Characterization of gas pressure balances at LPM

This paper describes development of gas pressure scale up to 18 MPa using dimensional and cross-float experiments in Croatian national pressure laboratory (HMI/FSB-LPM). Practical pressure scale for gas as transmitting medium is realised using a group of six gas-operated-piston-cylinder assemblies and diving bell manometer. For more than 15 years, traceability of pressure measurements has been achieved through German national pressure laboratory PTB. Recently, Croatian national length laboratory (HMI/FSB-LPMD) improved their measurement capabilities and LPM started independent realization of traceability for gas pressure scale through dimensional measurements. The purpose of this work was to develop a method for direct determination of effective area using basic SI units, determine stability of piston cylinder units from 15 years of measurement data history, compare results from cross-float and dimensional method and to establish prerequisites for independent traceability chain in LPM. Dimensional measurements were performed for two gas-operated-piston-cylinder units with diameters of 35 mm and 8 mm and their effective areas were calculated with relative standard uncertainty of 18 and 35 ppm, respectively. A comparison of these results with cross-float experiments is given, showing good agreement. Finally, measurements, calculation method, results of effective area and other important metrological characteristics such as deformation coefficient and fall rate are presented for the entire group of six gas-operated-piston-cylinder assemblies.