Gas meter

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

RMG specialises in gas measurement technology for pipeline network operators and industrial customers. Therefore, depending on the desired flow and pressure range, classical turbine wheel gas meters or ultrasonic gas meters are used for gas volume measurement.

The gas meters are mainly approved and tested for custody transfer metering for billing purposes; however, there are also cheaper variants available for internal use. Originally developed for natural gas applications, the gas meters are also increasingly in demand for use in hydrogen or in gas mixtures containing hydrogen and have been tested and approved for this purpose. Of course, they can also be used with other types of gas in an industrial environment or for biogas production and feed-in.

The ultrasonic gas meter is based on the measurement of the transit time of ultrasonic pulses generated by so-called transducers and transmitted through the gas flow. The geometric travel of the pulses is inclined against the direction of flow of the gas. The ultrasonic pulses "swim" in the moving gas flow. If the transit time of a pulse in the upstream direction is compared with the transit time in the downstream direction, a transit time difference is found which is proportional to the flow velocity of the gas. By arranging several such ultrasonic paths in one measuring device, one achieves an improvement in the measuring accuracy and has the possibility of completely recording the flow profile of the gas flow.

Since their market introduction about 20 years ago, ultrasonic gas meters have become the dominant technology in the field of large gas measurement. It is a very modern form of gas meter which has several advantages. For example, there is no mechanical movement in the meter and no parts protrude into the gas flow. Thus no pressure loss occurs. The measurement is very accurate and can be done bidirectionally (in both directions). Versions for high operating pressures (up to 300 bar) and large nominal diameters (standard up to DN600, special versions up to DN 1000) are available.

Turbine wheel gas meters are used for custody transfer metering. In these meters, incoming gas causes an axially arranged turbine wheel to rotate. Its revolutions are proportional to the operating volume of the gas flowing through it. The flow guide body in the meter is designed in such a way that a uniform flow profile is obtained for the turbine wheel in order to obtain accurate measured values.

In the classic turbine wheel gas meter, the revolutions of the turbine wheel are transmitted mechanically via a gear to a mechanical roller-type counter. Recently, versions with an electronic totalizer have also become available. Turbine wheel gas meters are characterized by outstanding reproducibility and long-term stability, which is one of the reasons why they are used as reference standards in all high-pressure gas meter test rigs.