Purity (gas)

The purity of gas is an indication of the amount of other gases it contains. A high purity refers to a low amount of other gases. Gases of higher purity are considered to be of better quality and are usually more expensive.

The purity of a gas is generally expressed as a grade prefixed with the letter N giving the "number of nines" in the percentage or decimal fraction. An N2.0 gas is 99% pure, and 1% (by volume) impurities. An N6.0 gas is 99.9999% (six nines) pure, with 1 part per million (1 ppm) impurities.

Intermediate values are formed using the common logarithm. For example, a gas which is 99.97% pure would be described as N3.5, since log₁₀(0.03%) = −3.523.