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Results are presented of an intercomparison of pressure measurements between the National Physical Laboratory (NPL), India, and the National Institute of Standards and Technology (NIST), USA, using piston gauge pressure standards over the range 6 MPa to 26 MPa. The intercomparison, using the NPL piston gauge pressure standard, with a nominal effective area of 8.4×10 -5 m 2 and the NIST piston gauge pressure standard, with a nominal effective area of 2.0× 10 -5 m 2 was carried out at the NPL. The intercomparison data obtained show a relative difference of 1 × 10 -6 in the zero-pressure effective area ( A 0 ) of the NPL standard as obtained by the NIST standard. At 6 MPa the relative difference in effective areas is 3.5×10 -6 ; at the full scale pressure of 26 MPa, the relative difference is 12×10 -6 . These differences are in excellent agreement with the statements of uncertainty of the respective standards as obtained from the primary standards of these two laboratories.

NIST pressure calibration services with nitrogen are now based on two transfer standard piston gages for which the effective areas have been determined by calibration with the manometer developed at NIST for gas thermometry. Root-sum-squared three sigma uncertainties for the areas for the two gages are 3.05 ppm and 4.18 ppm.