Adjusting nitrox mix in twinset?

[boulderjohn]

To some posting:

Quick please notify Stephen Hawking that there are 2 places physics breaks down, Black Holes and inside a doubles tank - but I do not think that Stephen Hawking owes a subscription to Penthouse to anyone on this site!

This is not a 'lay person' issue. Most of the mixing will be done on the fill, there will be some gas exchange during heating/cooling cycles and from Dalton's Law. tbone felt the need to explicitly state "You will kill someone". Never had I suggested mixing gases in separate tanks and then opening manifold. Come on tbone. My first statement (2) is how almost everyone is filling double tanks. If you want to twist what I said in reply (2) go ahead but there is always mixing going on, not just filling and using as was commented by 2airishuman. If this was not the case, we would get our doubles way out of wack all the time!

Now I can only handle the gases as Ideal only. The random interaction with the vessel wall, which has a surface area probably >3 sq ft. The drip tube has a surface area of less than 1/64 sq in or .0001 sq ft so for every 100,000 interactions a molecule has with the side wall, it will have 3.6 with the drip tube. So yes any movement due to Dalton's Law will be slow but will occur. This is not a 'theory' as some have implied but a simple fact due to Dalton's Law! Practicality will state it will not make the gas uniform immediately but it will over time. The higher the difference in quantity of molecules in one side will cause more to make the path to the other side. The overall affect on uniformity will be small and slow but does happen. Pressure itself is the movement of the gas with in the vessel. They are not standing still!

I have seen a number of comments that the manifold is small. Who cares, to the molecule, if I compared a mite (the molecule) on a flea traveling on a dog in a car through the Holland Tunnel (the manifold), the mite would be way too large (fairly certain but not checked). The slowness comes from the surface area difference and even then, how many go through the manifold or bounce back.. I do not think the molecule states "is my butt to big to get through that manifold" very often though.

I am not a physicist or chemist and I am long removed from college where I did problems like this but this is a simple problem that is often done in the lab environment. Being in a Scuba tank does not change physics.

I see. So can you explain the reason those of us who have tried this were fooled into thinking that this mixing had not occurred? When I joined the two tanks with a fill whip for several days and then analyzed the tanks, what caused me to misread the analyzer and think nothing had changed? Was I hallucinating, and, if so, what caused it? Rick Murchison did the experiment over several weeks--what do you think caused him to misread the results?

By the way, in your first post you said the mixing would happen quickly; now you say it will happen slowly. Those terms are obviously relative terms. Can you define what you mean by "quickly" and "slowly" so people know what they should expect if they want to blend two gases through either a manifold or a fill whip?

 

[tbone1004]

@packrat12 your first post has no issues from me because that is SoP. I have issues with your post #13, because we know that tanks don't really mix "quickly" when they are in the same tank because the gases striate themselves *proven if you put He on top of O2 and analyze, takes several hours and some jostling to get them to mix*. More importantly, gas doesn't really like to move through the manifold, yes per Dalton's law they will eventually do it, but it takes a LONG time for it to happen across the manifold as proven by various experiments that have been quoted previously.

seriously, if you are so adamant that it happens, fill a set of doubles, one side with air, one with some sort of nitrox. Let us know what mix goes in, and what the mix should be coming out. Put a regulator with an LPI on each side, and analyze each. You will see a different number out of each post and that number won't be the magical 50:50 balance that you claim.
Repeat this every 6 hours for the first day, then once a day until it comes out where you think it's supposed to be. I think you'll be surprised at how long it actually takes.
Per what you and @AlexL are saying, it will be the right number out of both posts instantly or at least within a few hours. That is not the case, but if you don't believe us, then by all means, prove us wrong

 

[cool_hardware52]

More importantly, gas doesn't really like to move through the manifold, yes per Dalton's law they will eventually do it, but it takes a LONG time for it to happen across the manifold as proven by various experiments that have been quoted previously.

