Gas correction for thermal mass flow controllers: calculate real flow or required setpoint from the gas conversion factor (GCF) for 14 gases.
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Why an N₂-calibrated MFC reads wrong on argon
Thermal mass flow controllers measure the heat transport of the flowing gas – which depends on specific heat capacity and density. An MFC calibrated on nitrogen but flowing argon therefore reads systematically wrong. The correction: Q(real) = Q(reading) × GCF(process gas) / GCF(calibration gas).
Worked example: sputtering system
An N₂-calibrated MFC is set to 100 sccm but flows argon (GCF 1.39). Real flow: 100 × 1.39 = 139 sccm of argon – almost 40 % more than displayed! Conversely: for exactly 100 sccm argon, set the setpoint to 100 / 1.39 = 72 sccm. The calculator handles both directions.
Key conversion factors (relative to N₂)
Monatomic gases run high: argon 1.39, helium 1.45. Diatomic near 1: O₂ 0.993, CO 1.00, H₂ 1.01. Polyatomic well below: CO₂ 0.70, CH₄ 0.72, NH₃ 0.73 – heavy molecules like SF₆ (0.26) or propane (0.36) at the bottom. The reason is molar heat capacity: the more degrees of freedom, the more heat per mole, the lower the MFC reading.
Limits
GCF values are approximations for the linear range. They differ slightly between manufacturers and strictly apply at low turndown. For tight-tolerance processes: use the manufacturer's table or have the MFC calibrated on the process gas.
