Not my answer but the one that I followed:
This question has come up before in several other forums, but there has never been a good clear answer. Overclocking, upgradability, and temperature aside, I would suspect that 4x2 would be slightly faster than 2x4 because there would be more interleaving. Interleaving means that the data is spread out across more memory chips. While some chips are waiting for their CAS timing cycle to complete, data access can occur from the other chips.
The thing is, it really depends on the northbridge (NB) implementation. The NB must be able to take advantage of interleaving across four DIMMs, and I don't know which NBs do and which don't. I suspect the most of the Xeon NBs can interleave across four DIMMs, in fact most of those NBs require DIMMs to be installed in groups of four. I'm not sure about the consumer NB lines such as P35, 965, and nForce 680i, the answer could be different for each chipset...
I know this does not directly answer your question. I am quite certain that 4x2 will never be slower than 2x4, it will either be the same or faster if the NB supports four-way interleave.
Comment: The only benchmark I can find (on the old AMD 939) showed performance slowing as a result of the second bank being populated due to the timing hit.  It's possible old controllers (or FBRAM controllers on servers) did benefit that way but newer consumer controller do not.

http://www.anandtech.com/memory/showdoc.aspx?i=2560&p=2


What is the general term for a persistent/ongoing magical effect in D&D?

Question: I'm trying to figure out an appropriate word to describe a magical effect of unknown origin in a D&D game. I can't call it a spell or spell effect since it's not known whether the effect is caused by a spell or some other kind of magic. The most obvious term that comes to mind is "enchantment", but in the context of D&D, this term is reserved specifically for magic that affects the minds of others, so it is not suitable as a general term for any magical effect. So what would an appropriate term be for a persistent magical effect?
Comment: Ryan, are you talking about a specific edition of D&D, or are you looking for a term that spans multiple/all editions?
Comment: I happen to be playing 5th edition, but the question seemed general enough to me that it hadn't occurred to me that the answer might depend on the edition. I suppose, if your answer is limited to a specific edition, just indicate that in the answer.
Comment: The question about terminology may be generic enough but it is a slippery slope as 3.X/PF and 5E treat how dispel magic affect them.
Answer: Unless you're explicitly searching for a synonym/alternative, I think 'magical effect' itself is appropriate- it covers both spell and non-spell cases. In addition, several game elements (in 5e, at least) refer to them as such:
Dispel Magic:

Choose one creature, object, or magical effect within range. [...]

Antimagic Field:

Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere.
[...]
Spells and other magical effects, such as magic missile and charm person, that target a creature or an object in the sphere have no effect on that target.

Nystul's/Arcanist's Magic Aura:

You change the way the target appears to spells and magical effects [...]
Comment: Ok, it looks like this is indeed the term used in official materials.
Comment: Yep, "magical effect" is indeed the general D&D (5e) term for effects caused by magic (whether or not the source is a spell).
Answer: Permanent Enchantment
Enchantment has two meanings in D&D.
The narrow one is a mind affecting spell.
The more common meaning is magic imbued in an object or creature.  A magic sword has an enchantment on it.  You could even say that the narrow meaning is just a specific case of the broader meaning because the mind affecting spell is a spell that imbues a creature with an effect for a specific amount of time.
For the other part of your question, "permanent" is the term for a spell that does not have an end point to its duration but is ongoing.  This is different from an Instant spell that makes a change to an object where the change is then permanent (the damage from a fireball persists after the spell and cannot be dispelled).
Answer: I would refer to it based on how the effect manifests itself. For example you could call it a dome, cloud, haze, mist, field, emanation, or any other more specific or appropriate adjective.
Comment: The querent is not looking for ways to describe magical effects, but ways to refer to magical effects with no need for context.


Can I settle at the Tenpenny Tower without destroying Megaton?

Question: I am curious if I could settle at the Tenpenny Tower without blowing up the Megaton bomb and becoming evil (and if this is possible even after I disarm the bomb and fail the Mr Burk's quest to detonate it). Can I? If yes - how?
Answer: You cannot.
Mr Burk will only give you the key to your room as a quest reward for setting off the bomb.  Even killing him doesn't get you the key.
Comment: Even if you cheat to get the keys, the room will not be assigned to you, which means you can't decorate it, store anything in it, sleep in the bed ... basically its just a room.
Comment: Ah, that's true.  I recall trying that.  I'll fix my answer.
Comment: I can't get on the wiki right now, but can't you take over the tower with the ghouls without destroying Megaton?  Or does that not grant you the room?
Answer: If you are playing on PC then there are mods for this. I use one called Tenpenny Suite After Saving Megaton where you get the suite after dealing with the ghoul Roy and his gang.
Answer: Though i have not tested this, You may be able to do this by cheating using the command console. Y U NO WORK pointed out even hacking the key does not give you ownership, but with many Bethesda games, you can skip events without actually doing them to obtain the key the "proper" way by using the command console, (cheating). Through the command console, you can give yourself any quest and set it to almost any point (called the stage). there are 3 parts to this command, and all 3 can be obtained from the wiki, with 2 of them from the quest page. you would just set your quest status for the quest to blow up megaton to be right after you actually blow it up. This command can be used as a workaround for bugged quests or to skip hard/annoying parts of the quest, though use at your own risk as its impossible to predict everything that can happen.
it would look something like this:
<code>setstage 00014e9e 60
</code>
setstage is the command to adjust your quest status, the next number is the ID of the quest Power of the Atom, and stage 60 is the stage where you collect your reward for blowing up megaton in the quest as taken from the Wiki page. (stage 50 is the stage where you press the button, so you probably have to go to 60)
The quest would likely continue as if you had blown up megaton (and people would likely also react like that), but the town would remain. But as noted, this is untested and just an idea. it may blow up megaton for you, but at least you (probably) wont get the bad karma for doing it. It may also cause other events to bug out as the state of the save file if megaton survives would be normally impossible.
Comment: @AdamSmith It's the style favored by Bethesda's programmers and used in the existing game's scripts. Consistency is always a good thing.
Comment: The leading zeroes are unnecessary, so you could use `setstage 14e9e 60` if you wanted. Or `setStage 14e9e 60` to use proper camelCase, which is meaningless for a console command but a good habit if you ever wanted to script things. Because basically, that's all the console is: a script that gets run line by line as you type them. Also probably good practice for SE answers, as like scripts they are meant to be read.
Comment: @KRyan It's only "proper" if the scripting language in question prefers camelCase. Kind of a silly distinction to make, since many do not....


