Developments in the Accurate Measurement of High Pressures

The measurement of steady high pressures in a fluid system with the highest accuracy demands the use of pressure balances (free piston gauges) of accurately known effective areas. This requires a precise knowledge of the way in which the effective areas of the piston-cylinder assemblies concerned vary due to the elastic distortion caused by the applied pressure. Two methods which have been directed to the solution of this problem are described. The first depends on a principle of similarity as applied to the deformations of two assemblies of the same general dimensions but constructed of materials having substantially different elastic moduli. The second method makes use of measurements of the flow characteristics of the pressure transmitting fluid using two pistons having a known difference of diameter. The distortion factors are shown to be representable as linear functions of the pressure, so that the effective area at pressure P is connected with that at zero pressure by expressions of the form A P = A O (1 - λ P ) where λ may be termed the distortion coefficient. The final accuracy of the measured distortion coefficients is about ± 2%, which corresponds to an uncertainty in effective area of about ± 1 part in 10 5 at 1000 bars increasing in proportion to the pressure at higher pressures. Some aspects of the practical calibration of pressure balances, carried out by direct balancing against assemblies calibrated by the methods described, are considered.