I tried to determine the mass flow 9sccm) of an old MFC using Alicat flow meter (Model: M-500SCCM-D/5M) using nitrogen gas. I obtained the mass flow values at different set points both under atmospheric pressure and vacuum; they deviate widely at higher values. The flow meter estimates the mass flow appropriately at STP as seen from the attached graph (500 sccm max value is obtained). But under vacuum conditions, it deviates quite a lot. In general, the MFC's are calibrated under STP conditions (because the mass flow is defined under STP) but are made to use with vacuum systems. How does one estimate a true flow rate under vacuum conditions if they are calibrated at STP? Any leads/discussions would help me.
And is it not that the MFC offers a bias in the flow delivered under vacuum conditions?
Pablo Constantino Thanks for your input. The temperature-controlled MFC's work is based on the deltaT between two coils. Under vacuum, the gas molecules might not have enough residence time to sufficiently heat the coils. Maybe, the MFC is biased under vacuum. Does it mean should I follow the orange curve (above) while reporting? Many of the publications report mass flow determined at STP, but the true flow rate might be different.
As a metrologist, I can only say that the best calibration, it that made in working condition. Measuring equipment are made to compensate conditions (above all those mentioned in factory specs). However controllers are not always supposed to have the same accuracy. That's why I I distrust more of a controller.
If orange curve represents better your working condition I would use that one.
For a complete security you should send to calibrate your measuring equipment to a competent service. Otherwise you will never be sure
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