Manage photo in sharepoint 2010

Question: I have a list for students. I want to keep photos of each student. How can i do this? Do I need to add a field to Student custom list? Please help me with correct approach. I am using a visual webpart to add items to the Students list. How can i upload photo using same visual webpart?
Answer: 1) You have to create a Column in you Student List and Set its type to Picture or HyperLink and select Picture from Dropdown.
2) Create a Picture Library named like Student Photos.
3) While Saving Student Data Upload Student Photo with it and Save that photo to Student Photos Picture Library.
4) Save link of that Photo to your Student List

Manage photo in sharepoint 2010


The existence of these tags reduce my [efficiency] and [productivity]

Question: The efficiency and productivity tags have no tag wiki, only 5 resp. 8 questions, and (especially the first one) are not used in a consistent way. I can see some point in focusing productivity on tips & tricks for using Stack Exchange, but we could also use tips for that (that tag lacks guidance too, by the way). Thoughts?
Comment: Proposed title: The existence of these tags come at a high [cost] and reduce my [efficiency] and [productivity]
Comment: Yeah, more tags can be burninated, but I don't want to risk the question becoming t̶o̶o̶ ̶b̶r̶o̶a̶d̶  needs more focus...
Comment: What about this title?: Overseeing the [cost] undermines our [efficiency] and [productivity] :D
Comment: [tag:efficiency] and [tag:productivity] might be of interest to those who like to talk about  [tag:cost] (another tag having a low number of questions and no tag excerpt)
Comment: Looking at the questions the first two seem a real rag-bag. I do not think any harm would come if they were eliminated. Many questions and answers provide tips so it is unclear what benefit having a small number tagged tips is so I would be inclined to eliminate that one too.
Comment: Although this did not attract much interest nobody seems to have objected to my suggestion so shall I go ahead and do the re-tagging or would you like to do it?
Comment: @mdewey yeah, this has been up for a week now and we seem to have consensus. You may do the honors; personally, I would avoid doing too many edits at once, as not to flood the homepage with bumps; 3-4 per day should be fine.
Comment: Under way. I also put in a brief wiki about tips.
Comment: Having mdewey burniate can [lead to edit rejections](https://meta.stackexchange.com/review/suggested-edits/76041), whereas if Glorfindel did them they wouldn't go to review.
Answer: Just to update here.
I started work on re-tagging as there appeared to be a consensus that this was worthwhile. Subsequently a dissenting view appeared here and some of my edits were rejected from the Suggested Edits queue. I have therefore concluded that the consensus was more apparent than real and have stopped the re-tagging process. I have contributed wikis for the two tags which seem to cover most of the cases tips and cost.
===== Original starts here =====
Let me make a concrete proposal for comment.
If there is a common theme behind the efficiency and productivity tags it is people trying to improve their personal efficiency in using the site. They are not asking how to make the underlying software more efficient. It is therefore hard to see any difference between that and asking for tips about how to use the site so I would propose that all three sets of threads could be combined as tips. I think one of the questions asks about tooltips which is slightly different but tagging is never perfect.
Answer: Thanks for all your efforts, these tags have been burninated!
Answer: I strongly disagree both the question and the answer. Instead of eliminating the tags, they should be fixed. Using SE sites to increase personal productivity and efficiency should be a top important topic on the metas.
Renaming these tags to "tips" is nearly so bad as deleting them.


Downgrading from from Unlimited to Enterprise edition in salesforce

Question: What are the consideration when Downgrading from  from Unlimited to Enterprise edition?Can you please provide your insights if you did a downgrade between instances? Has anyone done this type of downgrade? Is it hard and are there complications or is it just as simple as flipping a switch?
Answer: I suggest you review Salesforce Features and Edition Limits. The Unlimited Edition allows MANY more Validation Rules/object, custom fields/object, custom objects, includes free support, unlimited on-line training, has many more sandboxes, an unlimited number of custom applications (vs a maximum of 10) and much more. See How to select the right
Sales Cloud edition for more specifics and additional details that I've not covered as there's a huge difference between the two and much you'll need to check for if you're going to downgrade.
Comment: Is it hard and are there complications or is it just as simple as flipping a switch?
Comment: From my answer above, I'd hope it would be obvious there can be many complications that would require a lot of investigation before making the conversion. Its also something you'd want to discuss with your Account Executive and others who would be impacted with respect to your development & QA team because of the loss of sandboxes. Its not a trivial decision or task.


Putting Mata Shodasi Mahavidya picture with other gods and recite mantras

Question: Can Mata Shodasi Mahavidya picture be put in pooja room with other gods?
I don't know anything about MahaVidya but want to do Mata Shadoshi puja. Is there any special rule to do her puja and if not done properly, Will there be any side effect? Also can a girl chant her mantra?
Comment: It is not recommended to do pooja of Tantric deity without initiation. But you can still do bhakti puja to her.
Comment: @TheLittleNaruto i dont what happend to my reputation they have decreased from 20 to 13 now i am not able to join in chat since a week.... I tried geting some badges but reputation didn't increase... May be now i cant talk woth u guys anymore...
Comment: You need to gain reputation by asking right questions and answers. Looks like some found your questions not useful and downvoted.  For the least you can work on gaining rep by editing question or answer posts(Only if you found the posts need any corrections like Grammar or formatting). For each correct edit you would earn +2 reputation. :) Once you reaches to 20 rep, you can again be able to chat in chatroom
Answer: Sri Vidya mantras (particularly the Shodashi or Shodakshari mantra) are the most secretive among all mantras. So, the rule is to recite only after getting initiated from a Guru of that Parampara (of Sri Vidya).
Swami Shivananda, advises to recite a lot mantras in his book "Japa Yoga - A comprehensive treatise on Mantra Sastra ", but for Sri Vidya mantras, he specifically points out the need for initiation.

Sri-Vidya is the great Mantra of Tripurasundari or Bhuvanesvari or
Mahamaya. It is also called the Panchadasi or the Panchadasakshari,
for it is formed of fifteen letters. In its developed form it consists
of sixteen letters and is called Shodasi or the Shodasakshari. The
aspirant should directly get initiation of this Mantra from a Guru,
and should not start reading it for himself or doing Japa of it, of
his own accord. This is a very powerful Mantra and, when it is not
properly repeated, it may harm the Upasaka. So it is imperative that
it should be got directly from a Guru who has got Siddhi of this
Mantra.

