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Part 2: Cold Weather and Mass Air Flow Meter Accuracy (Stem Conduction)

Mar 01, 2013

In Part One, we looked at the effects of cold weather on mass air flow meters like Sierra’s Steel-Mass® Model 640S and our new QuadraTherm™ Model 640i. As discussed, cold weather typically doesn’t cause any physical damage to the flow meter, but it can affect the meter accuracy. Let’s look at how.

Flowmeters That Are Reliable and Accurate

In many applications, the gas in the pipe is hotter than the surroundings; in the Far North in the winter it can be dramatically so. We all know that heat flows from hot to cold, so, with a flow meter inserted into a pipe, it is only natural for heat to flow from the hot gas, up the probe stem of the flowmeter, then to the surrounding cold air. Since a thermal mass air flowmeter works by measuring the heat removed from the sensing element (mostly by the flowing gas), this heat conducted up the stem looks like flow. The bigger the temperature difference between the gas in the pipe and the outside air, the bigger this error. It can be up to 20%! Most companies do not manufacture an air flow meter that can account for this temperature difference.

Flowmeter With a Four-Sensor Advantage

Sierra’s new QuadraTherm 640i was designed with this problem in mind. The QuadraTherm uses four sensors instead of the traditional two, with two extra temperature sensors installed to measure the heat lost via stem conduction. By measuring and subtracting this heat loss out of the energy balance using its iTherm brain, The QuadraTherm 640i becomes independent of ambient temperature and achieves accuracies never before possible (up to 0.5% of reading).

Scott Rouse, Product Line Director
Written By:
Scott Rouse, Product Line Director
Sierra Instruments

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