Adjusting nitrox mix in twinset?

[NAM001]

"Deep specialty" training dives and I'm insisting that we go to max depths. I don't think they will allow 32 as that would be 1.6 ppo2, instead of 1.4

I can see both sides of the argument. I am assuming that DEEP is >100' for sake of argument. I dont think that 32 is going to be a bad decision as you are not going to be at 132 for the entire dive. However out of prudence and what i would call responsible training protocol I would approach the dive with the constraints of not exceeding 1.4 as if the 1.4 pp was not at a hard bottom and teh entire dive was at depth of 1.4 pp.. AND if an equipment failure occured that you retained the option to continue down and rest on the bottom with out exceeding 1.6. After all this is a training scenario after all. Personally a 110' dive would be good for me to evaluate the dive from an instructors view point. Much like AOW definition of deep being greater than 60' so you do your cert at 80 +/-.

 

[NAM001]

So, maybe I'm being either pedantic or overly hung up on terminology here, but doubles do not "mix" through the manifold. If the two cylinders in a twinset have somehow ended up with different mixes, they will stay that way and will not "mix" through the manifold even if the isolator is left open and even if a substantial amount of time has gone by.

Instead, what is going on here, is that bleeding from either post with the isolator open draws gas in equal amounts from each cylinder in the twinset. Filling using either post with the isolator open fills gas in equal amounts to each cylinder in the twinset. Proper blending depends on this, which is why it is vital to leave the isolator open during these operations. Done this way, each cylinder ends up with the same mix, because 1) each started with the same mix, 2) each had the same amount of gas bled, and 3) each had the same amount of gas filled.

When someone blends gases by adding gas to one cylinder in a twinset with the isolator closed, out of ignorance or carelessness, the cylinders will end up with different blends. The only three practical ways to clean up the mess are 1) to drain the twinset and start over, 2) to close the isolator, analyze each side, and bleed/fill the two cylinders individually as required to reach the target mix, or 3) to use a booster to force blending to take place.

They will self mix as long as the cross over is open. If you are concerned split the top off and fill half on each side with the crossover open. The air top off never gets to the one tank prior to mixing with the other tank. 1/2 the air goes into the tank the other 1/2 goes through the crossover so long at psi in both tanks are equal and it is so long as the crossover is open.

 

[packrat12]

@2airishuman that is correct. people have died from assuming that gases mix through the manifold. They will mix as pressures are changing *i.e. bleeding off or filling*, but they don't really mix by sitting around.

This statement is incorrect. You may want to read up on the behaviors of a gas including Dalton's law and the movement of gas. Each gas molecule , e.g. Oxygen, Nitrogen etc, exerts a partial pressure when combined is the pressure of the vessel. The gas molecules move from areas of higher partial pressure to areas of lower partial pressure for each gas. This means that they will uniformly mix. This time is fairly quick and the two tanks are connected via a manifold which is open.

 

[seeker242]

Personally a 110' dive would be good for me to evaluate the dive from an instructors view point. Much like AOW definition of deep being greater than 60' so you do your cert at 80 +/-.

I think that would be ok too. I generally don't go much below 100 anyway. But one of my goals for the class is to try to see if I get narced and if so how it effects me, etc. So with that in mind, I'm thinking the deeper the better!

 

[Lorenzoid]

Interesting discussion of how long it takes for uniform mixing here: Diffusion, theoretical question

 

[tbone1004]

This statement is incorrect. You may want to read up on the behaviors of a gas including Dalton's law and the movement of gas. Each gas molecule , e.g. Oxygen, Nitrogen etc, exerts a partial pressure when combined is the pressure of the vessel. The gas molecules move from areas of higher partial pressure to areas of lower partial pressure for each gas. This means that they will uniformly mix. This time is fairly quick and the two tanks are connected via a manifold which is open.

fairly quick is what, several weeks? The tanks don't even uniformly mix quickly if you are PP blending because of the different gas densities. The movement of different gases requires some sort of gradient, which they don't really have. The orifice is quite small *dipsticks aren't that big*, so for gas to move it has to take a VERY complicated path from one side to the other through a very small hole.

If that were the case, PP blending would be so much easier by putting helium in one side of a set of doubles, O2 in the other side *saving boosters*, and then topping off with air or nitrox. They would magically blend perfectly during filling and all would be well. Unfortunately it doesn't actually work like that, so we put O2 in both tanks, then put He on top of that, then whatever mix we are topping off with on top of that to give us our real gas mix.

TLDR, they won't mix in any practical amount of time. Fill and drain with the manifold open if you are changing mixes

 

[2airishuman]

This statement is incorrect. You may want to read up on the behaviors of a gas including Dalton's law and the movement of gas. Each gas molecule , e.g. Oxygen, Nitrogen etc, exerts a partial pressure when combined is the pressure of the vessel. The gas molecules move from areas of higher partial pressure to areas of lower partial pressure for each gas. This means that they will uniformly mix. This time is fairly quick and the two tanks are connected via a manifold which is open.

Ordinarily I'm not in the business of calling people out but in this particular case you're presenting misinformation that could get someone killed.

You're wrong. Go run the experiment yourself.

And as an aside, I would suggest you refrain from giving me reading assignments in physics. Just saying.

 

[packrat12]

1 - The gradient is not that much 32% - 28% and the gas is already 'on the bottom' of the tank so the overall difference is 4% or so.
2 - There will be gas flow during the filling process that will initially equalize some of that gradient.
3 - The heat exchanges will additional have some equalization.
4 - Dalton's law will take care of the rest.

5 - This was never a discussion about pure o2/he on one side vs the other or proper PP blending techniques. It still will mix due to Dalton's law, not only when being filled as I believe 2airishuman stated. Yes the higher the gradient, the longer it will take.
6 - Please care to reference someone 'dying' due to non-uniform mix which had the manifold open? I am sure I could pump gas in on the boat and jump in to create this but come on, even the ride out will cause gas exchange. I never have had to wait long - or roll my tanks to equalize them beyond 1% - 2% or so.

 

[PfcAJ]

This statement is incorrect. You may want to read up on the behaviors of a gas including Dalton's law and the movement of gas. Each gas molecule , e.g. Oxygen, Nitrogen etc, exerts a partial pressure when combined is the pressure of the vessel. The gas molecules move from areas of higher partial pressure to areas of lower partial pressure for each gas. This means that they will uniformly mix. This time is fairly quick and the two tanks are connected via a manifold which is open.

That is absolutely not accurate. The exchange across a manifold is incredibly slow due to the small orifice size.

Application is not always as clear cut as theory.

 

[NAM001]

I think that would be ok too. I generally don't go much below 100 anyway. But one of my goals for the class is to try to see if I get narced and if so how it effects me, etc. So with that in mind, I'm thinking the deeper the better!

Yes and if you can get your self narced you probably will not notice it because it will not be severe (statistically) . You may want to take some pics and then when you review them seehow much detail you remember about thaking them/.,

That is absolutely not accurate. The exchange across a manifold is incredibly slow due to the small orifice size.

Application is not always as clear cut as theory.

I agree if you had right tank with 32% and teh left tank with 24% and then opening the crossover. but you cant keep the air on one side to cause a fo2 difference with the crossover that is significant. what small amount is there has plenty of room throught the small orface to equalize.


Now here is another aspect if there were a difference of say 2% between tanks when you breath you would suck from one side and the other tank would replenish speeding up the equalizatoin. Or you would pull from both sides equally getting 1/2 breath of 32 from one side and 1/2 breath of 24 from the other side and the mix in the lungs would avg 28%. so its still not a problem.