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Industrial Thermal Mass Flow, The Heat is On…


Nov 03, 2011

One of the more interesting stories I have been told about Sierra is the genesis of the company’s flagship product, our model 640S Steel -Mass ® Immersible Thermal Mass Flow Meter.  In the mid 60’s, our founder, Dr. John Olin, was working on his PhD at Stanford.  One aspect of his research involved studying the fluid dynamics of gas turbine engines.  This necessitated measuring the velocity of the hot exhaust gases at various points.  A good way to do that is with a device called a hot-wire anemometer.  These devices have been around since 1914, when L.V. King published his famous King’s Law revealing how a heated wire immersed in a fluid flow measures the mass velocity at a point in the flow.  The problem was that the gases were so hot in this case that the wire would melt!  What was needed was an industrial version of a hot wire anemometer, with the heated element protected from the fluid being measured.  That was the early genesis of the 640S.

In our 640S, the “hot wire” is a precision-grade platinum RTD.  This wire is wound around a ceramic core, and this entire assembly is then swaged into a platinum/iridium sheath using a proprietary technology.  In operation, the “hot wire” is “immersed” in the surrounding gas and heated to 50°C above the gas temperature. This gas temperature is measured by a second non-heated RTD.  Once the gas begins to flow, the gas molecules convect heat away from the heated element causing the heater’s temperature to drop.  Control electronics monitor the temperature of the heater and the gas, and pump more heat (electrical power) into the heater so that the temperature differential is maintained at 50°C.  By King’s Law, the power added is directly proportional to the mass point velocity.

Today, 20 years later, Sierra offers a high temperature version of the 640S that will operate up to 750F.

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

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