Thermal mass flow controllers measure the heat capacity of the streaming gas – use a different gas than the calibration gas and the reading is off. The gas correction factor (GCF) fixes that: Q(real) = Q(reading) × GCF(process gas) ÷ GCF(calibration gas). Here is the reference table, relative to nitrogen (GCF = 1.000).
The GCF reference table (relative to N₂)
| Gas | Formula | GCF |
|---|---|---|
| Nitrogen | N₂ | 1.000 |
| Air | – | 1.000 |
| Oxygen | O₂ | 0.993 |
| Carbon monoxide | CO | 1.00 |
| Hydrogen | H₂ | 1.01 |
| Argon | Ar | 1.39 |
| Helium | He | 1.45 |
| Carbon dioxide | CO₂ | 0.70 |
| Methane | CH₄ | 0.72 |
| Nitrous oxide | N₂O | 0.71 |
| Ammonia | NH₃ | 0.73 |
| Tetrafluoromethane | CF₄ | 0.42 |
| Propane | C₃H₈ | 0.36 |
| Sulphur hexafluoride | SF₆ | 0.26 |
Guide values – factors vary by a few percent between MFC manufacturers; the respective manufacturer's table governs.
The physics: why monatomic high, polyatomic low?
At its core the GCF mirrors molar heat capacity: monatomic gases like argon and helium store heat only in translational motion – they cool the sensor less, the MFC underestimates the flow, the factor sits above 1. Polyatomic molecules like SF₆ or propane additionally absorb rotational and vibrational energy and cool more strongly – the factor sits below 1, down to just 0.26 for SF₆.
Worked examples in both directions
Reading → real flow: an N₂-calibrated MFC shows 100 sccm while argon flows: in reality 100 × 1.39 = 139 sccm. Target → setpoint: exactly 50 sccm of CO₂ should flow: set 50 ÷ 0.70 = 71.4 sccm. With calibration gases other than N₂, apply both factors – or simply use the MFC Gas Conversion Factor Calculator, which knows all 14 gases and both directions.
Limits of the factors
Strictly, GCF values apply only in the device's linear range and at the manufacturer's reference conditions. For gas mixtures the factor can be approximated as a mole-fraction average – but for processes with tight tolerances there is no way around calibrating for the actual process gas. Why this so often goes wrong in practice is shown in our article The 7 Most Common Mistakes in Handling High-Purity Gases.
