Purity Grade Desinga - Airgas.com

Specialty Gases
Pure Gases
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Guide to Purity Grade Designation
Airgas manufactures and distributes a full range of pure Specialty Gases through its network of more than 24 Specialty Gas production facilities and its more than 700 branch locations. (photo)

Each product has a documented purity specification and the quality assurance and control procedures employed guarantee that these specifications are met.

Purity Grades

Each product is available in a range of purity grades. These purity grades are identified by names such as "Research Grade", "Ultra High Purity", "Zero", "Prepurified", etc., that give an indication of relative purity such that "Ultra High Purity" is purer than "Prepurified". These terms, however, are not absolute. "Research Grade" Argon is not the same purity as "Research Grade" Methane or even "Research Grade" Nitrogen. The same purity name may be used for the same gas by different manufacturers, each offering the product with different specifications. The grade name therefore does not tell the user what the actual purity level is – it varies from product to product and manufacturer to manufacturer.

The important measures of purity are the overall percentage purity of the gas and the identity and levels of the impurities present.

A simple system used to signify the percentage purity is a digitized grade system. In this system the first digit signifies that number of 9’s in the percentage purity. The second digit indicates the next number that differs from a 9. Thus:

         Helium 4.5

There are four 9’s in the purity and the next digit is a 5 –

99995 as a purity percentage is 99.995%

Similarly Argon 6.0 is 99.9999% (Six 9’s and the next digit is a 0)

Often of more importance in a particular application is knowing what impurities are present and at what level. If Nitrogen is used as a zero gas for a ppm Oxygen analyzer, it is important to know the Oxygen level of each grade of Nitrogen when selecting which to use.

Each grade of gas will also carry a list of the impurities analyzed for and their maximum permitted levels.
Summary Purity Table


Percentage PurityGradeTotal Impurities
99.99%4.0100 ppm
99.995%4.550 ppm
99.999%5.010 ppm
99.9995%5.55 ppm
99.9999%6.01 ppm