And, there is no problem with being a woman and chanting the mantras of Sri Vidya. According to Tantras, everyone, including women has the right to be initiated. One of the Gurus of this particular Sri Vidya Paramapara is Lopamudra, the wife of Agastya Muni.
So, the real problem is to chant the mantras without getting initiated first.Being a woman is not a problem.
And, once you are initiated by a Guru, you will be taught the proper methods of doing a formal worship. Till then you can do a devotional worship, by offering lightened incense, lighted lamps, flowers etc. to the image. Place the image on an altar (covered with a red cloth) in your Puja place and while worshipping make sure to face either the East or the North.


How do I search for the surname SPEAK and not the verb speak?

Question: I'm researching family history and whenever I'm try to search for the surname SPEAK, I always get results with the verb speak included, particularly from newspaper articles, conferences etc. So how do I write the search parameters just to come back with the surname only? I'm drowning...
Comment: In short, you can't completely achieve this, as Google can't easily distinguish words by context. But you can exclude similar terms with a minus sign: '-speaks -speaker -speaking' etc which may help. If you are looking for a particular name then search for it in quotes: "William Speak" to find only results with that word pair. Also "Speak, William" and so on. Google also has an "AROUND" operator that aims to find specific words near each other. So 'William AROUND(2) Speak' should find 'William' and 'Speak' within two words of each other, in any order. And follow Chenmunka's advice!
Comment: You may get some help if you ask on WebApps.SE as they have questions regarding search engine tips and tricks.
Answer: Although somewhat limited, there is a tool called Case Sensitive Search that allows you to perform case-sensitive web searches on Google.
It can be downloaded from their site at http://www.casesensitivesearch.com/.
Basically, the tool is a web-application that scans through the case-insensitive results returned by a Google search and filters out just the results that match the case of your query.

Cautionary note: Now, I haven't used this tool for a few years, and I don't know anything about the people behind it (which is one reason that I was cautious about using it). The security software on my PC didn't flag any warnings though, so it is probably OK in that regard. Even so, I installed it when I was searching for a family named Garden, and removed it when I was done.

I mentioned that the tool had some significant limitations. The biggest of these was that it would only filter the first 1024 entries. However, that was a few years ago, and they may have improved it since then.
Comment: That website no longer exists. However I will try this from a WebApps.SE answer.

Here's help from Google itself. You can do this with Chrome too.

The Find bar (Ctrl + F) in Firefox offers a "Match Case" option to help you perform case-sensitive searches on a web page. If you type "RAM" in the find box, the browser will only highlight the phrase "RAM" on that page and not Ram or ram.
Answer: In this case, it helps to to a specialized search by limiting the types of results you are going to get or by adding other relevant keywords. In search engines you could try putting quotes around, such as "Speak Family" and limit the types of results. It may suite you more to search for specific names with other key words added, " 'Robert Speak' born in New York".
The most helpful I advice I can give you, however, is not to do search engine searches for the name "speak", but do search engine searches for specific resources, and then search for your name within those resources.
For example, you could do a search for "genealogy resources in New York" or "colonial families of rhode island" for example, and then from that list, FamilySearch might come up or a find-a-grave index. Go into those resource and then perform your search.
Another valuable option is to find a library in a bigger city and check out their genealogy department. You can do searches that will only include people's names, as they aren't going to bother indexing verbs. I hope this helps.
Comment: That's great advice when searching databases and I do adopt this process. I was looking for some way to narrow the search from newspaper archives or online books, where the verb speak is common.
Comment: I have used the Firefox finder window as suggested above by @Chenmunka and clicked on the "Case sensitive" button - which I've never noticed before. Doh!! It works very well indeed.


How can I go from Uyuni (Bolivia) to Puno (Peru) directly?

Question: How can I go from Uyuni to Puno directly? Is there a bus? and what's the travel time?
Answer: Short answer: You really can't.
Long answer: The most direct way (ie not taking a Potosi bus and connecting) is to take just two buses.
The first bus will be from Uyuni to La Paz.  This can take 12-15 hours depending on the time of day, the bus company, your driver (some will pull over for a few hours to sleep!) and stops (some stop in El Alto to drop off passengers). Bring warm clothes - it's a COLD long ride.
(My recollection is that it goes up route 30 to La Paz, not via Potosi, but I could be wrong - I certainly don't remember ever being in Potosi but it was in the middle of the night, to be fair)
Then from La Paz to Puno, you have two options - either the fast route around the lake by bus (5 hours), or via Copacabana, which takes longer but is more of a 'fun' experience - you take a ferry at one point.
I assume you're doing a salt flats tour? Speak to your tour company beforehand, as they can often organise a bus for you so that you can end your tour and hop on a bus to La Paz soon afterwards - they quite often fill up, especially if you want cama class instead of semi-cama seats.
Comment: @Washu It's just handy when I've done the route myself :)
Comment: you know a lot ;)
Comment: I'm a bit confused. I Haven't been there but I'm looking at maps out of curiosity having travelled routes that "don't exist" in other parts of Latin America I thought it might be fun to take a look... It looks like the most direct way *is* via Potosí to Nuestra Señora de la Paz. What am I missing?
Comment: @hippietrail - possibly not as clear as I'd hoped - going by Potosi meant 3 buses (ie to Potosi, to La Paz, to Puno) whereas the buses to La Paz direct mean fewer stops.  In addition, I'm almost certain when I went our vehicle went up the 30 road, rather than the 1 highway, skipping Potosi.  However I wasn't in the best state at the time (long story) so could be wrong.  I'll tweak my answer.
Comment: Ah OK. In asking I realized that we tend to use some of the same words when we talk about routes and bus connections, like "direct" that can make it a bit tricky. I guess the OP was asking both about the route and the fewest bus changes.


