NITROX

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. Nitrox is normally distinguished from air and handled differently. The reduced partial pressure of nitrogen reduces nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

 

The most common ways of creating Nitrox are:

 

Nitrox membrane systems use semi-permeable membranes to separate out a portion of the nitrogen in air. The resulting Nitrox mixture, containing between 24% and 40% oxygen (O2), is then delivered to the intake of a high-pressure compressor to fill scuba tanks or storage cylinders. The resulting nitrox mixture is analyzed for its approximate percentage of O2 before entering the breathing air compressor and again prior to usage to determine the precise percentage of O2. 

Membrane Systems
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Partial Pressure Filling

Pure O2 is purchased from a supplier and injected into the scuba cylinder or storage cylinder and then overfilled with oxygen compatible air. Tables are available that specify the amount of oxygen pressure to add for a specific starting pressure and final mix %. The fill lines and cylinders must be oxygen cleaned to prevent O2 fires or explosions. A manifold system makes this process much faster and easier.

Continous blending

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This is somewhat a hybrid of the two systems. Purchased O2 is injected into a mixing chamber attached to the compressor intake. O2 percentage is measured at the intake and at the output of the compressor. A metering valve is used to control the % of O2. The operator must use extreme care and continously monitor the compressor while it is running.

More resources on Nitrox

There are many good articles available on the web about Nitrox. I suggest that you take a look at the following.

This is a good place to learn the basics of Nitrox

This is a very good information about safely using Nitrox