Gas blending

Gas 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.

Shielding gases are inert or semi-inert gases used in gas metal arc welding and gas tungsten arc welding to protect the weld area from oxygen and water vapour, which can reduce the quality of the weld or make the welding more difficult.

Gas metal arc welding (GMAW), or metal inert gas (MIG) welding, is a process that uses a continuous wire feed as a consumable electrode and an inert or semi-inert gas mixture to protect the weld from contamination. Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material.

Modified atmosphere packaging is a technology used to preserve the quality of fresh produce so that it can be sold to markets far away from where it is grown, extend the marketing period, and help reduce food waste within the cold chain. The gas composition used to pack food products depends on the product. A high oxygen content helps to retain the red colour of meat, while low oxygen reduces mould growth in bread and vegetables.

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