Gas Meter

A gas meter is a specialized flow meter, used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

Several different designs of gas meters are in common use, depending on the volumetric flow rate of gas to be measured, the range of flows anticipated, the type of gas being measured and other factors.

These are the most common type of gas meter, seen in almost all residential and small commercial installations. Within the meter there are two or more chambers formed by movable diaphragms. With the gas flow directed by internal valves, the chambers alternately fill and expel gas, producing a near continuous flow through the meter. As the diaphragms expand and contract, levers connected to cranks convert the linear motion of the diaphragms into rotary motion of a crank shaft which serves as the primary flow element. This shaft can drive an odometer-like counter mechanism or it can produce electrical pulses for a flow computer.

Diaphragm gas meters are positive displacement meters.

Rotary meters are highly machined precision instruments capable of handling higher volumes and pressures than diaphragm meters. Within the meter, two figure "8" shaped lobes, the rotors (also known as impellers or pistons), spin in precise alignment. With each turn, they move a specific quantity of gas through the meter. The operating principle is similar to that of a Roots blower. The rotational movement of the crank shaft serves as a primary flow element and may produce electrical pulses for a flow computer or may drive an odometer-like counter.

...
Wikipedia