Abstract: A three-chamber gas meter having a two-chamber cartridge inserted into a casing, the interior of which forms the third chamber. The meter is of compact design and readily mass produced. The cartridge carries a valve plate which communicates to each of the chambers and also to the inlet and the outlet. A rotary valve connected to the valve plate will sequentially and selectively connect the chambers between the inlet and the outlet to measure the gas flow through the meter. A flag and crank assembly is operatively associated with the cartridge to connect the rotary valve and the diaphragms, which expand and contract to actuate the internally connected flag and crank assembly, thus to rotate the rotary valve. A pair of flag rods are journaled in the cartridge and included in the flag and crank assembly.