Design and Uncertainty Analysis for a PVTt Gas Flow Standard
John D. Wright, Aaron N. Johnson, Michael R. Moldover
A new pressure, volume, temperature, and time (PVTt) primary gas flow standard has been constructed at the National Institute of Standards and Technology with an expanded uncertainty of between 0.02 % and 0.05 % (k = 2). The standard covers a flow range of 1 slm^l to 2000 slm using two collection tanks and two diverter valve systems. The standard measures flow by rapidly diverting gas to a collection tank of known volume for a measured time interval.An overview of the significant and novel design features of the standard is given. The gas collection tanks are small in diameter and are immersed in a uniform and time stable controlled temperature water bath. Thus the system achieves rapid thermal equilibrium of the collected gas and low uncertainty measurements of the average gas temperature. The uncertainties related to the inventory volume are discussed in detail, and a novel operating method is described that leads to nearly zero mass change in and very low uncertainty contributions from the inventory volume. The gravimetric and volume expansion techniques used to determine the tank and inventory volumes are described. A detailed uncertainty analysis is presented as well as supporting comparison data between the two new flow standards.1. General Description of a PVTt Gas Flow StandardPVTt systems have been used as primary gas flow standards by the National Institute of Standards and Technology (NIST) and other laboratories for more than 30 years[l, 2, 3,4]. The PVTt systems at NIST consist of a flow source, valves for diverting the flow, a collection tank, a vacuum pump, pressure and temperature sensors, and a critical flow venturi (CFV) which isolates the meter under test from the pressure variations in the downstream piping and tank (see Fig. 1).The process of making a PVTt flow measurement entails the following steps:1) Establish a stable flow through the meter under test with flow through the bypass valve. 2) Evacuate the collection tank volume (Vr) with the vacuum pump.3) Wait for pressure and temperature conditions in the tank to stabilize and then acquire initial values for the tank (P: and T:). These values will be used to calculate the initial density and the initial mass of gas in the tank (m~).slm = standard liters per minute, reference conditions are 293.16 K and 101.325 kPa.