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There are several ways that industry professionals refer to oxygen grades. The Compression Gas Association (CGA) has identified seven grades of oxygen, A through G, which determine how pure the oxygen is. 
It is the purity of the oxygen (i.e., how much of any other gases are still present) and the way the oxygen cylinders are filled that separate oxygen grades.
There are four accepted “grades” of oxygen used in various industries: welding, research, aviation, and medical.


Medical oxygen is used for oxygen therapy and in hospitals. It is designated as a drug and therefore must satisfy FDA requirements for compressed medical gas. One of the requirements is that cylinders containing oxygen must always be completely evacuated to minimize the risk of contamination.
Liquid oxygen is stored in chilled tanks until required, and then allowed to boil (at a temperature of 90.188 K (−182.96 °C)) to release oxygen as a gas. This is widely used at hospitals due to their high usage requirements, but can also be used in other settings.


Aviation Breathing oxygen strictly regulated, and neither medical nor industrial grade oxygen can be substituted because of these standards. The CGA’s Grade E is commonly also called aviator’s grade, and this oxygen must also go through additional drying steps before it goes into a cylinder.


Welding oxygen is used in certain types of welding techniques, either to aid in the production of heat or as an additive to aid in the stability of the process. While in use, other contaminants may enter the oxygen cylinder, such as acetylene, which is used during one type of welding.


Research grade oxygen is 99.999% pure (“five nines,” also called grade 5) and is used in both chemical research facilities and specialty welding applications in the aerospace industry.

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