Prepare gas mixtures by the partial pressure method: calculate fill pressures for component and balance gas from a target concentration in ppm or percent.
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Preparing gas mixtures by the partial pressure method
By Dalton's law, for ideal gases the mole fraction of a component equals its share of total pressure. A mixture can therefore be prepared via the pressure gauge: p(component) = target fraction × fill pressure. This calculator returns the set pressures for each filling step – including dosing from a pre-mixture.
Worked example: 100 ppm CO in nitrogen
Target: 100 ppm at 150 bar(a) final pressure. Pure CO would need dosing to 0.015 bar – unreadable on a 300 bar gauge. The practical route via a 1 % pre-mixture: p = (100 ppm / 1 %) × 150 bar = 1.5 bar(a) of pre-mixture, then top up with nitrogen to 150 bar. Rule of thumb: the dosing pressure should be at least 1 % of final pressure, otherwise reading error dominates.
Filling procedure
1. Evacuate the cylinder (or keep a defined balance residual pressure – the calculator accounts for it). 2. Dose the component slowly to the calculated intermediate pressure. 3. Top up with balance gas. 4. Homogenise: roll the cylinder or store it horizontally for several hours – freshly filled mixtures are stratified!
Limits of the method
The calculation assumes ideal gas behaviour. At high pressures real gases deviate (compressibility factor Z) – several percent error at 150 bar depending on the gas pair. Temperature matters too: 1 K of warming during filling distorts pressure by about 0.35 %. Certified calibration gases are therefore made gravimetrically per ISO 6142; the partial pressure method suits working standards and function checks.
