Why is O2 used first to fill nitrox?

[oldenred]

Why is it thay O2 has to be first in the tank when filling nitrox during blending? Why can't they use regular air first since they know how mucch of each needs to be in the tank anyways. Wouldn't this be safer and avoid having to have tanks O2 cleaned? I tried looking this up but couldn't find anything on it. Thanks

 

[mac.calder]

There are a few reasons - compressors are cheap and efficient to run - gas boosters not so much. Gas will from from high pressure to low until they reach equilibrium - to drain the rest of the tank you use a booster, which "pushes" the gas to the destination cylinder - and at higher pressures it takes more and more energy (with boosters, generally provided by a high-pressure air supply) to "push" the gas in.

You would still need to O2 clean the valve of the tank even if O2 was added last as pure O2 would be passing through it at high pressure.... Additional to that, working with high pressure O2 is dangerous. The higher the pressure the more dangerous.

O2 first means you are dealing with "lower" pressure O2.

 

[Doppler]

Just to clarify what mac.calder wrote:

Oxygen typically comes in cylinders with a working pressure of less than 90 bar/2400 psi. Therefore, unless one has a gas booster (an expensive tool), the pressure in the supply cylinder must always be considerably more than that in the tank being filled. Hence the common practice of starting out with empty tanks when partial pressure filling.

And oxygen compatible components are still required... according to commo best practice.

 

[oldenred]

Thanks

 

[DogDiver]

Not even the US Navy will use oxygen at pressures greater than 3,000 psi. Too unstable.


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[clownfishsydney]

Doppler, oxygen is normally about 160 bar when full (at least in Oz). Your PSI is correct.

 

[Dr. Lecter]

Not even the US Navy will use oxygen at pressures greater than 3,000 psi. Too unstable.

I won't bother making you cite your source, since the Navy does a lot of stuff that would make civilian diving impracticable if we followed the same approach (and has done some other O2 rebreather stuff that'd be considered suicidal in rec circles).

Either way, unstable is incorrect. Higher pressure does mean that it can suffer the effects of higher velocity in a system, making O2 cleaning that much more important. But whether it's at 500psi or 5,000psi, O2 in a bottle is very stable. It's when you start moving it from very high pressures to very low pressures…like opening the valve with an unpressurized reg seated in it…that things tend to heat up. Whether there's any fuel for the heat and oxygen to ignite/oxidize is the kicker.

 

[Doppler]

Doppler, oxygen is normally about 160 bar when full (at least in Oz). Your PSI is correct.

Not enough coffee! Apologies... LOL