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Title: Development Of The Pvtt System For Very Low Gas Flow Rates
Author: Shin-Ichi Nakao, Yoshiya Terao, Masaki Takamoto
Source: Flomeko 2003
Year Published: 2003
Abstract: The new PVTt system as the standard for gas flow rate less than 5 mg/min has been constructed. This system has three unique aspects for a calibration of gas flow meters. In a PVTt system, the pressure downstream of a device under test (DUT) increases after a flow is diverted to a constant volume tank (CVT) side. It results in a change of a differential pressure working on DUT and a flow rate through DUT changes. Therfore, a flow meter which does not have a function to produce a constant flow rate can not be calibrated by a PVTt system. In this system, by introducing the automatic pressure controller (APC) to control a pressure downstream of DUT and to keep the differential pressure constant, any type of gas flow meters can be calibrated by the PVTt system. Also, as the initial condition in the dead volume uses as a trigger to stop a measurement, the initial and the final conditions in the dead are very close to each other so that the mass correction in the dead volume is not serious and does not need in some cases. And the third aspect is that the mass flow rate through DUT can be directly calculated from the changing rate of the pressure in the CVT at every moment. The CVT that cupper wool is stuffed has a characteristics like an adiabatic tank so that the temperature change in the CVT is very small, about 0.02 K/h during a measurement. The relative standard uncertainty of the PVTt system with these new aspects is 0.1 - 0.05 percent on mass flow rates between 0.1 mg/min and 5mg/min for present.

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