Evaluation of changes in cylinder volume due to gas filling and subsequent release

Abstract

This paper provides useful and important information in gas metrology especially where high-precision gas mixtures are essential. Buoyancy effects due to cylinder expansion are taken into account as a source of uncertainty in weighing for the preparation of primary or standard gas mixtures by gravimetric methods. Potential effects of cylinder construction materials, nominal volumetric capacity and filling gas species on cylinder volume during the preparation of primary standard gas mixtures remain unknown in the current literature. It is also difficult to find experimental evidence to demonstrate the theory that cylinder expansion upon pressurization is linear. We therefore devised an indirect measurement method to measure apparent volume changes and ultimately estimate changes in the volume of highly pressurized cylinders. We found that changes in the volumes of some gas cylinders varied significantly with construction material and nominal volumetric capacity. The relative volume changes for aluminium alloy cylinders were about 30% higher than those for manganese steel cylinders regardless of the nominal volumetric capacity and the filling gas species. We also provide experimental evidence verifying that changes in cylinder volume follow nearly linear patterns within the pressure range from 12?MPa to 0.1?MPa. Our estimate for the volume expansion of an aluminium alloy cylinder of 5?dm3 capacity with a pressure difference of 15?MPa is (15???2)?cm3. The reported value of about 20?cm3 in ISO 6142?:?2001 would be regarded as a rough estimate.

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