, , , Eric Harman, James Mehl
We describe our progress in the development of a novel gas flow standard using the acoustic and microwave resonances of a 1.85 m3, nearly-spherical, steel vessel at pressures up to 7 MPa. For flow calibrations using pressure and acoustic frequency measurements, the vessels cavity will be a calibrated gas collector; for controlled blow-down measurements, the cavity will be a calibrated gas source. Previously, using the cavitys microwave resonance frequencies, we determined the cavitys pressure- and temperature-dependent volume VBBB with the percentage uncertainty 0.011 %. (Unless stated otherwise, uncertainties are at a 68 % 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 the argons pressure and acoustic resonance frequencies determined the mass of argon Macst with an uncertainty of 0.023 %, even when temperature gradients were present. The percentage differences between Macst and independent gravimetrically-determined masses Mgrav fell in the range: Macst/Mgrav 1 = (2 to +4)104. Most of these differences were a linear function of pressure; therefore, they can be reduced in the future. Using the measurements of VBBB, the pressure, and the acoustic resonance frequencies of the enclosed gas, we demonstrated the principle of an acoustic flow standard by calibrating 4 critical flow venturis that NIST has used as transfer standards for 10 years. The differences between the new calibrations and many historical ones were within the long-term reproducibility of each CFV.
Flow Measurement and Instrumentation
flow standard, collection volume, calibrated volume, gas source, acoustic resonator, microwave resonator