Gas Selection Criteria
The type of gas required will be readily determined by the application. Selection of the correct purity of that gas needs a more detailed analysis of the application.
The principal selection criteria will be based on the effect of impurities in the application. Each impurity present in the gas should be reviewed to determine if it will impact the application and, if so, at what level does that impact become significant.
For example – 5 ppm of oxygen in Nitrogen to be used as a zero gas for an oxygen analyzer measuring in the percentage range will not be a problem. The same grade of Nitrogen would be totally unacceptable as a zero gas if the analyzer were measuring in the ppm or ppb range.
All impurities should be considered, not just the impurity of primary concern.
For example – "Zero" grade refers to a grade of gas that has very low levels of hydrocarbon impurity, and is therefore often used as a zero gas for Total Hydrocarbon Analyzers. It is also specified as a suitable carrier gas in gas chromatography using a Flame Ionization Detector (FID). This may not be a good choice, as there are other impurities in Zero grade gases that may adversely affect gas chromatography, such as oxygen and moisture which can lead to column deterioration. It is better to use a UHP grade which has the same low level of hydrocarbons, and also has guaranteed low levels of oxygen, moisture and other impurities.
Please call your nearest Airgas location for assistance in selecting the best grade for your application.
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