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How to Achieve Vapor Mass Flow Control with a Thermal Mass Flow Controller


Jan 09, 2012

I often get asked to describe the most challenging flow meter applications that Sierra has successfully handled. One that comes to mind is how we used a capillary thermal mass flow controller (designed to measure gas) to accurately control a liquid used in the semiconductor industry at extremely low pressure differentials of about 10 Torr.

Originally, I thought there was no way this could work because the liquid causes the capillary thermal sensor maximum output or “rail,” and the pressure differential is way too low for 5 SLPM equivalent N2 flow. But wait, it actually does work! Here’s how…

The liquid is bubbled in a vessel by heating it to 80°c and pulling a vacuum; this turns the liquid into a vapor. Then, it must be kept under vacuum and at 80°c to prevent it from condensing back into a liquid. And, if it stays a vapor, the sensor can detect it as non-condensing gaseous flow and not rail out.  This means the flow body of the mass flow controller needs to also be externally heated to 80°c.

The next step is to deal with the pressure drop. With a normal sized orifice in the valve to choke down and control the flow, the vapor would condense and turn back into a liquid, causing failure. The pressure drop would be much too high to achieve full-scale flow. The solution was to take a low flow meter body and match it up with a high flow valve typically used at up to 500 SLPM.  This provides a low enough pressure drop to reach full scale flow and does not cause the vapor to condense back into a liquid.

At Sierra, we are always striving for flow meter innovation and love to tackle tough stuff like vapor mass flow control. What can we do to help you? What’s your most challenging flow meter application? Share it in the comments below or contact us for advanced engineering support.

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

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