An approach to the evaluation of dimensional measurements on pressure-measuring piston-cylinder assemblies

W. Sabuga, T. Priruenrom

Abstract

To improve the consistency of 3-dimensional data describing the geometry of pressure-measuring piston-cylinder assemblies, a new approach, based on the leastsquares method, is proposed. It allows the results of diameter-, straightness- and roundness measurements to be linked with each other, with only minimum discrepancies between them. When processing the dimensional data, it is possible thanks to this new approach to weight them differently - according to their measurement uncertainties. The new approach was applied to three gas-operated piston-cylinder assemblies with nominal effective areas of 10 cm² and 5 cm² which are used at PTB as primary gas pressure standards for the range up to 2 MPa. The dimensional measurements were carried out by means of different instruments and the results were analysed. The discrepancies in the dimensional data sets obtained within the scope of the new approach are typically smaller than 16 nm, which agrees with the uncertainties claimed for each kind of dimensional measurement. The effective areas of the three piston-cylinder assemblies were calculated using the Dadson theory and then adjusted taking into account the results of both cross-float measurements carried out between them and pressure measurements carried out against a primary mercury manometer. Finally, relative standard uncertainties smaller than 2·10^-6 could be obtained.

Keywords

pressure balance, piston-cylinder assembly, dimensional measurement, least-squares method

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IMEKO-TC16-2007-074u.pdf

DOI

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IMEKO TC

TC16 - Pressure and Vacuum Measurement

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