Manufacturer calibration gases are expensive and have lead times. With a suitable filling system, simple test gases can be mixed in-house – using the partial pressure method. The Calibration Gas Blending Calculator (Partial Pressure) provides the filling pressures.
The principle per Dalton
In ideal gas mixtures, the mole fraction equals the partial pressure fraction: a 2% target concentration at 150 bar final pressure means 3 bar partial pressure of the component. Filling is sequential: first the minor component up to its partial pressure, then top up with the base gas to final pressure.
Step by step
- Define the target mixture: component concentration, final pressure, cylinder volume
- Read the filling pressures: the calculator shows up to which pressure the component is filled and how much base gas follows
- Use a pre-mixture: for small concentrations (below ~1%) the tool calculates a two-stage pre-mixture – filled directly, the partial pressures would be too small to dose accurately
- Account for residual pressure: if the cylinder is not empty, the calculator correctly subtracts the residual pressure
Worked example
2% methane in nitrogen, 10 l cylinder, 150 bar: fill CH₄ up to 3 bar, then N₂ up to 150 bar. That yields a calculated 3 Nm³ of test gas – enough for years of gas detector calibrations.
Error sources and limits
Three things spoil the accuracy: temperature drift (the gas warms up during filling – the pressure is only correct after temperature equalisation), real gas behaviour (with CO₂ and high pressures, the mole fraction deviates noticeably from the pressure fraction) and missing homogenisation (roll the cylinder after filling or let it rest for several hours). For accredited calibrations there is no way around certified calibration gas with a certificate of analysis – the in-house mixture is meant for function tests and non-critical applications. Safety rules for flammable and oxidising gases must be strictly observed.
