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 gas can be expressed as a percentage
In mathematics, a percentage is a way of expressing a number as a fraction of 100 . It is often denoted using the percent sign, “%”, or the abbreviation “pct”. For example, 45% is equal to 45/100, or 0.45.Percentages are used to express how large/small one quantity is, relative to another quantity...

 value or as a decimal fraction
Fraction (mathematics)
A fraction represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, we specify how many parts of a certain size there are, for example, one-half, five-eighths and three-quarters.A common or "vulgar" fraction, such as 1/2, 5/8, 3/4, etc., consists...

. The decimal fraction is an abbreviation of the percentage value, where the first digit
Numerical digit
A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...

represents the number of nines in the percentage value and the last digit represents the last digit of the percentage value. For example, a purity of 99.97% can be abbreviated as purity 3.7 and a purity of 99.9999% is the same as purity 6.0.
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