Nines (notation)
Nines are an informal logarithmic notation for proportions very near to one or, equivalently, percentages very near 100%. Put simply, "nines" are the number of consecutive nines in a percentage such as 99% or a decimal fraction such as 0.999. Their common uses include grading the purity of materials – especially precious metals and industrial gases – or measuring the availability of a service.
Nomenclature
The nines are a count of the leftmost digits 9 that appear in a proportion. For example, 90% would be described as "one nine"; 99% as "two nines"; 99.9% as "three nines"; and so forth.However, there are different conventions for representing inexact multiples of 9. For example, a percentage of 99.5% could be expressed as "two nines five" or as 2.3 nines, following from the logarithm definition.
A percentage of 100% would, in theory, have an infinite number of nines – though, in the context of purity of materials, 100% is virtually unachievable.
The number of nines of a proportion is:
Uses
Precious metals
The exact purity of very fine precious metals such as platinum, gold and silver can be of great interest. Based on the system of millesimal fineness, a metal is said to be one nine or one nine fine if it is 900 fine, or 90% pure. A metal that is 990 fine is then described as two nines fine and one that is 999 fine is described as three nines fine. Thus, nines are a logarithmic scale of purity for very fine precious metals. Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then "five", so 999.5 fine is "three nines five", abbreviated 3N5.Canada's Big Maple Leaf, a coin made of gold at 5N purity, stands as the purest gold coin ever minted, anywhere. The purest gold ever achieved was reportedly produced at the Perth Mint in 1957, at "almost six nines" purity, as measured by the Worshipful Company of Goldsmiths of London.
Gases
The nines scale is also used in other contexts, such as describing the purity of gases. The purity of a gas is an indication of the ratio of it to other gases in its mixture, as measured by volume. Thus, a high purity refers to a low amount of other gases, or impurities. Gases of higher purity are in many contexts 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, indicating the "number of nines" in the percentage or decimal fraction. For example, a N2.0 gas is 99% pure and 1% impurities by volume; a N6.0 gas is 99.9999% pure, with 1 part per million impurities.
Intermediate values indicate the digit following the last nine. For example, N4.6 estimates a purity level of 99.996%. An alternative representation uses the common logarithm: for example, a gas which is 99.97% pure would be described as N3.5, since log10 = −3.523.