Why O2 before Air for PPB Nitrox?

[rjack321]

Nowadays its ridiculously easy to make o2 compatible air. Most air is OCA with even just the stock filter on a compressor. It wasn't always this easy. Most filters fail for water long before they fail for hydrocarbons.
Net, If your shop is producing nitrox they should be making OCA even if the continuous blend or use a membrane. There is no reason not to have this level of cleaniless since its so easy.

 

[NAM001]

I can think of a multiple hypothetically reasons. And reasons for ease. My greatest physics reason is that the source of O2 is stored in a low pressure container compared to the diluting gas. If you are going to get heating of the gas then do it as you dilute the O2 content and not increase it. I believe that Helium is a low pressure container gas also. It makes sense to handle flammable gasses at the lowest pressure as if 1800 # is any less dangerous than 3000#. If you put any added gas in a tank it is a small percentage compared to the whole and that provides the longest period of diluting by stirring of the O2 as diluting gas is more or less sprayed into the tank.through he dip tube. personally I dont think the additional time makes a difference. much like being hit by a semi or a train the end is the same. I have never mixed trimix but I also think it may have to do with like the tank pressure of the O2 whether you have a booster pump to put the O2 in slowly and at the higher pressure. I have seen O2 added last. If I had to guess the general process was established with out the booster pump being an option and being able to use the most O2 form the source tank. Certainly the greatest chance of an ignition is when the source and destination delta pressure is the greatest. most often ( which is not many times) I see helium put in first then the O2 then topped off with air or nitrox 32 or some other mix. Im sure there are many methods to do this. The greatest issue i have had with making nitrox is wehat I can only call coeficient of expansion with O2. If I make a batch of 40% in an empty tank it used aps 25 O2 and air top off. so I put IE 750 O2 in the tank. I then let is sit for a half hour or more and return to fine the pressure is sometimes in the mid 600's and I have to top off with more O2 before air. If you do a fill of an AL80 and fill to 3200 is cools to somewhat at or above 3000. Fill rate has the greatest effect on gas heating. It to me is only reasonable that the critical step is to get the O2 right form the start or else you will have 2 gasses cooling and you will not get your desired mix. the 750 cools to 650 and the overall tank cools from 3200 to 3000. If you do the O2 first and let it cool you can be sure you have the right amount of O2 pressure at environment temps. Then who cares what the top off rate is. Keep topping off till you get the desired end PSI. The rates are different. I think O2 has a fill rate of what 50 psi per minute and air of 600 psi per minute. those are to fill rates two heating rates and that is a plan for the wrong final mix. Then you have too high of tank PSI to transfer more O2 to the tank with out a booster. I would guess that if the whole process was done at 50 psi a minute (slowest fill rate) the error at the end would be minimalized.

 

[NAM001]

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?

The key to the question is whether they are filling with grade E hyperfiltered air on regular fills or not. If nitrox is available then there is a very high chance that all air if nitrox grade air.

 

[rhwestfall]

Shop here has two whips. Actually, multiple, but one being passed through a hyper filter. They do EanX by PP. If you bring in a cleaned tank (so identified by the VIP sticker), you can get 21% top ups from the hyper filtered whip to (in theory) keep the tank "clean".

YMMV