Adjusting nitrox mix in twinset?

[PfcAJ]

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/.,

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.

Try it. Post results.

I seent it before

 

[boulderjohn]

A few years ago, I had two single tanks with different mixes of gas, and I wanted them to have the same mix, and blending them would be perfect. I connected them with a fill whip and opened the valves. After a couple of days, they had not changed at all.

 

[Jack Hammer]

I didn't blankly recommend it, i asked what depth. If you're planning on staying at 130ft for the whole time, then of course not, don't dive EAN32 at that depth. Bad idea. if you're planned depth is 100-120ft and you may hit 130 for a minute or so, then no, I don't have a problem with it. His instructor may, but I don't. I personally don't like going beyond 100ft without helium, but that's just me. I get narc'd easily.

For a "deep specialty" there is also going to be no decompression. That means at 130ft, on EAN32, NDL is going to be something like 10-15 minutes depending on algorithm/tables, etc. 15 minutes. If you subscribe to the NOAA CNS table, that's 33% of the max for a single dive. 15 minutes on AL80s at 130ft, estimated consumption of 45cf is about 900psi assuming they are full. Top off with air after that dive and you're back to pretty close to EAN28. Top off with EAN28 for the rest of the course if it's banked.
Odds are, the first dive is going to be longer than 15 minutes and a lot shallower than 130ft. No instructor would just take the first dive of a course to the max of the limits, that's insane. So yeah, I'd keep EAN32 in there for the first dive and top off with air as the dives get progressively deeper. If the first dive of that course is to 130ft for the duration of NDL's, then I'd find a new instructor

Tom, for the most part I agree with what you've said in the quote above. That wasn't neccesarily what was in your original response and can read very differently from what you may have intended.

 

[RayfromTX]

This is most interesting. It illustrates a lay persons understanding of physics and how different the reality can be. Admittedly my knowledge of gas theory is weak compared to my practical physics related to building science. My understanding of this is that absent differential pressure the mixing that would occur through the manifold would be by diffusion or effusion and the size of the orifice would determine the rate of exchange along with the relative sizes of the different molecules.

In simple terms, visualize a bunch of ping pong balls that are two different sizes bouncing around in large containment vessels and the vessels are connected by a small tunnel. The balls will sometimes take the path through the tunnel but only by chance and because there are more of the large size in vessel A and more of the small size in vessel B there is a greater chance that a large ball will move from vessel A because there are more of them. The deck is stacked in favor of large balls moving from vessel A to B and small balls moving from vessel B to vessel A. This will continue until they are equalized because at that point the odds of a large ball moving from A to B is equal and vice versa.

Now the problem with a small orifice and a torturous path is that the molecules are just bouncing around randomly and they have to find the tiny surface area of the orifice and keep randomly moving in the right direction. The odds of that happening are small compared to finding the relatively large surface area of the vessel wall and bouncing of of it. As the orifice becomes really small the collisions of the balls (molecules) hinders the movement in the direction of the opposite vessel. The time frame for complete mixing under these conditions becomes extremely long.

Don't think of the orifice as being a water leak which would drain the liquid quickly. Think of it as a giant wall with a dartboard on it and you are blindfolded and your blue darts can only go into the next room if you hit the bullseye and you don't even know which direction the wall is and you have a friend that has a pile red darts and he's blindfolded too and he's trying to hit the little hole and he keeps reaching down and picking up red darts and throwing them blindly. Eventually some of your darts are going to end up in his room and eventually they will start to accumulate and then some of the darts he picks up to throw will end up being blue darts but not many at first.

The orifice is a target, not a leak. Also, this is a simple description and I understand that in reality at a given pressure and temperature a volume can only hold a given number of molecules and that movement is a zero sum game.

I'd really appreciate if a physics guy or a chemistry guy could check my understanding and correct the orifices in it.

 

[Lorenzoid]

In all the previous threads on this I have seen, there has been no consensus.

 

[dberry]

Despite the theory (and I teach kinetic theory of gases at an introductory level), I suspect the pressures involved make this an extremely non-ideal system. Although we could turn to more sophisticated theories of fluid dynamics and such, ultimately I'm an experimentalist.

Someone with a twinset and an oxygen analyzer needs to do the experiment and post the results. Air in one cylinder, equal pressure of EAN 32% in the other. Wait a few hours, or a day, or whatever you think it'll take, and measure the O2 levels.

sidenote: @RayfromTX your description of molecules leaking through an orifice as being the result of random collisions with the walls until they hit the orifice is correct in terms of the kinetic theory of gases. The process of gas escaping through a pin-hole is called "effusion", and it can be calculated fairly accurately under conditions where the gases behave "ideally" (low pressure, high temperatures). But again, I suspect the situation in normal scuba rig deviates significantly from ideality.

 

[AlexL]

Someone with a twinset and an oxygen analyzer needs to do the experiment and post the results. Air in one cylinder, equal pressure of EAN 32% in the other. Wait a few hours, or a day, or whatever you think it'll take, and measure the O2 levels.

kind of surprised that an actual execution of this experiment, or at least a review of empirical results derived from it, is not part of tech 1 / doubles primer type classes. my money is on no measureable affect of blending between the two over short (<months) periods of time.

just to throw a wrench into the whole mix (ha!), when you breathe both tanks down with the isolator open, the two blends will be mixed together 'on the fly' as the tanks remain equalized in pressure, resulting in an intermediate blend at the 1st stages.

The only difference between theory and reality, is that in theory, there is no difference. -- yogi berra / J.L.A. van de snepschuct

 

[stuartv]

Someone with a twinset and an oxygen analyzer needs to do the experiment and post the results.

Uhhh..... ---->

A few years ago, I had two single tanks with different mixes of gas, and I wanted them to have the same mix, and blending them would be perfect. I connected them with a fill whip and opened the valves. After a couple of days, they had not changed at all.

 

[tbone1004]

@AlexL only problem with blending on the fly is you will be breathing against different gas densities as well as different flow paths. I.e. if you have helium in the right side, and oxygen in the left, equal pressures and breathe off of the right post, then off of the left, you will have two different mixes. You have a longer much more tortuous path through the whole manifold and have a very different gas density. So you have a lower proportion from the opposite side tank as you do from the tank on the breathing side, and the helium is going to have an easier time moving through due to lower density.

In the example above, instead of 50/50 as you'd expect, you'd get something like 70/30 and 40/60 from both sides while you're breathing *these are BS ratios btw, just emphasizing the point*

Several people have in fact died from this type of scenario btw. Cave Divers Forum likely has incident reports from it. Death by improper gas blending, not cool *said like Chuck from Last Man Standing*

 

[stuartv]

Does this mean that I should always analyze both sides of my twinset, instead of just analyzing one side and assuming the other is the same?

I mean, how would I know if someone filled it wrong (so that I ended up with different blends on each side), analyzed it and realized it was wrong, but only "fixed" it by adjusting one side. So, if I happened to analyze the same side they "fixed", then I wouldn't realize the other side was wrong.