The Piston Gage as a Precise Pressure-Measuring Instrument

Abstract

Abstract The errors which must be considered for accurate measurements of pressure with piston gages are discussed. A new design of piston gage is described which permits the operator to control the clearance between piston and cylinder at any operating pressure. Instrumental errors are thus substantially reduced, particularly those resulting from elastic distortion. A check list of errors inherent to piston gages is presented. These errors are to be considered when instruments are used to accuracies better than ½ of 1 per cent. The need for a check on the longtime variations of the piston gage is supplied by the fixed points, changes of state of some very pure materials which take place at definite pressures. Such are the melting pressure of solid mercury at 0 C, the melting pressure of water at 30 C, the transition between crystalline states of bismuth. As an indication of its possibilities the experimental model of the controlled-clearance piston gage was used to measure the melting point of mercury at 0 C. Results were in satisfactory agreement with Bridgman’s value.

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