A new electronic viscometer based on Rayleigh wave mechanics

The operation of a new viscometer is described, based on Rayleigh wave mechanics, where a thin walled tube of 5 mm diameter is immersed to a fixed depth of 60 mm in a fluid and is made to undergo axial vibrations generated by a vibrating piezoelectric plate which supports the tube. The motion of the tube is damped by the fluid, and the piezoelectric crystal feedback circuit gives a voltage inversely proportional to the square root of the viscosity × density product. The design and performance characteristics of the patented viscometer are described. The instrument is portable and gives a measurement of viscosity × density for a test time of 1 min, over a wide range of values, and can be modified to permit continuous measurements as a function of time. The results are accurate for a Newtonian fluid, while giving an apparent viscosity × density at low shear rate for non-Newtonian fluids. A probe measures the fluid temperature during the test. Only a small amount of fluid is necessary for measurement. Rayleigh wave mechanics theory is shown to be obeyed by the instrument. The electronic circuit uses a phase-locked loop to drive the thin walled tube at a resonance of about 1.5 kHz. The processor feedback signal is large, varying from 2 V down to ∼0.5 V DC when measuring SAE 10 to 60 engine oils. There is an onboard computer to process the data. The computer gives a prediction of viscosity at 40 °C, or can be modified to predict viscosity at any reference temperature, for a paraffinic or a naphthenic mineral oil, as well as for various synthetic oils. For a particular fluid, it is possible to build data reduction into the computer, for example, equations for density as a function of temperature can be stored in the computer to reduce the viscosity × density result to viscosity and to density.