Recent advances in technology on two fronts, 1) the fabrication of large diameter pistons and cylinders with good geometry, and 2) the ability to measure the dimensions of these components with high accuracy, have allowed dead-weight testers at NIST to approach total relative uncertainties for dimensionally-derived effective areas near 3.5 ppm (1ς. This paper describes a 35 mm diameter piston/cylinder assembly (known within NIST as PG-39) that serves as a pressure standard in which both the piston and the cylinder have been accurately dimensioned by Physikalisch Technische Bundesanstalt (PTB). Both artifacts (piston and cylinder) appeared to be round 50 nm and straight within 100 nm over a substantial fraction of their heights. Based on the accurate diameters at 20 C provided by PTB ( 15 nm) and on the good geometry of the artifact, the uncertainties for the effective area were estimated to be very low even after possible crevice effects were included. The area based on the measurements from PtB coincides with the area measured by the NIST Precision Engineering Division in 1990 within 1.0 ppm and with recent values obtained by the NIST Ultrasonic Interferometer Manometer also within 2.5 ppm.After analyzing the dimensional measurements several ways we have determined that the effective area for this gauge at 20 C and zero pressure is Aeff.20 = 1007.925, 2 (1 2.8x10-6) mm2; (k=2). The temperature coefficient for the area was measured and found to be (8.754 0.03) x 10-6/K (k=2). The pressure coefficient is estimated to be (6.5 4.0)x10-12 Pa-T (k=2). Thus when using the gage at a reference temperature of 23 C and at pressures between (0.05 and 1.0) Mpa the effective area should be taken as:Aeff.23 = 1007.951,7 mm2 (1+6.5 x 10-12 P/Pa), with a relative expanded uncertainty u(A)/A = [(2.8x10-6)2 +(4x10-12 P/Pa)]1/2; (k=2).
Citation: Journal of Research (NIST JRES) -
Volume: 108 No. 2
NIST Pub Series: Journal of Research (NIST JRES)
Pub Type: NIST Pubs
dead-weight tester, piston gage, piston/cylinder assembly, pressure measurement, primary pressure standards