Why O2 before Air for PPB Nitrox?

[giovyledzep]

This is probably a basic question answered in any Gas Blender course, but it came to my mind and I can't find the answer online.

Why is the procedure for partial pressure blending of Nitrox to put 100% oxygen in the empty tank first and then top up with air?
Wouldn't it be "safer" to add the required amount of air (provided it's properly filtered) first and then top up with 100% oxygen so that there is not a high concentration of oxygen in the tank at any moment?

 

[michael-fisch]

The danger area is the valve with it's 3 or 4 sharp 90° turns. Running HP O2 through this will occasionally cause minor combustion frying the nylon valve seat.
At lower pressures the danger of this happening is lower.
Next problem, it's next to impossible to get O2 tanks with more than 2400psi from the suppliers in the USA. Southern Europe the max is 200bar. Germany and a few northern European states have 300Bar O2 available. Without a booster how are you ever going to use up the supplied O2 and having/using a booster is an added expense.

Michael

 

[RainPilot]

You would only get one or two fills if you were lucky before you needed to use a booster. By putting in O2 first you can get a lot more fills before you needed to boost.

 

[tbone1004]

couple reasons.

O2 historically only comes in 2200psi bottles, so you would need a gas booster which is very expensive. Putting the O2 in first means it's a lot less expensive to fill this way.
More importantly is while the ppO2 in the tank won't be so high, the ppO2 in the fill whip and valves is going to be very high which drastically increases the risk of something going boom. The tank is low priority in terms of things that will explode, the valve is the highest risk. Add to that, if you have a high pressure steel tank, pumping O2 at close to 4000psi is extremely dangerous in and of itself

 

[giovyledzep]

Makes sense. Thanks everyone.

One more question: I read everywhere that the air that you want to mix with O2 has to be "O2 compatible", and if your tank was oxygen clean but you had a "normal" air fill, you cannot do a nitrox fill for your next dive. You need to bring the tank in for O2 cleaning again.
While it sounds like a responsible way to do things, it sounds pretty unnecessary and it's not what I see in dive shops. I had tanks filled with nitrox, then air, then nitrox again with no problem and by the time they take to fill I can assume they do PP blending.
Plus nobody asks what you previous fills were, they just look at the VIP sticker, if it says O2 clean, great, who cares if you had it filled with normal air or not at any point.
Is this ok?

 

[formernuke]

The regular air they run if they are doing PP blending is the same air they put into the tank after adding the O2. It is filtered to blend with pure O2 so it does ruin your O2 clean.

The issue is if you go to a place that doesn't have that extra filter.

 

[DeepSeaExplorer]

It hasn’t been that long that 3,000 PSI O2 has been commonly available, 10 years or so.

So you would only be able to fill the tank to whatever the maximum O2 pressure was, typically 2,200 PSI.

 

[jgttrey]

It's your responsibility with your nitrox/O2 clean tanks to keep them that way. Not doing that puts the blender at risk.

The fill station should not put regular air in a nitrox tank. They can put hyperfiltered air, sometimes called EAN21, in there - no problem. It isn't "air" per se that is the problem. It's that regular scuba air is not necessarily filtered to O2 compatible standards. A shop that fills nitrox cylinders will usually have the hyperfiltered air as standard.

So, if you get your tank filled somewhere else with regular Grade E air that is not suitable for a nitrox cylinder, it's on you to get it cleaned before letting anyone partial pressure blend into it.

 

[OkByMe]

Makes sense. Thanks everyone.

One more question: I read everywhere that the air that you want to mix with O2 has to be "O2 compatible", and if your tank was oxygen clean but you had a "normal" air fill, you cannot do a nitrox fill for your next dive. You need to bring the tank in for O2 cleaning again.
While it sounds like a responsible way to do things, it sounds pretty unnecessary and it's not what I see in dive shops. I had tanks filled with nitrox, then air, then nitrox again with no problem and by the time they take to fill I can assume they do PP blending.
Plus nobody asks what you previous fills were, they just look at the VIP sticker, if it says O2 clean, great, who cares if you had it filled with normal air or not at any point.
Is this ok?

I think what you are seeing in the dive shops is the NITROX fill from a bank, NOT pp blending. A big difference between introducing 32% and 100% into a possibly contaminated tank or valve.

 

[rsingler]

by the time they take to fill I can assume they do PP blending.

It may be that they're still not PP blending, and they're not banking the nitrox, so it takes a little extra time to get the fill stick concentration perfect before they start compressing...