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Capillary Thermal, How Exactly Does it Work?


Jul 20, 2011

I get many questions from customers about how a Capillary Thermal Mass Flow Meter, like our Smart-Trak 100, works. So, I decided to do a brief overview of the principle of operation to accompany the video below.

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The total flow through a tube or piping system enters the CTFM where it passes through a Laminar Flow Element (LFE) creating a pressure drop across it and in turn a small amount of flow, typically in the sccm range, is fractioned off of the total flow and passes through the capillary flow sensor. With a good LFE, this ratio of flow through the sensor and flow through the LFE remains constant.

The sensor construction consists of a hollow SS capillary tube with two Platinum Resistance Temperature Detector (PRTD) coils wrapped around the outside of the tube. These two identical PRTD coils have a constant and equal amount of current through them and are in turn heated up. With no flow passing through the sensor tube both coils are heated equally and thus have the same signal across them. As flow starts the heat gets transferred from the upstream winding to the downstream winding, the greater the flow the more heat gets transferred downstream. This creates a temperature difference between the two PRTD coils and thus produces a different signal across them that is directly proportional to the difference in the two temperatures, and is directly proportional to the flow. This difference in signals of the two coils is sensed by the electronics and is used to create an output signal.

Since this heat is carried from the upstream winding to the downstream windings by the actual molecules of the gas passing through the inside of the sensor tube, this type of flow meter directly measures the mass flow of the gas and is independent of temperature and pressure.

For a much more detailed principle of operation discussion including diagrams and formulas click here.

Mark Blackbourn, Engineer
Written By:
Mark Blackbourn, Engineer
Sierra Instruments

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