The nines scale is also sometimes used in describing the purity of bottled gases. 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. For gasses, the number of nines is usually written after the letter N, rather than before it. 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