Try this one and search for " 1910.133(a)(5) "
http://osha.gov/
Filter Lenses for Protection Against Radiant Energy
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Minimum(*)
Operations Plate thickness-inches Plate thickness-mm Protective
Shade
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Gas Welding:
Light Under 1/8 ............ Under 3.2 ......... 4
Medium 1/8 to 1/2 ........... 3.2 to 12.7 ....... 5
Heavy Over 1/2 ............. Over 12.7 ......... 6
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Oxygen cutting:
Light Under 1 .............. Under 25 .......... 3
Medium 1 to 6 ............... 25 to 150 ......... 4
Heavy Over 6 ............... Over 150 .......... 5
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Footnote(*) As a rule of thumb, start with a shade that is too dark
to see the weld zone. Then go to a lighter shade which gives
sufficient view of the weld zone without going below the minimum. In
oxyfuel gas welding or cutting where the torch produces a high yellow
light, it is desirable to use a filter lens that absorbs the yellow
or sodium line in the visible light of the (spectrum) operation.
Footnote(**) These values apply where the actual arc is clearly
seen. Experience has shown that lighter filters may be used when the
arc is hidden by the workpiece.
Hope this helps,
John Wright