Dry Gas Meter Calibration and verification - An application note

‘Gas totaliser meters’, or ‘dry gas meters’ are widely used in a variety of applications. One very common use is in the sampling process for environmental monitoring and stack emissions sampling. Legislation in this area is becoming ever tighter for these applications, where the performance of the gas meters has to be verified through regular calibration. A dry gas meter consists of two or more chambers formed by diaphragms. Applied gas flow is directed internally by moveable valves. The gas meter chambers alternately fill with and release gas, produc­ing a continuous flow through the meter. The movement of the diaphragms during gas fill/release is converted by a mechanism into rotary motion. This rotary motion drives a counter, which expresses the volume of gas flowed. Most modern flow reference instruments capable of the accuracy required to verify these devices express flow rate in terms of mass flow. Gas meters are devices measuring volumetric flow. Volume flow is susceptible to changes in gas temperature and barometric pressure, so during calibration these factors have to be measured and corrected for. This ensures a meaningful comparison with the mass flow reference device. The other major factor when calibrating a gas meter through comparison with a reference mass flow rate is time. To determine the flow rate measured by the gas meter, its volume sample is converted to flow rate by timing the sample. Suitable sample times are based on factors such as the gas meter Qmax, and the flow rate being applied compared to this Qmax. For example, if the meter Qmax is 10m3/Hr and the applied flow is at 0.5m3/Hr, to achieve a large enough counter sample for a meaningful comparison the sample time would be many minutes. If the flow is at 9m3/Hr, a large enough sample would be obtained relatively quickly. Chell Instruments have developed systems sold Europe-wide, designed specifically for the calibration of volumetric gas totalisers with very low uncertainties. These systems take into account pressure, gas temperature, time and flow. All corrections are displayed and applied within a certificate template. These templates can be issued to each gas meter, and give comprehensive traceability for each during your audit.

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