How to exclude posts from category posts count

Question: I have a custom loop to display posts based on a custom field. My problem is that if i have 2 posts and exclude 1 from the loop the category posts count (categories list widget with show count enable) still remains 2.
Answer: Assuming you have a list containing the post IDs you want to exclude, you could start with something like (PHP 7.3):
function exclude_posts_from_category( array $cat_args ): array {
    $exclude = array(); // TODO Add post IDs to exclude.
    $cat_args['exclude'] = $exclude;
    return $cat_args;
}
add_filter( 'widget_categories_args', 'exclude_posts_from_category', 10, 1 );

You can read about the widget_categories_args filter here. Additionally, you can alter the $cat_args array using a combination of arguments that can be found in the documentation of the wp_list_categories() function.
I did not fully test this code, so let me know in case you're having any trouble.
Answer: If you want to exclude posts from all queries, if you are not an "admin", and excluding the "single" query - so you can open the post, you can use this:
add_action('pre_get_posts', 'exclude_posts_from_all_queries');
function exclude_posts_from_all_queries($query) {
  $exclude_posts = array('9');
  if ( !is_admin() && !is_single() ) {
    $query->set('post__not_in', $exclude_posts);
  }
}

Just add the Posts ID number to $exclude_posts array.
Hope that it helps,
Cheers
Comment: Thank you for your answer but still shows the total number of posts. Let me be more clear. I use the "Categories" widget and i have enable "Show post counts". I want all excluded posts not counting.
Comment: Did you logget out from WP? you have to be logged out because of the conditional `is_admin()`. Also clear your cache. Or check your blog on another browser that you have not used...
Comment: I already did all these but still the category posts count remain the same.
Comment: I also found the same question with no clear answer: [http://wordpress.stackexchange.com/questions/16581/how-to-hook-get-terms-to-only-show-count-of-posts-that-have-custom-meta]


How deep will sandpaper penetrate?

Question: This may not be important, but it was something that I would find pretty neat to know. And the question is simple, how 'deep' will a given grit (36,80,120,etc) tear into the wood. I do not think there is much more to be said.
Comment: is your question "how deep can I sand down wood based on the grit?", or "how deep can loose abrasive grit penetrate and get stuck in my wood piece?" I find the wording a bit confusing.
Comment: @FlorianCastellane I am basically just asking how much a single piece of X grit will remove with one pass.
Answer: You can see the diameter of the average grit in a piece of sandpaper here:
https://www.gessweincanada.com/category-s/11328.htm
Assuming the grit is embedded at least 1/3 of this diameter, you would "cut" into the wood about that far per row of "grit".  So it depends on the length of your sandpaper (a 1 inch piece of 80 grit sandpaper is going to cut less than a 10 inch piece per stroke, probably about 10:1 if I had to guess).
But really sandpaper does not "cut", it more tears or smashes the wood fibers.
Answer: This is a "How long is a piece of string?" question.
Lots of factors:

What type of sandpaper is being used (open coat or closed, abrasive type* etc. etc.)?
How hard is the sandpaper being pressed into the wood?
How fast is the sandpaper moving?
A key factor obviously, how hard is the wood?
And last but by no means least, how long are you sanding for?

In theory there's no limit to the depth you can sand to, with any grit within reason, but obviously in practice there are limits in terms of cost (paper wears out, or fills or corns develop), user patience (who hasn't gotten bored sanding?) and the time available (sanding is rarely the fastest way to do something in most home woodworking environments).
With powered sanders what's feasible to do with sandpaper have opened up enormously — just look at the type of stock removal someone with a belt or disk sander is comfortable doing versus someone who has to sand by hand — but the traditional way(s) the same task was done are worth investigating and trying out for oneself..... bearing in mind that in time almost everyone seeks to reduce the amount of sanding they do to a minimum.

*Different abrasives have different hardnesses and the individual grains are different shapes, some with sharper edges than others, and each type wears down in its own way. So some papers initially cut fast but quickly lose efficiency, others can be slower to begin with but the falloff in sanding efficiency can be much slower.
Comment: (pathetically-off-topic-comment): <<"How long is a piece of string?">> - well in most modern programming languages it is just a matter of calling the length function on the instance of the string.
Comment: @AndreiRînea LOL
Answer: What exactly do you mean by "penetrate"?
Since the sandpaper will rip small particles from the wood you can actually cut into wood as deep as you want as long as the sandpaper is still sharp and the sanding dust does not block the sandpaper.
For example you will remove top one or two millimeters from a parquet floor using a belt sander to remove the varnish and small dents and scratches in the wood. This is done using a sandpaper belt which surely is thinner than those two millimeters.

How deep will sandpaper penetrate?


Is it possible for a Chrome extension to write to a file in Chrome's system directory?

Question: I've never really written a real Chrome extension. I did one that was just a dropdown list of links a while back, but that doesn't really count. Anyway, rather than dive head first back into the massive collection of tutorials on writing Chrome extensions, I figured I'd make sure my idea was possible first.
My plan is this: I want to take the currently playing track in Pandora's player, and send the track information to a source outside the browser (in my case, a Python script).
Getting the info itself shouldn't be hard at all; their new HTML5 interface makes that part super simple. The problem is communicating that information once I've gotten it. I'd like to avoid having to upload the data to a web server and have Python pull it from there.
Obviously, Chrome won't make it very easy for extensions to send out DBus messages or anything, but is it possible to write to a file in Chrome's system directory somewhere, and have Python pick it up? Am I being completely ridiculous in even asking such a question?
EDIT:
I thought I'd report back on what I actually ended up doing for anyone who happens by this question in the future. Since I only needed this extension to work on a single computer (mine), I just installed mod_python for Apache and had my extension send its data straight to my Python script waiting on localhost. This has the advantage of not needing to poll a URL or file from Python, as I had originally assumed I'd need to. It also avoids introducing the potential security risks involved with NPAPI.
Answer: The only way to do this, that I can think of, would be with an NPAPI extension. NPAPI lets you include a native binary with your extension that your plugin is then able to communicate with. You can do pretty much anything you like in that binary.
However, heed the warning on page I linked to above:

NPAPI is a really big hammer that should only be used when no other
  approach will work.
Code running in an NPAPI plugin has the full
  permissions of the current user and is not sandboxed or shielded from
  malicious input by Google Chrome in any way. You should be especially
  cautious when processing input from untrusted sources, such as when
  working with content scripts or XMLHttpRequest.
Because of the
  additional security risks NPAPI poses to users, extensions that use it
  will require manual review before being accepted in the Chrome Web
  Store.
Answer: This is an old question, but here's another option:
If you write to localStorage from your extension, you can access Chrome's Local Storage directory, where every site and extension is conveniently given its own file. In Linux, the directory is located in ~/.config/google-chrome/Default/Local Storage, and in Windows it's at %LocalAppData%\Google\Chrome\User Data\Default\Local Storage. I don't have a Mac to check on, but it shouldn't be hard to find.
The data is stores as what looks at a glance to be JSON inside SQLite. I haven't personally tried turning it into something useful, but the information is all there. I just did a quick test, and the file seems to be updated as soon as you write to localStorage, so you could put a filesystem watcher on it and get a notification any time it changes.


