Two reasons make it necessary to measure gas volumes in industrial and economic environments. Firstly, of course, when the gas changes hands, i.e. when the gas is invoiced. Secondly, when the production or consumption of gas in industrial processes must be determined in order to be able to assess, control and optimise the processes.
All gases are highly compressible, i.e. with increasing pressure the volume decreases and they expand strongly with increasing temperature. The indication (or measurement) of a gas volume is therefore only meaningful if the corresponding pressure and temperature are indicated at the same time.
In order to be able to compare gas quantities, users refer to gas under the nationally agreed standard conditions, e.g. in Germany at p= 1,013.25 mbar and T= 0°C. However, this standard volume cannot be measured directly in practice; the gas volume is always measured under the operating conditions currently prevailing at the measuring point. If pressure and temperature are determined simultaneously, the standard volume can then be calculated from the operating volume using these values (so-called volume conversion).
Direct mass flow meters based on the Coriolis principle or thermal anemometry occupy a special position among gas meters. As the name suggests, these meters determine the mass of a quantity of gas and not its volume. However, these instruments are only used in special applications and are therefore not discussed further.
In contrast, all other commercially available gas meters, hereinafter referred to as "gas meters", measure the operating volume. If the gas meters are used in commercial transactions for billing purposes, they must be approved according to the European Measuring Instruments Directive MID and calibrated regularly.
Depending on the operating conditions and applications, different technologies are preferably used:
- The household sector is dominated by diaphragm gas meters
- Rotary piston gas meters are also frequently used in the industrial sector