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The Type of welding you are doing, the brightness of the arc.
A darker helmet shade, compared to gas welding. A shade of 10 for 70 amps, it works up from there, or you could buy a auto-darkening hood which you can see out of, but when you strick the rod on the metal it darkens to the right shade for you to still see out of.
If by gas welding you mean-oxyacetylene welding. The difference is just that arc welding requires a darker shade of lens. When oxyfuel gas welding (OFW) usually a shade 5 lens is recommended. When arc welding, a minimum of shade 10 lens should be used (actually shade should be determined by amount of current used).
The helmet holds the lens. Lenses come in different shades (darkness). Use the correct shade. Most welding applications call for a shade #10. TIG weldors usually use a shade #9. Plasma applications can call for a shade #11.
the welding amperage being used
Shielded welding needs a gas to shield the weld from the atmoshere air. Unshielded uses a flux the burns when welding to shield the weld from the atmoshere air.
Welding helmet which has eye protection and a face shield .
Gas Tungsten Arc Welding Gas Metal Arc Welding Flux Cored Arc Welding
The amount/brightness of light given off by the process used. Shade #4 or 5 when cutting or welding using the Oxy-Fuel process is usually sufficient. Shade #9, 10, or 11 when stick, MIG, or TIG welding depending of the material being welded and Amperage used. Aluminum reflects more heat and light than most other metals so a darker shade may be called for. TIG welding produces less smoke letting more light escape the weld area, darker shade, possibly. A person whose eyes are sensitive to bright light would probably use a darker shade. I've used shades #9, 10, 11, and 12 when welding. The #12 lens when TIG welding on a ships' propeller using 300 to 400 Amps. Water cooled torch, 5/32" Tungsten. A lot of heat/light was reflected from the surface of the metal. Use as dark a shade that still allows you to see clearly. Remember, the higher the number the darker the shade.
why soaking need for welding
Welding process that uses both flux core, within the wire, and gas to protect the molten weld pool from contamination.
The coating.