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Pure Oxygen under high pressure conditions aids combustion in a huge way. High pressure O2 hitting hydrocarbons (grease/oil for example) can spontaneously ignite.

When filling nitrox cylinders, one method is called partial pressure blending - basically you put pure O2 into the cylinder and top up with air. During this process you have pure O2 going through the tank valve and into the cylinder.

O2 cleaning is undertaken to ensure that both the cylinder and valve are absent of these hydrocarbons.

Juz
 

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Clean

OK, the best way of explaining this is probably that for a cylinder to be in O2 service (so you can get a nitrox fill) it must be both O2 Clean and O2 compatible. O2 clean means that the item in question has been cleaned of all residue that could act as the fuel in the fire triangle. Now O2 compatible means that all of the components are certified for use with O2, and as such do not 'readily' combust in O2.

If the piece of scuba gear you are using is being used with less than 40% nitrox (40% O2 60% N2 instead of air 21% O2 79% N2) then it may be used without being in O2 service, although it is advised for it to be done.

If the scuba gear is going to come in contact with anything above 40% O2 then it MUST be in O2 service. Now if you have a 32% fill in your cylidner this is usually done using partial pressure blending where the 32% is achieved by putting O2 in the cylinder so although the cylinder is only going to give you 32% your cylinder still needs to be in O2 service.

Hope this helps and makes sense
 

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Nigel Hewitt
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To add some feel to the business consider this:

It is by adding a good oxidiser that you convert those charcoal bricks that will never light on your barbie into gunpowder, the stuff Guy Fawkes tried to blow up Parliament with. What ever your views on the rights and wrongs of blowing up Parliament you really don't want to do this to your diving equipment so if you are providing an excellent oxidiser (pure oxygen under pressure) it is a good idea to make sure there is nothing burnable in there.
 

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To add some feel to the business consider this:

It is by adding a good oxidiser that you convert those charcoal bricks that will never light on your barbie into gunpowder, the stuff Guy Fawkes tried to blow up Parliament with. What ever your views on the rights and wrongs of blowing up Parliament you really don't want to do this to your diving equipment so if you are providing an excellent oxidiser (pure oxygen under pressure) it is a good idea to make sure there is nothing burnable in there.
Nigel - You have a very modest sense of humour.
 
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