This invention relates to an improved transducer for use in a vibratory viscometer; which transducer is particularly useful for, but not limited to, the inline detection under flow conditions of viscous and elastic properties of fluids being processed.
Vibratory viscometers are well known in the art and generally employ a transducer which has an immersible portion which is vibrated with a small amplitude. Fluid viscosity/density/viscoelasticity can be determined from the frequency and/or amplitude changes in the vibration and/or the power required to sustain the vibration when the immersible portion of the transducer is immersed in a fluid.
Such transducers generally comprise (i) an immersible tip, (ii) an electromagnetic drive and (iii) an electromagnetic or piezoelectric pickup. Transducers of this type are described by J. G. Woodward in "Vibrating Plate Viscometers", Electronics, February 1952; and by J. D. Ferry in "Viscoelastic Properties of Polymers", published by John Wiley & Sons, New York, 1970.
Prior Oscillatory Viscometers
The oscillatory viscometers described below exemplify the prior art, and have transducers which interact with the fluid being measured by "surface loading" of an oscillating portion of the transducer; said term having been used J. D. Ferry in his aforementioned publication.
The oscillating surface generates shear waves in the liquid or other fluid. Viscoelasticity is measured in terms of the characteristic impedance of the liquid.
In the vibrating plate viscometer described in the aforementioned publication of J. G. Woodward, the moving end of a vibrating reed supports a plate which detects viscous resistance of the fluid in which it is immersed. The reed is driven electromagnetically and the oscillations are picked up by a barium titanate piezoelectric block. The reed is clamped at one end, exposing the driver and pickup to fluid vapors. The viscometer measurements are confined to viscous loss determined from the decrease in amplitude of oscillation observed when the plate is immersed. The plate vibrates in a direction essentially perpendicular to its major surfaces.
A. Konno, S. Malsino and M. Kameko, in Japan Journal of Applied Physics 189 (1968), reported their measurements of storage modulus and viscous loss by oscillating an immersed very thin microscope slide at 100 Hz. This apparatus, driven by a moving coil, is reported as strictly a research instrument. The internal workings of the transducer were exposed to fluid vapors.
A viscometer known as Le Viscosimetre "Pivert" is sold by the Societe Francis de Service, 8 rue Nobel Zl, 45700 Villemander, France. The sensor tip of this viscometer (also called "Sofraser") is a U-shaped stainless steel needle. One leg of the U-shaped needle is mechanically driven sinusoidally at 125 Hz., which is near the mechanical resonance frequency of the transducer. Both legs of the needle are clamped at nodal points. Frequencies are measured at both legs. The phase difference between the frequencies is transformed into a voltage or current related to the viscosity of the liquid in contact with the needle. The manufacturer states that installation conditions can decrease accuracy and therefore should be carefully checked to be sure of optimum accuracy. The range of viscosity measurement is 0.5 to 15,000 mPa.s (cPs). The environmental limits of utilization are 200.degree. C. and pressure to 100 bars.
The Dynatrol Viscosity Detector is sold by Automation Products, Inc., 3030 Max Roy, Houston, Tex. 77008. Although this instrument does not employ a blade sensor, it does employ welding at the node point of a resonating system in order to isolate the oscillating probe from the driver and pickup. Viscosity-density product is detected by an immersed 5 inch long stainless steel probe. The probe is made from a stainless steel rod and bent very much like a hairpin. Both ends of the rod are welded at nodal points to a supporting plug. One end of the probe penetrates into the housing where it is electromagnetically driven to resonate in flexure at 120 Hz. The other end of the probe also penetrates through the plug into the housing where there is an electromagnetic pickup coil. The decrease in amplitude of vibration of the immersed probe due to interaction with the fluid is electronically converted to viscosity-density product.
The Labor-Viskosimeter QV35 is marketed by Bopp & Reuther GmbH, Car-Reuther-Strasse 1, Postfach 310140, D-6800 Mannheim 31, West Germany. This instrument uses an oscillating transducer to measure viscosities of laboratory samples of liquids by means of a quartz crystal sensor oscillating in torsion at 55 KHz. The damping effect of the fluid being measured on the amplitude of oscillation is converted into viscosity. The temperature range of this instrument is limited to -50.degree. to 150.degree. C.
