Why use shielding gas in the first place?
Oxygen while welding creates oxygenation. Oxygenation offers brittle welds that look bad and act worse. Many customers require welds that act perfectly and look great. Shielding gasses make these great welds.
Shielding gas is selected according to the material being welded. The following guidelines may help:
Argon - the most commonly used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminum, and titanium.
Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include the risk of hydrogen cracking in carbon steels and weld metal porosity in aluminum alloys.
Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture are the high cost of gas and difficulty in starting the arc. http://www.twi-global.com/technical-knowledge/job-knowledge/tungsten-inert-gas-tig-or-gta-welding-006/
Make Argon Last Longer when Using Purge Gloveboxes like Sparky
Argon is heavier than oxygen. It "settles" in the chamber. If you gently lift open the hood and do not disturb the gas much a lot of your Argon remains in the chamber. Less Argon needed for re-purge.
You Tube References
Tig Welding Stainless Steel - Walking the Cup vs TIG Finger