Effects of viscosity, temperature, and rate of rotation on pressure generated by a controlled-clearance piston gauge

Abstract

The calculation of the pressure generated by a controlled-clearance piston gauge depends upon the jacket pressure corresponding to zero clearance between the piston and cylinder Pz. The dependence of Pz on the viscosity of the pressure transmitting fluid, the temperature, and the rate of piston rotation have been measured. The four fluids used in this study have viscosities ranging from 0.7 to more than 10 000 cP. The value of Pz is nearly independent of viscosity below 60 cP. Above 60 cP, Pz depends strongly upon viscosity. Variations of Pz with temperature and rate of rotation are more severe at higher viscosities. The results suggest that the best choice of fluid is the one having the lowest viscosity at the operating pressure. Such a fluid can be selected on the basis of having the most nearly linear plot of Pz as a function of pressure of the candidate fluids. These results are also a clear indication that, for the most accurate pressure measurements, a controlled-clearance piston gauge must be characterized using the same operational and environmental conditions with the same fluid as are used in normal operation.