Gas flow rate measurement devices are usually calibrated in secondary flow laboratories and the lower level measurement standards are conducted to the traceability chain by the primary or national measurement standards which are established and maintained at the national metrology institutes of that country. Since the SI unit for gas flow rate is derived from the meter and the second, the uncertainty related to a reference system is expected to come from the dimensional and time calibration of that standard. Reference systems implementing the principle of combined gas law like a PVTt system (pressure-volume-temperature-time) does not obey this generalization and pressure-temperature measurements play an active role in uncertainty calculations. A system that utilizes the method of weighing the collected gas in a vessel offers an advantage over volumetric methods since there is no need for density corrections. Either it is a gravimetric or volumetric standard, to reach the lowest uncertainty possible is one of the important flow standard design considerations. In this paper the effects of design and operation of the bell prover, piston, PVTt and gravimetric systems are discussed with results of their relevant overall uncertainty.