Randy - As long as the difference is reasonably predictable, you can develop quite good setpoint profiles to control your BT effectively. I was bothered with the 110°F differential for a while, but I just set the ET target to desired BT + 110. So my first crack happens around 510°F ET or 395-400 BT. The problem I had before was that the difference between BT & ET varied throughout the roast (the BT & ET converged at first crack, and then the BT exceeded ET - A physical impossibility!). That makes designing a profile a pain in the arse.

Ultimately, if you find that the BT thermocouple is giving you good enough control, then you're good. You might try monitoring ET temperatures with the BT control to make sure you're not driving the ET heat too high too early in the roast. but if that were the case you'd likely see scorching or tipping on the beans.

At some point I think the uneven heating in a popper will become more than just an ET measurement issue. As the temperature difference between the hottest and coolest parts of the roaster/heater become greater there will be a point that it will effect the quality of the roast.

I have noted also that lower ambient temperatures increase the problem. This makes sense as there is a larger delta across the heated air input, more work for the heater to do.

I don't know what that point is and it would be difficult to test. 20F, 60F, 100F? At some point here it will begin to affect the roast quality. The ideal is to have total immersion in a uniform heated environment. While no roaster quite achieves the ideal, still, the closer the better.

Just think about gently feathering the finish on a light City+ roast. ......somewhere between 400-500F ET....

OK - I fired up the controller and it says the P is set to 105, I to 2 and D to 0. The I offset is 82.5 - Control period is the smallest it will take, 1 sec.

This keeps the temperature within about +/- 6 degrees F when at about 420 BT/530 ET. I would like to get it tighter than that, but that tuning was the best I could find.

Regarding the Automation Direct software -- It will not load the Delta controller. :( It says the SOLO unit was not found. The recording software works to record the PV so I know it's communicating. It just can't control the controller.

OK - I fired up the controller and it says the P is set to 105, I to 2 and D to 0. The I offset is 82.5 - Control period is the smallest it will take, 1 sec.

This keeps the temperature within about +/- 6 degrees F when at about 420 BT/530 ET. I would like to get it tighter than that, but that tuning was the best I could find.

Regarding the Automation Direct software -- It will not load the Delta controller. :( It says the SOLO unit was not found. The recording software works to record the PV so I know it's communicating. It just can't control the controller.

Thanks for the info, will try tweaking my settings some more tonight.Bummer about the Automation software not working with your Delta controller.I was thinking about trying to buy another controller, so I could easily monitor both ET and BT. The nice thing about 485 is you can have multple devices on the same line, so you can put another controller on the 485, set it up with a different address, and use it to monitor another thermocouple (at least in theory).

I was going to look for either an automation direct, or Delta controller, but I can't use a Delta, based on your post.

Now, I will have to see if I can luck into another deal for an automation direct controller, I got lucky on ebay for the one I just got.

Sorry if I got your hopes up with the additional features of the solo software.

Frost,I just finished insulating my popper, in the hopes of reducing the temp difference in the roaster. I'm sure the bottom, where the hot air enters the chamber, is the hottest, but I am hoping that my insulated popper is better in that regard. I am thinking I should make a taller chimney (it's one soup can tall now, can double that :), and maybe add another layer of insulation on the cans.

So, after losing the PID data logs from two roasts, I sat down this morning, to figure out what I was doing wrong.

In their logger program, you have to tell it to save to file BEFORE you start. I'm used to doing if after, at the end, but that did not work too well with this program. :(

I do have some hand noted temps from my last roast, and I saw a difference of between 80 and 100F between BT and ET. Not sure why it seems less now then I saw before, but I took 5 data points, and that's what I got.

Yes - the Delta software works the same. It is buggy and not very intuitive.

I do have some hand noted temps from my last roast, and I saw a difference of between 80 and 100F between BT and ET. Not sure why it seems less now then I saw before, but I took 5 data points, and that's what I got.

I saw you had a 100°C difference before, but that you then moved the TC to a more "average" spot. Is it possible you're seeing differences in offset because of ambient temp differences? Or perhaps even humidity? Anything else diffferent this time? Again, so long is the differential is not variable from roast to roast or too variable during the roast, you can do BT profiling effectively from it.

I have been reading about your controller and I am putting together one of my own similar to the one illustrated in your post. Very helpful information-could you possibly post complete wiring diagram to include switches and the like. Thank you in advance.

Hi Lee -- I don't have anything more updated than what I posted. Here are a couple things I found out in the months following the build:

1) Make sure you use real quick connect terminals and don't try to attach a quick connect terminal to a solder terminal. Some of the parts I ordered or tried reusing (e.g. the fuse holder which was salvaged from the router speed controller) had solder connects that were unmatched in thickness from the quick connects, so there was added resistance which can cause the wire insulation to melt and potentially catch on fire. See this thread for the gory details.

2) I also installed a small fan in the side of the case below the switch to cool the case. I do not think it's really that necessary but a good idea to keep the temperatures in the case stabilized.

3) Use a 15A power cord - most of the ones from computers are 10A, and the wire heats up scarily hot.

Here are some parts that I ordered from Mouser after I completed the BOM:

The slide switches for the thermocouple input were junk. I replaced them with toggle switches: click here

Also, I moved the MET thermocouple sensor from the chamber location because it was not giving good readings. There is too much variance in the MET at each point in the chamber where the fins enter the popping area. I installed a stainless steel mesh "floor" into the popping chamber, and threaded the thermocouple wire into the mesh so it measures the MET just below the mesh which I find gives a better indication of the temperature of the air hitting the beans. I ordered a swatch of mesh from here and cut a circle of the mesh just larger than the chamber, and wedged it in so it was about 1/3" above the floor of the popping chamber, and slightly convex (in relation to the beans). That also helps prevent scorching and tipping.

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