maximum solo 401K contribution

Question: My net profit from a freelancer job is $2656. The IRS "self-employment estimated worksheet" calculated that my SE tax is $375.27, and "Deduction worksheet" calculated that after SE tax deduction, my maximum solo 401k contribution is $2648.37 After I pay SE TAX, the net profit reduces to $2280.73. Can I really contribute more than the profit after SE tax?
Comment: Is this a pre-tax 401(k) or a post-tax 401(k)?
Comment: This is a pre tax Solo 401k.
Answer: If you contribute the complete amount to your 401k, you don't own taxes on it. As a result, you can claim back the taxes you already paid, in your annual tax-filing, and yes, you can put the complete amount, including the temporarily 'missing' taxes, in the 401k (assuming you have enough cash on hand).
Comment: Really. I asked a question in a previous post and the bloggers said that SE, self employed tax must be paid first, then I can contribute whatever money left up to maximum allowable amount based on IRS. Your answer was my original ideal plan that I gave up after getting old response from my previous question. You suggested I can contribute all after expenses to my SOLO 401k, right? Thanks.
Comment: @soodala2k you've got "two" wage numbers: one that your **income tax** is calculated on (which is reduced by 401k contribs), and another that FICA/Medicare are calculated on (and which are **not** affected by 401k contribs).

maximum solo 401K contribution


What phrase to use in this situation?

Question: I am working on a statistic problem. I define "A" as the number of all the products in the warehouse, but some of the products are not qualified and I need another parameter to describe the number of the qualified products, for example, "B". Then, what should I say about B? Could I say that B is the number of all the products except for the unqualified ones? Is "except for" a proper expression? May I have other words or phrases that fit the situation better? Thanks a lot!
Answer: Try using 'A' as all products in the warehouse and 'B' as all qualified products in the warehouse.
If instead we use an exclusion method, such as 'B' as all products in the warehouse excluding all unqualified products, it suggests that 'B' contains products that are qualified plus products that have not yet been processed to a qualified or unqualified state.
Comment: With or without the exclusion method, the potential existence of products that are neither qualified nor qualified must be considered, and therefore we should decide which group would include them, if they exist.
The exclusion method merely emphasizes that such a group could exist, otherwise why do we not use the simpler 'B' as all qualified products form.
Comment: Thanks for the explanation. Do you mean that, if I use an exclusion method, B could include some products that are neither qualified or unqualified?
Comment: I think I take your point. Thanks again for your quick response.

What phrase to use in this situation?


Discrepancy in table 2.1 Pg 49 in the book The Intelligent Investor by Benjamin Graham

Question:
The above pic shows inflation at 5 years intervals. I am finding a bit hard to understand this table. For e.g. calculation of percentage rise in wholesale prices in 1960 seems to be wrong. Actual rise
(100.7 - 97.2)/97.2 = 3.6% but the table says its 9.2%. I found such discrepancies in other values as well. Also, I am not able to grasp the footnotes at the table (specifically point a and b). Can someone please explain?
Thanks!
Answer: Footnote a means the following:

Price levels are annual averages
The table sets the year 1957 as the base level (100) for both wholesale and consumer prices
Indexing the year 1967 as the base level--instead of 1957--would lower 1970 consumer prices from 134.0 (as displayed in the table) to 116.3 and 1970 wholesale prices from 117.5 to 110.4
"for the stock index" is a misprint and belongs in footnote b immediately below

Footnote b means that the S&P stock index was normalized to a value of 10 during the interval from 1941-1943. That choice was probably influenced by World War 2, just as the year 1946 was used instead of 1945 (to avoid the distortion effect of temporary war time price controls).
There are a lot of errors in the five-year percentage changes throughout the table, for wholesale and consumer prices as well as stock index earnings and prices.


Must a CFII be instrument current to provide instrument training?

Question: Can a CFII conduct training for an instrument student in VFR conditions if the CFII isn't instrument current?
Comment: Welcome to aviation.SE! I edited your question to make it a little clearer but if I got something wrong then don't hesitate to edit again or just roll back my changes. I assumed you're asking about the US; whenever you ask about rules or regulations please always tell us which country or regulations you mean. The [tour] may be helpful if you're new to the site.
Comment: That... is a really good question!
Answer: The requirements for providing instrument instruction are in 14 CFR 61.195(c) and they don't mention currency at all. There are also various FAA legal interpretations on whether or not CFIs (not CFIIs) require an instrument rating to provide instrument training (see this question), and what instrument training requires a CFII in the first place (see this question) and none of them mention currency either.
So no, there is no requirement for a CFII to be instrument current when providing instrument training.
Having said that, 61.65(d)(2)(ii) requires instrument students to do a cross-country under IFR with an instructor. An instructor who isn't instrument current couldn't provide that specific training flight, although obviously the student could use a different instructor. And many CFIIs like to take their instrument students into IMC to let them experience how different it is from being under the hood.
All in all, it's in a CFII's best interests to remain instrument current but it isn't required.
Comment: I think it's best to be current before taking on an instrument student so the CFII is sharp and giving the best possible instruction
Comment: @DanielRackley I don't agree. Everything else being equal, a CFII who is actively training students would be a much better choice as an instructor than someone who is current but hasn't trained anyone in a while.


Are there advantages to using packaged wood finishing cloths instead of old towels and shirts?

