Why do you need to specify the gas flow range on a Mass Flow Controller?

The Full Scale (FS) Range and Gas on a Mass Flow Controller (MFC) directly correlates to the transmitted output (analog or digital) of the device. In an analog device, the maximum output value (such as 5 vdc or 20 mA) will be equivalent to the FS value. The accuracy of most MFCs is a function of this FS Range. For analog devices, it is commonly ±1% of FS. For digital devices, it is commonly published to be ±(0.5% of Reading + 0.2% FS). Selecting a FS Range close to the maximum flow rate for your application optimizes accuracy. This is a good practice. In addition, the gas must be specified. Most MFCs use thermal based sensors. These sensors actually measure the molecular flow rate rather than the mass flow rate (by way of measuring the differential in heat transfer as gas passes through a by-pass tube). Various gas molecules transfer heat differently, and thus the specific gas must be known for optimal calibration.

This post is an excerpt, with permission, from Teledyne Hastings Instruments.
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