According to the metering principle, gas metering devices can be divided into direct metering and indirect metering. There are many metering chambers in the direct metering gas meter, and the amount of gas passing through is directly measured and accumulated according to the volume of the metering chambers. There are two kinds of direct metering gas meters: dry type and wet type. Membrane flowmeter and Roots flowmeter belong to dry type and are widely used in small and medium flow measurement. Indirect metering gas meter has no metering room. It uses some physical characteristics of gas flow to convert it into flow, and then introduces time factor to get the cumulative value. For example, orifice flowmeter using air pressure difference, turbine flowmeter using air velocity, vortex flowmeter using air to hinder the formation of vortex, etc. These flowmeters are mainly used for large flow measurement. According to the gas pressure in the pipeline, the gas metering device can bear low pressure, medium pressure or high pressure respectively.

The widely used membrane gas meter (see figure) is equipped with a flexible membrane, which can expand and contract with the gas in and out. Metering chambers are formed on both sides of each film. There is a slide valve above the metering chamber, which can alternately connect the inlet and outlet of the metering chamber and the gas meter. When the gas enters the metering chamber, the film expands, compressing its adjacent metering chamber and exhausting the gas. This repetitive and alternating action is transmitted to the recording device through the mechanical structure, and the gas throughput is accumulated.