Pressure and temperature measurements in the annulus between the piston and cylinder of a simple dead-weight piston gauge

Abstract

Precise and accurate pressure measurements are obtained using appropriately designed and operated piston gauges. Elastic distortion of the piston and cylinder is the leading cause of inaccuracy in measurement of higher pressures. The distortion depends upon the ratio of the pressure in the annulus between the piston and the cylinder to the pressure under the piston. Heretofore, as the proper value of this ratio or a method to determine it was unknown, the practice has been to assume a value of 0.5 when calculating distortion effects. In this work, the pressure and temperature of the fluid in the annulus have been measured along the working length of the piston and the cylinder. The model for the pressure ratio proposed by Bass on the basis of the clearance between the piston and cylinder is in excellent agreement with the pressure measurements. The appropriate value of the pressure ratio for the calculation of the distortion is marked by a sharp decrease in the slope of the pressure ratio curve. The corresponding value on the length axis is the appropriate location to calculate the area of the gauge from the dimensional measurements. The temperature measurements indicate that the temperature gradient along the cylinder is due solely to the flow of fluid up through the annulus and, for the most extreme case, was less than 60 mK.