[COLOR=#000000]LONG time as in forever. The mixing "front" in a typical manifold is about .090" diameter. This "tube" is at about 12 inches long. (Center to Center + the internals of each post.

There is *no* pressure differential between the cylinders with an open Isolator. Nothing happens *very* slowly.

As tbone has noted well qualified individuals have done the experiment, IIRC they used 100% in one bottle and air in the other. The results were effectively *no* change after weeks....

Tobin[/COLOR]

 

[PfcAJ]

[COLOR=#000000]LONG time as in forever. The mixing "front" in a typical manifold is about .090" diameter. This "tube" is at about 12 inches long. (Center to Center + the internals of each post.

There is *no* pressure differential between the cylinders with an open Isolator. Nothing happens *very* slowly.

As tbone has noted well qualified individuals have done the experiment, IIRC they used 100% in one bottle and air in the other. The results were effectively *no* change after weeks....

Tobin[/COLOR]

Well it's short on a cosmological timescale

It's all about perspective!

 

[seeker242]

 

[dberry]

I don't think citing Dalton's Law of Partial Pressures is particularly useful in this context. That law is used to describe a gas mix at equilibrium. The mixing issue is one of kinetics. EVERYTHING will eventually reach equilibrium (i.e. 2nd Law of Thermo - entropy wins, eventually.) The question is how fast the system reaches equilibrium.

@packrat is correct in pointing out that gas molecules are ridiculously tiny compared to the size the orifice, and that there are no big-butt gas molecules involved here that might get physically slowed down "squeezing" through the hole. He is also partially correct that the probability of the gas molecules escaping through the orifice is related to the ratio of the area of the hole to the area of the containing volume. The molecules bang randomly into the walls; once in a while they bang into the wall a "miss" - i.e. enter the orifice. This is the process of EFFUSION, which requires the orifice is small compared to the mean-free-path of the molecules (i.e. how far a molecule travels before bumping into another molecule.) Thus effusion doesn't really worry about a molecule bumping into another molecule as it escapes.

But under higher pressures the mixing becomes a process of DIFFUSION (i.e. the progress of a molecule "trying" to mix is hindered by bumping into other molecules. Now add into the problem the non-ideal property of gas VISCOSITY and things slow down considerably.

There is a classic chemistry demo for gas diffusion in which you take an air filled tube (ca. 2 cm in diameter and 1 meter long) and place a drop of ammonia solution in one end and a drop of HCl solution in the other. The gases diffuse through the air and form a film of white ammonium chloride where they meet. The usual point is to show the lighter NH3 molecules diffuse further than the heavier HCl molecules, but the relevant point here is that it takes minutes for any significant number of the molecules to make it down the tube to the meeting point. That is not "complete" mixing, just the very beginning. And that is at 1 bar with a tube diameter of 2 cm. Crank up the pressure in the tube (increased hindrance to diffusion) and decrease the diameter of the tube relative to it's path length and it's not hard to imagine complete mixing could take a very long time.

Twists, turns and deviations from a straight line reduce gas conductance rates (adds resistance, in series), so tank-to-tank mixing through a manifold has additional problems.

Here's a video of this classic demo.

 

[RayfromTX]

I'm going to stay at >100 years. I'm not going to >1000 years. I'm comfortable right there. You can't prove that it will take more than 1000 years but I'll bet proving > a year will be easy. It will take a year though. What shall we do while we wait? Go diving?

If you want to cheat the test put the twin set outside oriented so that one tank sees morning sun and the other sees afternoon sun. You might get it done in about a year then. Convection is way faster than diffusion and way way faster than effusion.

Do I understand the concept Don?

 

[2airishuman]

I'm not sure how much this matters but I do believe that there is a genuine risk of blending problems leading to accidents so I'm still posting.

Just as a reminder, what I wrote upthread was:

[D]oubles 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.

