A novel method to rapidly and accurately calibrate gas flowmeters such as those used to measure natural gas flowing in pipelines has been developed by scientists at the National Institute of Standards and Technology, Gaithersburg, Md.
The method take advantages of a fundamental physical principle: When a sound wave travels through a gas with regions at different temperatures, the sound wave’s average speed is determined by the average temperature of the gas.
Accurate calibrations of gas flowmeters are of interest to meter manufacturers and calibration laboratories, and the development could potentially impact all segments of the natural gas industry.
Conventional calibrations are typically conducted by flowing a gas stream through the meter being calibrated during a measured time interval. The quantity of gas that passes through the meter is determined by collecting the gas in a large tank of known volume and measuring its average temperature and pressure, which in turn, reveals the amount of gas.
Collecting the gas in large tanks — NIST uses a 953.50 ft3 (27 m3) tank — generates different temperatures in different parts of the tank, which make the average temperature difficult to measure. Those gradients persist for hours or days, making a fast reading inherently inaccurate.
To get around the problem, current practice entails calibrating many small meters, one at a time, and then using them in parallel to calibrate a larger meter. This is done with a smaller collection tank where temperature differences are reduced to produce more accurate reading. These multiple calibrations are time consuming.
NIST's innovation replaces the problem of accurately measuring the average temperature of a large volume of gas with the easier problem of accurately measuring the average speed-of-sound in the gas.
In one recent proof-of-concept paper published in Measurement Science and Technology, NIST researchers deduced the internal shape, thermal expansion and volume of a 79.25 gal (300 liter) collection tank by measuring what microwave frequencies resonated (formed standing waves) within the evacuated tank.
In a second set of experiments, they filled the tank with argon gas and measured the frequencies of the acoustic resonances. From the frequencies and the pressure, they deduced the mass of the argon in the tank.
Finally, they heated the top of the tank to establish a temperature difference across the gas of 4 pecent of the average gas temperature. The temperature difference changed the acoustic resonance frequencies and the pressure; however, the mass of the argon, as deduced from the frequencies and the pressure, was unchanged within 0.01 percent.
This result implies that the acoustic resonance technique could be used to measure the collected gas, even in the presence of a temperature gradient such as those that occur in a much larger tanks located outside the well-controlled environment of a laboratory.