Thermal mass flow controllers are traditionally calibrated for a specific gas, a desired flow range, and a set of operating conditions. Over time, the use of conversion factors based on a ratio of specific heats between gases came into use as a way for users to configure a single mass flow controller for multiple gases. This method of configuring a mass flow controller for multiple gases is still common today - you're using it if your device lets you select a gas by: rotating a knob, pressing a button on a display, or sending an RS232 command to the device.
Accuracy is the primary issue with this method of conversion. Converting flow rates between the calibration gas and another gas based on a ratio of specific heats can result in a mass flow control error of 5-6%. This error is the result of the conversion method because it ignores other property differences that exist between gases in the real world. If you're changing the gas on your device with one of the actions above, ask the manufacturer of your mass flow controller what the accuracy of the device is for a gas other than the calibration gas.
P.S. If you're told that such a device is linear in all the available gases and thus the mass flow accuracy doesn't change when the gas is changed, RUN! This is not physically possible. Feel free to contact us for comparison data.
MultiFlo® by Brooks Instrument is a leap forward in configuring a mass flow controller for multiple gases because it converts based on gas differences in specific heats, densities, and viscosities.Multiflo-Capable mass flow controllers cut the conversion flow control error in half compared to devices that convert gases based on a specific heat ratio alone.
You can find more information on MultiFlo®-Capable mass flow controllers on the Brooks Instrument by clicking here.
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