Thermal mass flow controllers do not measure the flow itself but the heat capacity of the gas streaming past. If a gas other than the calibration gas is used, the MFC reads wrong – the MFC Gas Conversion Factor Calculator corrects this with gas correction factors (GCF).
The principle
Q(real) = Q(reading) × GCF(process gas) ÷ GCF(calibration gas) – the GCF describes how strongly a gas influences the thermal measuring principle compared to nitrogen. Monatomic gases like argon (GCF ≈ 1.39) and helium (≈ 1.45) sit well above 1, polyatomic ones like CO₂ (≈ 0.70) or SF₆ (≈ 0.26) well below.
Step by step
- Select the calibration gas: usually N₂ – stated on the MFC's nameplate
- Select the process gas: the gas actually flowing (14 gases included)
- Set the direction: "what actually flows?" (reading → real) or "what must I set?" (target → setpoint)
- Enter the value and read off: the calculator returns the factor and the corrected flow
Worked example
N₂-calibrated MFC, process gas argon, reading 100 sccm: in reality 100 × 1.39 = 139 sccm of argon flow. Conversely: if exactly 100 sccm of argon are to flow, set 100 ÷ 1.39 ≈ 72 sccm. Forget the correction and your argon dosing is off by almost 40% – an expensive mistake with process gases in coating or semiconductor manufacturing.
Limits of the factors
GCF values are guide values and differ between MFC manufacturers by a few percent, because measuring principle and sensor construction play a role; strictly they also only apply in the device's linear range. For processes with tight tolerances there is no way around two things: use the factor table of the respective manufacturer – or have the MFC calibrated for the process gas outright. For conversions, replacement devices and sanity checks, however, the GCF calculation is the standard tool.