Question: Some manufacturers offer packaged "wood finishing cloths". I've always used old towels, old shirts, or paper towels both for applying certain finishes as well as when wiping off excess. Do "wood finishing cloths" offer much advantage over using whatever is relatively clean and laying around for either of these applications?
Comment: I'm voting to leave open. I disagree that it's primarily opinion based, as the answers have objective points (ie, guaranteed lint free).
Answer: I've used both and haven't seen any real advantage to the manufactured ones. In fact, I seem to get better mileage out of my polyurethane by using a bit of old sweatpants material cut and sewn into a tube. It fits my fingers well, holds more finish before having to re-apply to the rag, and the outside doesn't deposit lint; it's convenient, and seems to use less finish than the manufactured fabrics since you're not soaking the whole rag, just the surface area you're using to wipe with.
Answer: Main advantages of the manufactured ones is that you know they're low-lint, you don't have to worry about color bleeding out, and if you're doing a lot of woodworking you might run out of houehold rags.
On a slightly related note: microfiber cloth seems to be as good as a "tack cloth" for removing dust before finishing, and can be washed and reused many times.
Answer: I once found a roll of cheesecloth in the dumpster at work and took it home for finishing projects.  I liked it because it was rolled up like paper towels and the individual pieces were separated (about the size of a roll of paper towels). I could just grab one when I wanted it.
They worked well but no better than the cut-up T-shirts or old cotton socks that I usually use.  By the time I retire a pair of socks, there's little fuzz left to become lint.  If there is any lint from a T-shirt, it seems to come from the cut edges which I manage to mitigate by folding the cut edges under when I roll up a piece.
Answer: I would say there are 3 considerations

Lint:  The smallest amount of lint will make a finish bumpy. Provided you know which types of cloth will or won't leave lint, I see no benefit in the manufactured ones.
Absorption:  Some fabrics have too much of a synthetic fiber content and won't hold enough of the finish to be useful. They can have a 'smearing' effect.  You'll know as soon as you try something like a sock with poly blend.
Supply:  I have no lack of household cloths.  My favourites are dish cloths and dish towels.  If you have a supply problem, then you can either buy manufactured ones, or spend $2.00 on some flea market t-shirts.

So the first two considerations are a matter of experience.
The third is the only reason I would suggest buying manufactured cloths.


Reporting ESPP ordinary income on shares purchased living in Europe

Question: I have shares acquired through an American company I was working for. They were purchased through the company ESPP plan. I was living and being paid in Europe at the time ( I am not a US citizen). 10 years later I have moved to the US full time and will be shortly selling these shares. Am I liable for US ordinary income tax on these shares, even though they were acquired while not a US tax payer?
Comment: Thanks for the info. I knew I was going to have to pay CGT when I sell the shares, but the ordinary income tax obligation was a mystery. Selling the shares before moving wasn't really an option as the share price was very low.
Answer:
Am I liable for US ordinary income tax on these shares, even though
  they were acquired while not a US tax payer?

No, you're not liable for the US ordinary income tax.
If you are a US tax resident already, you are liable for the US capital gains tax.
Since the holding period was more than 1 year, that means preferential tax rates, but you're still going to pay taxes on your gains on these stocks even though they were acquired when you weren't a US tax resident. To avoid that, you should have discarded them before moving (and maybe bought back if you wanted to continue holding them).


Why does light take the path taking the least time?

Question: I know this question has been asked many time but my question is bit different here. What i mean by the above question is that I recently got to learn that light actually does not exclusively take the path taking least time. Light, infact, also takes nearby path other than that taking the least time but the reason why we see light taking least time is that the other path interfere with one another and cancel each other out. Now my question is that exactly how does this interference and cancellation occur? Also why does this interference and cancellation also not occur with path taking the least time? I am not very advanced in math so please if anyone can try to theoretically explain this or try to go easy on the math, I woud greatly appreciate it. Thanks.
Comment: TLo is asking how the waves that take the paths that are not least time interfere with each other to cancel out. It is a clear and standard question. It has nothing to do with seeing light spread uniformly (where you see scattering), not with light as particles. Light is both particles and waves, the truth is in the electromagnetic theory classically and QED quantum mechanically. The explanation is best done with a simplified exposition of how Feynman path integral formulation works, or the Huyigings (spelling?) principle classically and how the different wavelets cancel out
Comment: I think that if we envision light as being photons, the least is actually the most probable path.
Comment: Photons actually do take every path. Turn a light on and then go stand at different points of view.  You'll see that the path of least time is not exclusive, it is everywhere.  It's easier to derive this with individual photons instead of waves.
Comment: Related: https://physics.stackexchange.com/questions/375170/how-does-fermats-principle-of-least-time-follow-from-huygens-principle
Answer: Fermat's principle is one expression of it, you can see it derived at
https://en.m.wikipedia.org/wiki/Fermat%27s_principle
It is a consequence of Huygens's principle, which is explained and where you see the wavelets cancelling out in the article atwikipedia at https://en.m.wikipedia.org/wiki/Huygens–Fresnel_principle. It is also derived there. It shows that rectilinear or plane wave propagation (i.e., the simplest mathematical way to figure it out from A to B, taking A and B at infinity) is also explained that way. Fresnel showed that in 1816.
The Feynman QM formulation is in his Lectures in Physics, you can google more. It has also been generalized in field and quantum field theory to the principle of least action, actually for any field, not just light. See for instance https://en.m.wikipedia.org/wiki/Principle_of_least_action


Martin Luther's 95 Theses

Question: Did Martin Luther actually nail a copy of his 95 Theses to a church door? What do we know and not know about this?
Comment: This is not a trivial question, it is a controversial and debated one. The article in the English Wikipedia does not address it at all.
Comment: Have you read the Wikipedia page on the [95 Theses](https://en.wikipedia.org/wiki/Ninety-five_Theses)?
Comment: @fbd If this question is worth re-opening, it would be worth showing some initial research as well.
Answer: The de.wikipedia has dedicated a whole section to the historicity of this incident. In the introduction:

Die Historizität des Thesenanschlags, bei dem Luther seine 95 Thesen
am 31. Oktober 1517 eigenhändig an die Tür der Schlosskirche in
Wittenberg genagelt haben soll, ist umstritten.

The historicity of the incident that Luther shall have nailed the 95 theses on the church door in Wittenberg at 31. Oct. 1517 is debatable.
Although in de.wikipedia the article has not properly used citation in situ to reveal the sources of individual sentences. The debatable nature of the theses-nailing is shared by a number of websites, including luther.de:
http://www.luther.de/leben/anschlag/

Daß Luther an besagtem Tag seine Thesen mit lauten Hammerschlägen an
die Tür der Schloßkirche zu Wittenberg genagelt haben soll, gehört
aber wohl in das Reich der Legenden (Die Legende vom Thesenanschlag).

