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Under Pressure, Part 3: How to Avoid ‘Droop’ (Hysteresis) with Flowmeters

Sep 25, 2012

If you’ve been reading my series on pressure regulation, you know my goal is to help you achieve the best possible gas mass flow control with your flowmeter. Previously, I’ve discussed undersizing and seat-drop lock. Now I want to move onto something that can really get you down – “droop.”

Pressure Regulation with Flow Meters, how to avoid droopRegulators may exhibit hysteresis, also known as droop. Droop is a function of the loss in max outlet pressure with regard to flow rate and is often characterized as the ‘range’ of change in outlet pressure across normal operating conditions (before asymptotically approaching choked flow). So, the hysteresis of a regulator is the difference in outlet pressure for a given amount of diaphragm depression between approaching the setpoint from a higher outlet pressure or from a lower outlet pressure. In short, there will be a slight difference in the actual output for a given amount of depression pending climb or fall to setpoint. Or there will be a slight difference in post ride height on the regulator for a given outlet pressure setpoint if you approach from the top or from the bottom. This means that the more up and down you go, the more you should pay attention to whether the setpoint is at the top or the bottom of the meniscus, so to speak.

Proper line regulation, be it supply pressure regulation or back pressure regulation, is fundamentally key to a properly tuned flow system process. All instruments operate most accurately under steady-state conditions, so awareness of regulator behavior and proper tuning will save you time and money during transient conditions.

At Sierra, we are always happy to help you with your application. Feel free to contact us with any questions you may have about pressure regulation in flowmeters, gas mass flow control applications or other topics.

Morgan Zealear, Engineer
Written By:
Morgan Zealear, Engineer
Sierra Instruments

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