Primary Pressure Standards Based on Dimensionally Characterized Piston/Cylinder Assemblies
James W. Schmidt, K. Jain, A P. Miiller, Walter J. Bowers Jr., Douglas A. Olson
NIST has characterized two large diameter (35.8 mm) piston/cylinder assemblies as primary pressure standards in the range from 0.05 to 1.0 MPa with uncertainties approaching the best mercury manometers. The realizations of the artifacts as primary standards are based on the dimensional characterization of the piston and cylinder, and models of the normal and shear forces on the base and flanks of the piston. We have studied two piston/cylinder assemblies, known at NIST as PG38 and PG39, using these methods. The piston and cylinder of both assemblies were accurately dimensioned by Physikalisch Technische Bundesanstalt (PTB). All artifacts appeared to be round within 30 nm and straight within 100 nm over a substantial fraction of their heights. PG39 was dimensioned a second time by PTB, three years after the initial measurement, and showed no significant change in dimensions or effective area. Comparisons of the effective area of PG38 and PG39 from dimensional measurements, against those obtained with calibration against the NIST Ultrasonic Interferometer Manometer (UIM), are in agreement within the combined standard (k = 1) uncertainty of the dimensional measurements and the UIM. A cross-float comparison of PG38 versus PG39 also closed with the dimensional characterization within their combined standard uncertainties and with the UIM calibrations. The expanded (k = 2) relative uncertainty of the effective area is about 5.8x10-6 for both assemblies.