The incindent that Luther at the said day nailed his theses with loud hammerstrokes on the door of the Schloßkirche in Wittenberg belongs to the realm of legends.
More info comes from Frankfurt Allgemeine:
Schwang Luther 1517 tatsächlich den Hammer?
In the article it wrote that there is no definitive answer to this day. 300 publications have been dedicated to the subject.
First, Luther himself never mentioned the nailing. The whole nailing thing comes from the testimony of Luther's comrade Philipp Melanchthon and a note from Luther's secretary Georg Rörer (this note was rediscovered in 2006 with lots of media coverage). Both refered to one nailing event. The "Pro-nailing" historian believe that two accounts are independent and Rörer's account appearred even in Luther's lifetime. The "Against-nailing" historian asserted that yes, they were from Luther's comrade, but not from the witnesses of the event and what is more, they were penned 20 years after the said days. And they believed the whole thing is to build a myth.
What is sure is that Luther sent his theses to two bishofs at that day and later also to others. Also, the theses were written to provoke a theological discussion about indulgence. It was common in those days to print these theses in Wittenberg. And according to the theology faculty it was also common at those days to paste the prints on church doors. So if Rörer is a reliable source, Luther's theses were also nailed at several churches. So nailing theses at the door was at that time more like an announcement thing rather than a war-declaration.
Comment: Sorry for the confusion between gate and door. Answer edited.
Comment: I think there is a big difference between a "door" and a "gate." Nailing your "indulgences" to an actual door of a Church would get an Achtung however....
Comment: Tür means "door"; Tor means "gate". The translations given in this answer, and hence also its conclusions, are wrong.

Martin Luther's 95 Theses


ccd application circuit

Question: I'm trying to use a ccd linear image sensor (µPD3753) and I'm using the application circuit example in its datsheet, however I'm new at using these type of sensors. Can someone please explain why they're using a hex inverter and not just one (this is needed because the sensor has 2 clocks and they must be inverted in order for the device to work) also why is the pnp transistor needed.
Datasheet
Answer:
"why they're using a hex inverter and not just one"

...because you need 3 (not just one; their basic function is buffering the signal and just inverting by the way) and they come in packages containing six.

"why is the pnp transistor needed."

...because the CCD output signal is high impedance (cannot provide much current). The transistor circuit is an emitter follower that leaves voltage about the same (just adds about 0.6V but that doesn't matter at all in this application, because the signal will have a more or less arbitray offset anyway because of "dark current") but has low output impedance (can provide much more current).
Answer: IC's always define the test setup for input and output parametric results.
It says in so many words in the datasheet that this is not mandatory but it is how their IC was tested. It is always a good idea to define source and load impedance for every spec and here they choose what they thought was ideal.
Since the 5V 74HC04 drivers are approximately 50 ohms and the Clock Φ1,Φ2 inputs are 300 pF typ. having a 47 Ω terminates the driver during the transition to minimize ringing from the capacitive load.
The Vout has a Zout spec. of 0.5kΩ typ to 1k max,  so having an emitter follower buffers the output signal and with a PNP makes it short circuit proof to gnd only.  You can choose whatever impedance you need for your load.
It is consistent with the same attention to detail in the schematic to using 2 caps for supply decoupling and RC decoupling to Vod on IC and  emitter follower.

ccd application circuit


Physical commodity trading quantitative risk return model

Question: I am very new to commodities, I was previously in portfolio management/optimization (Black Litterman Markowitz etc). I am now a Buy-Sell analyst for Petrochemicals, and need to understand the basic concepts of physical commodity trading in order to create a model supporting trading decisions. I have been through Investopedia, IHS, Forbes, and a few other sources with a comb but am yet to find any actual simple examples of incorporating variables like risk appetite or political risk into a risk-return model for a single transaction. Any help/ advice on where to start is highly appreciated, I am completely lost.
Answer: Your question piqued my interest. While not specific to commodities, this looks like a good starting point for quantifying political risk:

Practically, this means taking the following steps (according to Rigobon 2003):
Step 1: Define the treatment group, or a set of "event" days on which the variance of the unobservable factor is high, such as the Italian referendum on December 4.
Step 2: Choose a set of "non-event" days to serve as a control or comparison group. Common practice suggests choosing non-event days one or a few days before the event days, so as to minimize the influence of risk factors other than the political one. Political risk may also change on these days, but (by assumption) they change less than on event days. This assumption is a leap of faith, but hopefully a reasonable one.
Step 3: Apply a standard econometric technique known as instrumental variable (IV) regression. Consider regressing changes in one variable of interest (e.g., the CDS spread of Spanish sovereign debt) on changes in a second variable (e.g., Italian CDS spreads), that is then instrumented by a proxy.3 This proxy is the same variable but with opposite sign on non-event days.
Unlike a traditional event study, this approach does not apply the unrealistic assumption that political risk only changes on event-days. In situations like the Italian referendum, in which periodic polls revealed changes in voter preferences, the traditional event-study method may underestimate the magnitude of the political risk.
source: Quantifying Political Risk on Financial Markets—Italian Case Study
Comment: thankyou verymuch, am now reading this! Assuming I am able to quantify the risk of an event, how do you think I should go about factoring it into a trade deal? thankyou for the sexsigma reference I didn't think of checking their insights!
Comment: I read the Case study in the link, very informative. The complexity I have yet to find any concrete resources for, is how do Physical commodity traders like Trafigura or Glencore actually plug this quantified political risk into a VaR and Risk-Return model to make a decision. For portfolios the vanilla is the Markowitz or Black-Litterman, right? so what is the base quantified models for Physical Deals and Risks? do I minus it? multiply ? that's where I am maybe ignorantly stuck.
Answer: Commodity prices depend on global supply and demand but not on the perception of the market regarding an adequate risk premium for a specific asset class.
Have a look on this Claude Erb and Campbell R. Harvey, "The Tactical and Strategic Value of Commodity Futures"
Answer: I think you might benefit from reading a few resources.

Hélyette Geman's book Commodities and Commodity Derivatives is an excellent overview from a quantitative perspective.
Morgan Downey's Oil 101 is a good explanation from an oil trader's perspective of the oil market.
For petrochemicals, pipelines, refinining or other issues, William Leffler's "in Nontechnical Language books are great. Petrochemicals in Nontechnical Language is written by Leffler and Donald Burdick and could prove useful for you.
If you find yourself getting into natural gas or power, Bob Shively and John Ferrare's Understanding Todays {Natural Gas, Electricity, Global LNG} Business books are informative and usually up-to-date with recent practice.

