The Reference Gauge technique for static expansion ratios—Applied to NPL medium vacuum standard SEA3

Static expansion pressure standards are typically characterised in terms of the ratio between the density of an expanded sample of gas and that of an initial sample. For standards that perform a series of expansions through several system vessels most of the existing techniques to measure this expansion ratio involve disturbance to the system. They usually also involve operation with higher pressures and with valve sequences that are not encountered under normal operational conditions, thereby introducing the risk of systematic errors. A modification of the Reference Gauge technique for expansion ratios is described which has none of these disadvantages. The technique is described with reference to the medium vacuum standard SEA3 at the UK National Physical Laboratory.