MFC gas conversion factor (GCF) calculator
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KEY POINTS
A thermal MFC measures flow by the heat a gas carries, so gases with different specific heat and density read incorrectly. With an N₂-calibrated MFC, actual flow is reading × GCF: to flow 100 sccm of SiH₄ (GCF about 0.60), set about 167 sccm. C₄F₈ and SF₆, with GCFs around 0.2, can flow only about one-fifth of the N₂ full scale.
Converting between calibration gas and process gas
The factors are typical values commonly used for thermal MFCs. They differ by a few percent between makers and sensor designs, so for accuracy use the MFC maker's factor table or actual-gas calibration (multi-gas, multi-range functions).
Conversion factor for a two-component mixture
Estimating the conversion factor from physical properties
Defaults are for SiH₄ (density 1.433 g/L, cp 0.3189 cal/g·K) and give a value close to the table's 0.60. Use this to estimate a rough factor for gases not in the table.
Gas conversion factors
| Gas | GCF (N₂ = 1) | Actual flow limit on an N₂ 500 sccm range |
|---|---|---|
| N₂ nitrogen | 1.000 | 500 sccm |
| Ar argon | 1.390 | 695 sccm |
| He helium | 1.450 | 725 sccm |
| H₂ hydrogen | 1.010 | 505 sccm |
| O₂ oxygen | 0.993 | 497 sccm |
| Air | 1.000 | 500 sccm |
| CO₂ carbon dioxide | 0.700 | 350 sccm |
| N₂O nitrous oxide | 0.710 | 355 sccm |
| NH₃ ammonia | 0.730 | 365 sccm |
| CH₄ methane | 0.720 | 360 sccm |
| SiH₄ silane | 0.600 | 300 sccm |
| SiH₂Cl₂ dichlorosilane | 0.400 | 200 sccm |
| Cl₂ chlorine | 0.860 | 430 sccm |
| HCl hydrogen chloride | 1.000 | 500 sccm |
| HBr hydrogen bromide | 1.000 | 500 sccm |
| BCl₃ boron trichloride | 0.410 | 205 sccm |
| CF₄ carbon tetrafluoride | 0.420 | 210 sccm |
| CHF₃ trifluoromethane | 0.500 | 250 sccm |
| C₂F₆ hexafluoroethane | 0.240 | 120 sccm |
| C₄F₈ octafluorocyclobutane | 0.164 | 82 sccm |
| SF₆ sulfur hexafluoride | 0.260 | 130 sccm |
| NF₃ nitrogen trifluoride | 0.480 | 240 sccm |
| WF₆ tungsten hexafluoride | 0.250 | 125 sccm |
| Xe xenon | 1.320 | 660 sccm |
Why thermal MFCs need conversion factors
A thermal MFC heats the sensor tube the gas flows through and measures the heat carried by the gas from the temperature difference upstream and downstream. At the same mass flow, gases with different specific heat and density carry different amounts of heat, so an MFC calibrated with nitrogen reads incorrectly with other gases. Polyatomic molecules with high specific heat (SiH₄, CF₄, C₄F₈ and others) carry more heat at the same flow, so their GCF is below 1.
Tool recipes are written in actual gas flow, so with an N₂-calibrated MFC the setpoint must be the desired flow divided by the GCF. For gases such as C₄F₈ and SF₆, with a GCF around 0.2, only about one-fifth of the N₂ full scale can flow. Modern MFCs store the characteristics of many gases and convert automatically once the gas is selected.
FAQ
How do I convert an N₂ MFC setpoint for another gas?
Divide the desired process gas flow by the gas's GCF. For example, 100 sccm of SiH₄ with a GCF of 0.60 needs a setpoint of about 167 sccm.
How is the GCF of a gas mixture calculated?
GCF_mix = 100 / Σ(a_i / GCF_i), where a_i is each component's volume percent.
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Last updated: October 23, 2026