The SMU Primary Mercury Manometer and its Comparison with Three Manometers of Different Design

After the CCM Medium Pressure Working Group intercomparison indicated that the SMU primary mercury manometer differed by more than 20 ppm from other national standards, attention was paid to determining and eliminating the possible sources of these deviations. These efforts included replacing or redesigning some of the manometer components. The modified manometer design is presented in this paper. A piston gauge has been calibrated in the absolute mode using nitrogen as a pressure media, with the primary standard manometers at four national standards laboratories: Slovenský Metrologický Ustav (SMU), Bratislava; Amt fur Standardisierung, Messwesen und Warenprufung (ASMW), Berlin; Physikalisch-Technische Bundesanstalt (PTB), Braunschweig; and the National Institute of Standards and Technology (NIST), Gaithersburg; over a period of six years. Each of these manometers is of a different design. This has provided a basis from which to determine how the design changes affect the SMU manometer performance. The results indicate that the previous systematic deviations have been eliminated in the upper pressure ranges. The results for all four manometers agree to within their claimed uncertainties, however at the lowest pressures significant differences still exist and require further investigation.