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Pure gases are classified by grade.
|Grade||Minimum purity||Total impurities|
The first digit of the grade classification indicates the 'number of nines' purity, e.g. N3.0 = 99.9% purity.
The second digit is the number following the last nine, e.g. N4.6 helium has a guaranteed minimum purity level of 99.996%.
Concentration conversion table
|Percent||Parts per million||Parts per billion|