Nitrox PPO2 blending waiting period for homogenisation of mix

[JahJahwarrior]

I have heard some people partial blend in the following manner:
1: add o2 to the correct amount.
2: slowly add in 200-300 psi of air to dilute the mix somewhat
3: very quickly add in about 500 psi of air, to "shake up" the mix
4: finish filling at the slower rates reccomended in mixing courses.

I've also seen people rolling tanks around on the ground to mix the gasses.

 

[JBRES1]

To fill a cylinder via PP fill requires more time than filling via banked gas.
I fill my cylinders at 50 psi per min for o2, and then top off with compressed air at 400 psi per min. So it takes me about 15 min to fill a cylinder.
As for the blending of the gas, If I test the mix just after I fill the cylinder often it is not the same as it is 2 or 3 hours later. The o2 content is just a little higher right after the fill, I'm talking about a 2% higher content.
Here is one to think about.
I fill my doubles with pure o2 to start with , filling thru the right post on the manifold.
Then I close off the isolator and top off each cylinder with compressed air.
I let the set sit for about an hour, then open up the isolator to balance off the cylinders.
I then test the o2 content of each cylinder, and record on the cylinder. The o2 content is very close, but never the same. I have let the cylinders sit with the isolator open for days, and the 2 cylinders do not balance off in o2 content even after sitting. Go figure !

Jim Breslin

 

[jdh]

I fill my doubles with pure o2 to start with , filling thru the right post on the manifold.
Then I close off the isolator and top off each cylinder with compressed air.
I let the set sit for about an hour, then open up the isolator to balance off the cylinders.
I then test the o2 content of each cylinder, and record on the cylinder. The o2 content is very close, but never the same. I have let the cylinders sit with the isolator open for days, and the 2 cylinders do not balance off in o2 content even after sitting. Go figure !

Jim Breslin

Did you come up with this on your own or does someone teach this stuff?

 

[kawaboy]

Hello all

Thanks for all your replies/comments. There certainly have been some useful insights. Thanks again.

I'll have a word with the LDS in question and ask for his reference for this waiting period. I hope to have something interesting to report back.

Cheers

Joe

 

[rjack321]

Pick the most likely for your LDS from the following list of possibilities:

• Didn't want to fill it right then, so they had you come back later
• Didn't know how to fill it right then, so they had you come back later
• Misinformed
• Low experience level
• You now have a mix of 21/35

All the best, James

Rick, I do not have a picture of one - I simply forgot to put the <sarcasm> tags around my post. I'll try to make it a bit more obvious next time as my lame attempt at humor must have missed the mark here.

My point was that he needs to find a new shop - this one can't be bothered with giving him the truth as to why he needs to wait for his nitrox fill.

He needs to wait because they don't have it ready for some reason... most likely they have not done it yet. But to simply tell him that he needs to wait for it to properly mix? c'mon now - we are not talking about an oil and water mix where after time they will separate.... we are talking about 2 gasses (of similiar weight, not like helium) under pressure which mix instantaneously in the cylinder. With seconds, each gas will occupy all space available to it in given it's partial pressure. It's not like the oxygen is going to hang out at the top of the cylinder (because it is not significantly lighter) and the nitrogen will lag at the bottom.

Anyway, I'll get off my rant now... anybody please feel free to correct me if I am in the wrong here, but again, this shop is simply giving him a BS line for some reason... I suspect they either do not know better, or they have just not done the work.

Okay, this time I am really done with the rant.

Guys with all due respect...

Gases do stratify. If you have mixed up 4 al40s (standing upright) with 50% with a compressor like mine (3cfm) you'd know from experience that your mix is not going to be accurate for quite awhile.

Yes the density of air and O2 is <similar> but its not exactly the same and the air top is not the same temperature as the O2 which was added yesterday either.

Or worse 2x 2200psi 240cf O2 cylinders that I introduced into my bank. I used the O2 inside until reaching 310psi then topped with air to make 32%. Standing upright with the non-extended length dip tube. Filling those at 3cfm... All done.... and they have air by analysis. Standing motionless, 4 days later I had 32%.

EAN can definitely statify. How long it takes to mix depends on the fill rate used and the shape of the tank. I don't PP blend very often (EAN50 in AL40s pretty much) and I find lying them on their side for an hour or so is adequate for my mix to come out as expected.

 

[4sak3n]

Go old school, good for pony tanks:

What happens if you have a 40cf pony?

 

[wedivebc]

The gas they're using in the aquarium is very, very dense and WAY heavier than Oxygen and Nitrogen. And of course they're all heavier than Helium.

Cute demo, but does nothing to refute my point.

-Charles

You are aware that the gas in a scuba tank is about 200 times denser than gas at 1 ATA and may not act in quite the same way.

 

[fdog]

Guys with all due respect...

Gases do stratify. If you have mixed up 4 al40s (standing upright) with 50% with a compressor like mine (3cfm) you'd know from experience that your mix is not going to be accurate for quite awhile.

Yes the density of air and O2 is <similar> but its not exactly the same and the air top is not the same temperature as the O2 which was added yesterday either.

Or worse 2x 2200psi 240cf O2 cylinders that I introduced into my bank. I used the O2 inside until reaching 310psi then topped with air to make 32%. Standing upright with the non-extended length dip tube. Filling those at 3cfm... All done.... and they have air by analysis. Standing motionless, 4 days later I had 32%.

EAN can definitely statify. How long it takes to mix depends on the fill rate used and the shape of the tank. I don't PP blend very often (EAN50 in AL40s pretty much) and I find lying them on their side for an hour or so is adequate for my mix to come out as expected.

You are absolutely correct; but the OP was talking about a recreational single with 32%.

Not a T.
Not a 240 cf cylinder.
Being filled at 14 psi/minute.
Not 50%.
Not being topped off with oxygen.

Probabally, its an 80 at typical shop fill rates.


<shrugs> I fill O2 at 1 psi/second, and top with air at 150-200 psi/min, and I haven't had a post-fill analysis be off by more than a couple tenths of a percent when analyzed at the dive site.

Thats for typical 32% fills. Anyone well outside that box knows it and accomidates for it. But, as you so rightly note, the "typical for me" may not be the typical for someone else.


All the best, James

 

[charlesml3]

You are aware that the gas in a scuba tank is about 200 times denser than gas at 1 ATA and may not act in quite the same way.

I am aware of that. I'm also aware that the all of the gasses in the tank are at that same density and all the gasses in the tank are very close to the same weight. Therefore they will not stratify. If the dip tube doesn't agitate the mix enough it may take some time, but they will eventually spread evenly through the tank.

I'm still waiting for some kind of explanation as to how "rolling" or "shaking" a tank could make any difference whatsoever. The can of paint analogy is bogus. The paint in the can is WAY heavier than the air gap between the can and lid. That's the shake space.

-Charles

 

[Rick Murchison]

I have to say that I have analyzed tanks right after an O2 topoff and had them rich -- ten minutes later, they have settled down to what they really are.

Particularly with doubles, at 200+ ATA there is plenty of the "last gas" left in the manifold and dip tubes to skew an initial analysis, and this can last a long time.
Try releasing about "one full breath" before the analysis and I think you'll find this problem goes away. (Nitrox)
(By the way, just as a basic procedure, I put any oxygen required in first. If I need to do any topping it's gonna be with 'anything but oxygen.')
Rick