This is an area of great confusion. I believe a lot of it has to do with old thinking that has become outmoded. Remember that there was a time when no recreational agency allowed the use of nitrox, and there was a time that no presentations on nitrox were allowed at the annual DEMA convention. It was considered too dangerous. Now even OW students can use nitrox on their 4th certification dive.
I just bought two new tanks, and I read the warnings that came with the valve carefully. I noticed the care with which they were worded. In their totality, it was clear that the valves came to me ready to be used for a pure oxygen fill, although it did not say so specifically. What it did say was that if you use the valve on a tank being used for that purpose, make sure the tank is properly cleaned.
So what about filling a nitrox tank with air? The fear is that if you do use it with air, the air will contain contaminants that could ignite in the presence of pure oxygen. Pure oxygen is introduced into a nitrox tank if it is filled via the partial pressure method. The fear is that if you just hook the tank up to the compressor and fill it with air, it will thus become contaminated and create a danger for the filler. Let's look at the steps of the partial pressure blending method for nitrox to see the problem with this line of thinking.
Step 1: Add oxygen to tank
Step 2: Hook the up to the compressor and fill it with air.
So how is it that hooking the tank up to the compressor to fill it with air introduces contaminants if that is all you are doing, but it doesn't introduce contaminants if oxygen was added first?
The answer is it doesn't. There are different qualities of air, with different requirements for filtering out contaminants. If the compressor you are using is filtered enough to create nitrox via partial pressure, then it is pure enough to give you an air fill without the need for oxygen cleaning afterwards. I have no qualms about putting air in my tanks.
---------- Post added November 28th, 2013 at 09:59 AM ----------
Any idea why that is? I can't imagine air contaminating a O2 cleaned (Simple Green cleaned) tank.
As mentioned above, it depends upon the level to which the air is filtered.
Imagine if you could put in the air typically found in Los Angeles without a good filter!