Diaphragm gas meters or positive displacement gas meters are the most common type seen in most installations and almost all residential installations. There are normally two chambers formed by a movable diaphragm. Valves can direct the gas flow and the two chambers fill from the gas source and empty into the gas load, alternating their roles so that gas delivery to the load is essentially continuous. As the diaphragms expand and contract, levers connected to cranks convert the linear motion of the diaphragms into rotary motion which then drives the counter.
A Gas turbine meneter uses a turbine contained within the gas pipe. These meters allow the highest gas flow, but they have the lowest rangeability. The positive displacement meters can accurately measure (at ±1%) gas flows over a very wide range from their maximum rated flow rate all the way down to 1/100 of that flow rate or less, turbine meters are only accurate down to between 1/12 and 1/33 of their maximum rating depending on such things as the absolute pressure of the gas
Additionally, to convert from volume to thermal energy, the pressure and temperature of the gas must be taken into consideration.