In part VI of my series on natural gas flow meter types, I discuss the thermal mass flow meter, which is considered one of the four new-technology flowmeters.

Natural Gas Flow Measurement

I shared information on other gas meters in earlier posts, including Coriolis meters, diaphragm meters, rotary meters, ultrasonic meters, and turbine flowmeters. Sage Metering is the manufacturer of high-performance thermal mass flow meters that measure gas mass flow.

The thermal mass flow meter is considered one of the new-technology flow meters, part of a small group consisting of Coriolis, magnetic, ultrasonic, and vortex meters. These devices were introduced after 1950 and have been in a rapid product development era and high growth mode. According to a recent study by Flow Research, they predict that this market will grow approximately 7.5% through 2018, with the fastest growth in developing-Asian countries, China, and the Middle East.

Thermal Mass Flow Meter | Part VI of Natural Gas Flow Meter Types
Bob Steinberg discusses the thermal mass flow meter as one of the four new-technology flowmeters in a continuing series on natural gas flow meter types.

Thermal Mass Flow Meter

Thermal flow meters measure mass flow, not volumetric flow, and use heat to compute the measurement. Unlike the Coriolis meter, which relies on fluid momentum, the thermal mass flow meter depends on the gas’s heat-conducting or thermal properties. There are two different ways of measuring how heat disperses. One method is referred to as the “constant temperature differential,” while the other is the “constant current” process. Both ways rely upon the principle that higher velocity flow brings about more cooling in the flow stream.

There are two sensors in the constant temperature differential mode, a heated sensor and a second sensor, which measures the gas temperature. Mass flow is calculated based on the electrical power needed to maintain a constant temperature between the two sensors.

There is also a heated sensor and a second sensor in the constant current mode, which senses the temperature of the gas flow. In this method, the power to the heated sensor is constant, and mass flow is calculated based on the temperature difference between the sensors.


This style of meter excels in low-flow applications and has a fast response time. The thermal meter can measure low-pressure gases that the Coriolis meter is unable to quantify. It is ideal for emissions monitoring and stack flow measurement. The meters are also available in insertion style models, which facilitate applications of larger pipe size measurement. Additionally, they also have a relatively low cost as compared to the other meter alternatives. Take a look at the 9 Advantages of Thermal Mass Flow Meters.


Thermal mass flow meters are mostly used to measure a gas flow of known gas composition and work in a few liquid applications. They cannot measure steam, and accuracy varies depending on the manufacturer. On average, they are 1%–5% accurate, with the Sage thermal mass meter’s flow accuracy being +/- 0.5% of full scale and +/- 1% of reading. You may also be interested in How to Overcome 9 Limitations of Thermal Flow Measurement.

Natural Gas Flow Meter

The Sage natural gas flow meter excels in natural gas flow measurement for applications in building management systems, combustion control, and submetering. If you are interested in more material on measuring natural gas flow using a thermal mass flow meter, read Natural Gas Flow Meter by Sage Metering for Energy Conservation.

Thermal Flow Meter Theory and Working Principle

To learn more about the thermal mass flow meter theory and working principle, take a look at this brief video.

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