Progress towards a gas-flow standard using microwave and acoustic resonances

Abstract

We describe our progress in developing a novel gas flow standard that utilizes 1) microwave resonances to measure the volume, and 2) acoustic resonances to measure the average gas density of a collection tank/pressure vessel. The collection tank is a 1.85 m3, nearly-spherical, steel vessel used at pressures up to 7 MPa. Previously, using the cavity's microwave resonance frequencies, we determined the cavity's pressure- and temperature-dependent volume VBBB with the expanded uncertainty of 0.022% (coverage factor k = 2, corresponding to 95% confidence level). This was the first step in developing a pressure, volume, speed of sound, and time (PVwt) primary standard. In the present work, when the shell was filled with argon, measurements of pressure and acoustic resonance frequencies determined the “acoustic mass” Macst that agreed with gravimetric measurements within 0.04%, even when temperature gradients were present. Most of these differences were a linear function of pressure; therefore, they can be reduced by further research. We designed and implemented a novel positive feedback system to measure the acoustic resonance frequencies. Using the measurements of VBBB, pressure, and acoustic resonance frequencies of the enclosed gas (nitrogen or argon), we calibrated 3 critical flow venturis that NIST has used as working standards for over 10 years. The two independent flow calibrations agreed within the long-term reproducibility of each CFV, which is less than 0.053%. Furthermore, the feasibility of a dynamic tracking technique using this feedback loop was tested by comparing ΔMacst computed under no-flow conditions and ΔMacst computed by the rate of fall or rise during a flow. This was done for flows ranging from 0.11 g/s to 3.9 g/s.