Mixed GasesGas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases. Gas mixtures are usually specified in terms of molar gas fraction (which is closely approximated by volumetric gas fraction for many permanent gases): by percentage, parts per thousand or parts per million. Volumetric gas fraction converts trivially to partial pressure ratio, following Dalton's law of partial pressures. Partial pressure blending at constant temperature is computationally simple, and pressure measurement is relatively inexpensive, but maintaining constant temperature during pressure changes requires significant delays for temperature equalization. Blending by mass fraction is unaffected by temperature variation during
the process, but requires accurate measurement of mass or weight, and calculation of constituent masses from the specified molar ratio. Both partial pressure and mass fraction blending are used in practice.