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Title: Z/Z-METER, On-Line Measurement Of Compressibility-Ratios For Reference Values Of Volume At Operational Conditions
Author: A.J.M. Herwijn
Source: Flomeko 2003
Year Published: 2003
Abstract: One of the methods to realize reference values for Volume of e.g. Natural Gas at increased pressures is via a calibration under a pressure difference (i.e. with expansion). In such a process, applied since the sixties in the Netherlands 1, 2, the un-known meter or Meter-under-Test (MuT) is compared through expansion with the known meter or Reference Meter (RM) at low-pressure. In a normal calibration-process the difference in pressure between the two compared meters is relative small and the resulting difference between the compressibility at the Reference Meter ZRM and the compressibility at the Meter-under-Test ZMuT has hardly any effect on the ratio ZRM/ZMuT and can be neglected (the ratio is thus considered to have a value of 1). However, in a calibration under expansion a comparison is made of a gas-flow under significant differential pressure-conditions and one of the most important contributions of uncertainty stems from the real gas constant Z. So, the ratio ZRM/ZMuT is of importance, rather than the absolute values of the compressibility factor. The Z/Z-meter is one of the current technical developments at NMi VSL-Flow 3, 4. The uncertainty contribution in the conventional method of calculation, described e.g. in M-GERG 5, 6 of 1991, is rather high (0,1% for the compressibility factor) and the resulting uncertainty of the ZRM/ZMuT ratio is 0,14%.

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