So, that's what we're talking about here. If you have a twinset, and there are different mixes in each cylinder in the twinset, they will stay that way and will not "mix" even if the isolator valve has left open and even if you wait days, weeks, or months.

3 - The heat exchanges will additional have some equalization.

I'm not sure what you mean, but unless one cylinder is heated more than the other, it won't result in any gas exchange.

But even if you paint one cylinder black and the other white and leave them out in the Texas sun, it's going to take weeks for any substantial blending to take place. The math on that is relatively easy based on the two temperatures, the fill pressure, and the number of cycles. I'll leave it as an exercise.

4 - Dalton's law will take care of the rest.

Dalton's law states that the total pressure is equal to the sum of the partial pressures of each gas. It has zip zap zero to do with diffusion.

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.

This discussion is about whether gases will mix through the manifold. They don't. It doesn't matter what gas blends are in each cylinder. Start with 32% and 36% and after a week you'll still have 32% and 36%.

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.

This is true in most cases but can't be relied upon.

I believe that some of the accidents have historically involved the isolator also being left shut, unintentionally, during the dive.

That aside, the whole point of having an isolator valve is that you can close it when circumstances require. Then you're breathing the unblended gas. (I think the question of whether an isolator valve provide a net improvement in safety, and the question of whether anyone is alive today who would not be but for the fact that they had an isolator valve to close, are both very interesting questions. Another thread, perhaps)

You will also breathe unblended gas if both posts are drawing gas at the same time, for example, during an air share.

There are some other corner cases, such as blockage of one dip tube, or one cylinder being warmer than the other at the start of the dive, so that it cools as the dive begins, which could lead to a period when unblended gas from the initially-cooler cylinder would be delivered to both posts.

@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.

I'm not sure I agree with this line of reasoning. The pressure stays the same in both cylinders if the isolator is open. As long as the temperatures of the cylinders are the same and the isolator is open far enough that there isn't a pressure difference between the cylinders, they will both deliver equal volumes of gas.

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.

If you're going to be thorough you would have to close the isolator, analyze each cylinder, then open the isolator.

I confirm that the isolator is open when I drop the twinsets off for fills, and again confirm it is open when I pick them up. That is enough for me, because it guards against reasonable mistakes that might occur when a tech-savvy fill operator is filling a twinset.

Never had I suggested mixing gases in separate tanks and then opening manifold.

I'm not sure what you actually are suggesting, but you seem to disagree with my statement that: "doubles do not mix through the manifold."

Now I can only handle the gases as Ideal only.

The reason they do not mix can be mostly explained while still treating the gases as ideal gases although the possibility of collisions between molecules does make the problem worse.

The random interaction with the vessel wall, which has a surface area probably >3 sq ft. The drip tube has a surface area of less than 1/64 sq in or .0001 sq ft so for every 100,000 interactions a molecule has with the side wall, it will have 3.6 with the [dip] tube.
[...]
The slowness comes from the surface area difference and even then, how many go through the manifold or bounce back.

Right, the problem is compounded by the fact that the geometry of the path through dip tube, valve, crossbar, isolator, valve, and dip tube has a number of reflective surfaces that will tend to send most of the molecules back the way they came. The fact that this is hard to model is what makes the overall problem hard to approach from a theoretical standpoint. The closest reasonable approach would be to have some number of intermediate vessels (3? 5? 7?) connected by orifices to each other, and then calculate the diffusion for each pair.

Being in a Scuba tank does not change physics.

I think we agree on that.

Where I'm at is that, experimentally based on tests other people have performed, the gases don't mix -- not to the extent and on the timescale that matters practically for diving -- and the theory backs that up.

 

[stuartv]

 

[tbone1004]

@2airishuman they should breathe equal volumes of gas, but the won't. The gas densities are going to be different which causes one to flow easier than the other, basically less viscous. One tank has a longer distance to travel as well. It may not be significant, but it is there. Try it some time