Primary pressure standards in the atmospheric pressure range are often established using mercury manometers. To a lesser extent, controlled-clearance dead-weight testers, in which one component (normally the piston) has been dimensionally measured, have also been used. The recent advances in technology on two fronts: (i) the fabrication of large-diameter pistons and cylinders with good geometries; and (ii) the dimensional metrology capability of these components, have allowed some dead-weight testers at the National Institute of Standards and Technology (NIST) to achieve total relative uncertainties (2 ) in generated pressure near 10 ϫ 10 -6 (10 ppm). This paper describes recent developments at the NIST in which accurate dimensional measurements have been translated into effective areas. It is anticipated that total relative uncertainties in generated pressure may decrease to 5 ppm (2 ) when recent dimensional measurements are incorporated in the newest gauges.
Primary pressure standards in the atmospheric pressure range are often established using mercury manometers. Less frequently, controlled-clearance dead-weight testers in which one component (normally the piston) has been dimensionally measured have also been used. Recent advances in technology on two fronts i) the fabrication of large-diameter pistons and cylinders with good geometry; and ii) the ability to measure the dimensions of these components, have allowed some dead-weight testers at NIST to approach total relative uncertainties (k = 2) in dimensionally-derived effective areas near 5 × 10−6. This paper describes a single piston/cylinder assembly (NIST-PG201WC/WC) that serves as both a primary gage in which both piston and cylinder are measured dimensionally and a controlled-clearance primary gage (employing the Heydemann-Welch method). Thus it allows some previous assumptions about the modeling of dead-weight testers to be checked. For the gage described in this paper the piston/cylinder clearance obtained from the two analyses have relative differences of 4 × 10−6 to 7 × 10−6 over the pressure range 35 kPa to 175 kPa. Some implications of these results will be discussed. From the dimensional characterizations and auxiliary measurements we have determined that the effective area for this gauge at 20 °C is: Aeff,20=1961.0659mm2(1+3.75×10−12P/Pa+3.05×10−12PnormalJ/Pa),where P is the system pressure and PJ is a control pressure. The estimated relative uncertainty in effective area is 8.2 × 10−6 +1.4 × 10−11 P/Pa (k = 2). The temperature coefficient for the area was measured and found to be (9.06 ± 0.04) × 10−6/K. Thus using the gage at a reference temperature of 23 °C yields an effective area: Aeff,23=1961.1192mm2(1+3.75×10−12P/Pa+3.05×10−12PnormalJ/Pa),with almost no increase in the uncertainty over that at 20 °C.
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