Dalton's law states that each component of a gas mixture contributes a partial pressure in proportion to its share of the total. For an ideal gas that share is also the mole fraction, which is what ppm by volume means.

Both pressures must be absolute and in the same unit, which is why only one unit selector appears. The ratio itself is unitless, so the choice of unit affects the displayed answer but never the concentration.

Where this stops being exact

The equality between pressure ratio and mole fraction holds for ideal gases. At the pressures used to fill a calibration cylinder, typically 100 to 200 bar, real gas behaviour breaks it, and the component actually present differs from the manometric target by a percent or more depending on the gas pair. This is why certified calibration mixtures are prepared by weighing rather than by pressure, following the gravimetric method of ISO 6142-1, and why a manometric fill should be verified by analysis rather than trusted from the gauge. Near atmospheric pressure the ideal treatment is accurate enough for most purposes.

The calculation also assumes the components do not react, dissolve into each other or adsorb onto the cylinder wall. Adsorption matters for polar and reactive gases at low concentration, where a passivated cylinder is the usual answer, and it acts to lower the delivered concentration below the calculated one.