Part 3: Cold Weather and Mass Air Flow Meter Gas Temperature (Gas Properties)

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In Parts One and Two of this series, we looked at the effects of ambient temperature on mass air flow meters such as Sierra’s FastFlo™ 620S and our new QuadraTherm™ Model 640i. As discussed, cold weather has few physical effects on an air flow meter, but can degrade accuracy due to stem conduction, unless accounted for, as does the new QuadraTherm 640i. While we have talked about the temperature of the surroundings, we need to also look at the temperature of the gas in the pipe, since this too can be affected by the surrounding temperature.

Influence of Temperature on Mass Flow Controllers

Thermal mass air flow sensors are famous for being “independent of temperature and pressure.” Since these are mass flow meters and not volumetric meters, this is mostly true, but there is some influence by temperature. As mentioned previously, thermal mass flow controllers work by measuring the heat conducted away by the flowing gas. The amount of heat lost is a function of gas heat transfer properties such as heat capacity, density, viscosity and thermal conductivity. All of these properties have some dependence on temperature and pressure. Most thermal flow meter manufacturers do not take this into account and assume a constant process temperature and pressure. Using its unique QuadraTherm sensor and iTherm brain, Sierra’s new QuadraTherm Model 640i continuously calculates the values of these heat transfer properties at process temperature, ensuring the most accurate reading no matter what is going on inside or outside of the pipe!

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