Risk appetite or aversion are often absent from commodity models because the contracts are often used to hedge risks. That said, if investors became a large presence in a certain market, a factor model with a commodity or multi-asset-class index (similar to the CAPM) might be useful. I suspect you might do better just trying to model the range of suppliers and their GPMs so that you can infer the supply function (similar to the "powerstack" function in power markets).
Answer: Besides taking the suggested reading materials above and skimming them for help, I have been in commodity markets probably 15 years.  I can tell you that there are NO commodity risk programs I've found that will answer your questions we all have as quants.  The typical quant will invent their own method combining concepts from the financial markets which are typically covered in school.  I've seen some of the strangest modeling in commodities including a model combining interest rate models with volatility surface models and models never combined in any research paper.  All I can suggest is that computing power today is so high most problems can be "solved" i.e. estimated via Monte Carlo.  Whether that leads you to a Python 3D array (correlations - what time period - EWMA - Garch - up to you - vol surface lookups, 1 week or a year model etc.).  Just backtest whatever creation you come up with in and out of the sample used to create it before using it and see how it performs before using it.  I'm also looking to boost my quant skills as a risk quant I have to examine / replicate / replace trader desk models.  If I knew of a good course or certificate I'd tell you, but I can't find any.  I work with some super creative quant traders no one can understand their mix of financial market models with commodity models either.  Apparently the are elite PhD graduates a.k.a. genius mathematically
Comment: Thanks for the answer @Matt , so technically I am "supposed" to be one of the Elite mathematic math wiz PhDs, I did a Applied Math in Surgical Robotics PhD at an Ivy League school. However, the PhD thing only gives the (average) of us more confidence in exploring areas we dont know and believing we can pick them up. I am sure there are some PhDs which by far out-math me however, theoretical Math and Physics PhDs being one.
Answer: To Anyone else looking for a base model to go off on, I was not able to incorporate the more high level (market risk, political risk), etc I was looking for. However, Trafigura does publish their value at risk model (VaR) for a single transaction: https://www.trafigura.com/media/1490/2014_trafigura_the_economics_of_commodity_trading_firms_section_iii_english.pdf
This was the single most useful article I found. To incorporate the additional risks, I would theorize using this risk as a baseline against which to go, and adding constants/variables in as blackbox variables (whatever metric you chose as political risk), and with your own company/public historical data, reverse identifying a good weight for the given variable. I have moved no from Petrochemical risk and am now a Data Scientist, so have not tested this theory


How could an AI be used to improve the teaching and learning of mathematics?

Question: I have been working with AI methods. I am thinking about how my daughter (and also other kids) could learn mathematics with the help of AI. For example, how could an AI be used to show the mistakes that a kid does during the learning path?
Answer: Fundraising is probably the most realistic answer with the biggest effect. Similar to simply adding blockchain to the company name helps.
For the question of real application, I should first say that I'm a machine learning engineer who works as a consultant. I had mathematics as a minor subject during bachelor's and Master's degree, which is like 80% of a bachelor's degree in mathematics. Mainly the bachelor's thesis is missing. I also gave private mathematics lessons.
I would distinguish learning mathematics at school from University. It's completely different. However, for this question it does not make much of a difference. It's also not super important that you ask it for math. It's about learning in general.
Machine learning tools can support for several minor tasks:

Figuring out how a symbol is called (I build write-math.com for that as a bachelor's project)
Figuring out what a symbol means
Getting relevant papers / parts of the lecture notes for a given problem
Helping to digitalize stuff for easier sharing / content creation
Learning things by heart: there are some nice apps like Anki which help you to learn arbitrary stuff by heart. The learning algorithm could potentially be improved by AI


how to compile single LESS file in magento2

Question: I am working on magento2 theming part. Here we have some predefined and my custom LESS file. Whenever we compile LESS file, it generates two css file -- styles-l.css, styles-m.css . But we do not know from which LESS file, my css is going to styles-l.css and styles-m.css. Is there any way to find out this issue? I want to see from which file (line number too) my code is coming ? Is there any way to solve these issues ? Any help is  really appreciated. Currently i am doing deploy static data. It compiles all LESS files. I want to compile single LESS file.
Answer: You have to install the Grunt for your server, check below url to setup the Grunt .
http://ericnish.io/blog/compile-less-files-with-grunt/
Inside the Gruntfile.js you can add the code like below example
For single File compile us the following code
<code>files: {
          "css/main.css": "less/main.less" // destination file and source file
        }
</code>
Answer: Compile only 1 file makes no sense.
But you can use SourceMap for your compiled CSS to see in which line & file is your sourcecode.

Enable SourceMap-Support in Chrome Browser:

http://dannycroft.co.uk/enabling-source-maps-in-chrome/


Simple Module in Craft CMS 3

Question: How do I create a simple module for one website in Craft?. I don't have much experience in Craft CMS.  I have tried from 3 days but I cannot get an answer. I'm following all the tutorials that I can find, but none I feel are clear. Any recommendations on the simplest of examples, even if it's just outputting a variable.
Answer: Start with the manual: https://docs.craftcms.com/v3/extend/module-guide.html
This blog post covers module development in more detail:
https://nystudio107.com/blog/enhancing-a-craft-cms-3-website-with-a-custom-module
It includes a link to generate a scaffold module, which should work out-of-the-box.
Comment: thanks alot. that help me so much @Richard
Answer: I recently reviewed a fellow developer's module and posted a recording of the review online. It's not necessarily building a module from scratch, but I did very briefly cover the different parts of a simple module Class, and also specifically how to access and save FreeForm submissions and Craft entries, so hopefully it could be of some help. The only part I did not really cover in the video was hooking it all up in the composer.json and config/app.php files, but that should be fairly explanatory from the docs.
Comment: but how to run this module. i am totally new in craft @jalen

Simple Module in Craft CMS 3


How to export all Google Docs in a folder in one single PDF?

Question: I have a folder with hundreds of short Google docs (product descriptions).
I am wondering if there is an easy way (like "push button") to export all these in a PDF that simply assemble all the PDF into one single document.
Answer: I believe there is no integrated Google Drive feature for that at the moment. Essentially what you are trying is exporting server-side merged files. What you could do instead is download all the files in questions (in PDF for instance) and merge them locally. There are applications like pdfsam that can help you do that.
Answer: https://takeout.google.com/settings/takeout/custom should do the trick.
I apologies for the late reply.
Comment: Could you please post the relevant parts of the answer here.  Answers that are just a link are not very helpful.