Development of the PVTtPVTt system for very low gas flow rates

A new PVTtPVTt system (pressure–volume–temperature–time system) has been constructed to establish a standard for a gas flow rate of less than 5 mg/min. This system has two unique aspects for the calibration of gas flow meters. First, this PVTtPVTt system can calibrate any type of flow meter by introducing an automatic pressure controller to keep the downstream pressure of a flow meter constant, although most PVTtPVTt systems can calibrate only flow meters whose output is not influenced by the change of the differential pressure working on them. Secondly, this system does not need a mass correction in the dead volume in most cases, because the difference of the mass of gas in the dead volume between the final condition and the initial condition would be negligibly small as the initial pressure is used as a trigger to stop the measurement. It is presented in this paper that a mass of gas gathered in the constant volume tank (CVT) is the most important parameter on an uncertainty analysis of a PVTtPVTt system and the uncertainty of a flow rate must be described with the collection time. In this PVTtPVTt system, a realistic relative standard uncertainty of a flow rate of 0.01 mg/min is 0.21% when the collection time is 40 h and a realistic relative standard uncertainty is 0.0001% at a flow rate of 5 mg/min with the collection time of one hour.