The Model 1800 Viscometer sold by Combustion Engineering, Inc., P.O. Box 831, Lewisburg, W.Va. 24901 utilizes an oscillating sensor blade. Short pulses of current are applied to the top of the blade, which is composed of a magnetostrictive alloy. The blade protrudes through a metal diaphragm. Each pulse causes the blade to vibrate at its natural frequency of 28 KHz. When the amplitude of vibration of the immersed blade has fallen to a preset value that relates to the viscosity of the liquid, another pulse is automatically applied. The change in pulse rate is proportional to the square root of viscosity-density product. The range of viscosities measured is from 0 to 5,000 centipoise x grams/cm..sup.2 in ranges of 0-50, 0-500, and 0-5000. This viscometer was produced for many years by Bendix Corporation, and was first described as the Ultra-Viscoson by W. Roth and S. R. Rich in Jr. Applied Phy. 24 940-950, July 1953. The Bendix Ultra-Viscoson is described on page 308 of Viscosity and Flow Measurement by S. R. VanWaser, S. W. Lyons, K. Y. Kim and R. E. Cowell, Interscience Publishers, New York, 1963. Shortcomings included very high frequency of measurement, fragility of very thin strip (blade), lack of sensitivity at very low viscosities, need to flick the strap from time to time to relieve strains in the magnetostrictive alloy; and since strips were easily bent, replacement blades needed to be available.
Hermetic sealing between driver and pickup is a feature of the "Vibrating Sphere" and "Viscoliner" oscillating viscometers of Nametre Company, 101 Forrest St., Metuchen, N.J. 08840, the assignee of the present application. However, the spherical and cylindrical sensors are somewhat obstructive to flowing fluids, particularly slurries. In order to reduce the obstruction to flow, the diameter of the pipe in which the fluid flows needs to be increased so as to satisfactorily accept the oscillating sensor. Since the mode of oscillation has two degrees of freedom, care must be taken to chose between in-phase and out-of-phase torsional motion. Further, the oscillating surface generates diverging shear waves that may be so long in wavelength that they are not conveniently reflected for accurate measurement of highly viscoelastic fluids.
U.S. Pat. No. 4,729,237 describes a tuning fork transducer, each of the two arms of the fork having welded to it a blade that is oscillated in the liquid. It is claimed that one blade on one arm of the tuning fork gave less accurate viscosity measurements than having blades on each arm. This viscometer is a laboratory instrument. The specification describes vertical motion of a sample container to immerse the two blades in the liquid.
Objects of the Present Invention
For a long time there has been great need for small inline blade sensors that do not impede flow and that are intrinsically separated from the fluid being measured. Examples of these needs include transport of mineral slurries such as powdered coal and powdered lime where particle concentration must be controlled. The consistency of food fluids such as ice cream, coffee and bread dough needs to be monitored and controlled by means of reliable rugged viscosity sensors. Better control of viscoelastic properties of polymer fluids can be achieved by employing suitable sensors. Accordingly, an object of the present invention is to provide an inline blade sensor for a vibratory viscometer that impedes fluid flow to a substantially lesser extent than prior art sensors. Another object of the invention is to provide such a sensor in a configuration that facilitates isolation of the drive and pickup portions of the transducer from the fluid.
Other objects of the invention are to provide transducer-sensor devices that are relatively small, are rugged, easily inserted into industrial process pipes and equipment; that operate over a broad range of viscosities under hot and cold conditions and at high pressures; that can be speedily installed and quickly operated in remote and dangerous locations; that are dependable over long stretches of time; that can be easily removed, cleaned and reinstalled; that are capable of providing viscous loss and/or transducer sensor frequency signals for conversion into loss modulus and storage modulus values; that are capable of being vibrated over ranges of frequencies as well as at mechanical resonance; and that are adaptable for use in very viscous and very elastic liquids.