A piston gauge is a round piston fitted in a matching cylinder; the piston is loaded with weights of known mass and density. The piston is marginally smaller than the cylinder, and fluid fills the gap between the two components. In 2006, NIST established two, 36 mm diameter piston-cylinder artifacts (referred to as PG38 and PG39) as primary standards through careful dimensional characterization of their diameters, modeling of the forces acting on the artifacts, and experimental comparison of the artifacts to each other and to the NIST mercury manometer. Establishing a pressure scale based on the primary standards required careful intercomparisons between those primary standards and ten secondary standard piston gauges. The secondary standard piston gauges have successively smaller diameters to allow extending the range from 1 MPa (the upper limit of PG38 and PG39) up to 17 MPa.
A pressure scale traceable to primary standard piston gauges provides two significant advantages over a scale traceable to the mercury manometer. The first is that the upper limit of 1 MPa for PG38 and PG39 allows determining pressure-induced distortion in the secondary standard piston gauges, which is not possible using the mercury manometer. Distortion is a major contributor to uncertainty at higher pressures. The second advantage is that calibrations against piston gauges require much less time than that against manometers, making it feasible to perform more comparisons of the secondary standards to PG38 and PG39.