Weighing gas with sound and microwaves

January 26, 2015 by Mike Moldover, National Institute of Standards and Technology
Weighing gas with sound and microwaves
Schematic diagram of a gas-filled pressure vessel. The red-to-blue shading represents the temperature gradient in the gas, with the higher (red) temperatures near the top. The ovals represent a standing sound wave; its frequency is mostly determined by the average temperature of the gas. The wavy line represents a resonant electromagnetic wave; its frequency is mostly determined by the length of the tank. Wavelengths are not to scale.

NIST scientists have developed a novel method to rapidly and accurately calibrate gas flow meters, such as those used to measure natural gas flowing in pipelines, by applying a fundamental physical principle: When a sound wave travels through a gas containing temperature gradients, the sound wave's average speed is determined by the average temperature of the gas.

Accurate calibrations of gas flow meters issues are of urgent interest to meter manufacturers and calibration labs, with potential impact throughout the industry.

Conventional calibrations are typically conducted during measured time intervals by flowing a gas stream through the meter being calibrated. The quantity of gas that passes through the meter is determined by collecting the gas in a large tank and measuring its and pressure, which in turn reveals the amount of gas.

However, the process of collecting the gas in large tanks generates temperature gradients (different temperatures in different parts of the tank), which make the average difficult to measure. Those gradients persist for hours or days. Thus a fast reading is inherently inaccurate.

To get around that problem, current practice entails calibrating many small meters, one at a time, and then using them in parallel to calibrate a larger meter. This produces a more accurate reading, but is inherently time-consuming, and therefore expensive.

NIST's innovation replaces the difficult problem of accurately measuring the average temperature of a large volume of gas with the easier problem of accurately measuring the -of-sound in the gas.

In one recent paper, NIST researchers deduced the internal shape, thermal expansion, and volume of a 300 liter collection tank by measuring which microwave frequencies resonated (formed standing waves) within the evacuated tank.

In a second set of experiments, described in a forthcoming paper, 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 % of the average gas temperature. The 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 %.

This result implies that the acoustic resonance technique could be used to measure the collected , even in the presence of a gradient, such as those that occur in a much larger tanks located outside the well-controlled environment of a laboratory.

Explore further: Fill 'er up! A look at how much money car owners are saving

More information: "Microwave determination of the volume of a pressure vessel." M R Moldover et alĀ 2015 Meas. Sci. Technol. 26 015304 DOI: 10.1088/0957-0233/26/1/015304

Related Stories

Methane is leaking from permafrost offshore Siberia

December 22, 2014

Yamal Peninsula in Siberia has recently become world famous. Spectacular sinkholes, appeared as out of nowhere in the permafrost of the area, sparking the speculations of significant release of greenhouse gas methane into ...

Recommended for you

Matter waves and quantum splinters

March 25, 2019

Physicists in the United States, Austria and Brazil have shown that shaking ultracold Bose-Einstein condensates (BECs) can cause them to either divide into uniform segments or shatter into unpredictable splinters, depending ...

Study suggests trees are crucial to the future of our cities

March 25, 2019

The shade of a single tree can provide welcome relief from the hot summer sun. But when that single tree is part of a small forest, it creates a profound cooling effect. According to a study published today in the Proceedings ...

How tree diversity regulates invading forest pests

March 25, 2019

A national-scale study of U.S. forests found strong relationships between the diversity of native tree species and the number of nonnative pests that pose economic and ecological threats to the nation's forests.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.