Hasving a tank with one mix in it and another with another mix in it will take a significantly long time to equilize to the same mix. This is a different process than having a twin set with 1000 psi of 32 mix in each and topping off both tanks with air using one post. The higher pressure from the compressor will fill both ttanks at the same time and give you the same mix in both tanks. It would be a different story if you put air in one tank and then I moved from one tank to the other, but that is not the case you are putting air in a common manifold and higher pressure goes to the lower pressure which is both tanks. It is the same as closing the cross over and using 2 whips connected to both posts and coming from the same compressor output. Either way when you breath you are not breathing off the tank you are breathing off the manifold that is supplied by 2 sources (Tanks) Just as iff you had a manifold like this. 2 post manifolds function same as this with 2 reg connections in the middle instead of one. So toppiing off fills both tanks as once. If that was not true the crossover valve would not be part of a 2 post manifold.
Adjusting nitrox mix in twinset?
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AlexL
Contributor
thats how it would work if you had a pair of infinite volume, constant pressure sources with low differential pressures on the order of a few psi across the entire assembly, and a very restricted manifold, but thats not what we're discussing.
convert that over to volume and rethink your statement. if you take a pair of 100 cf tanks, fill em with X and Y mixes, and then breath 20cf off one post with the isolator manifold open, you're claiming that you're going to end up with 95cf in one, and 85cf in the other?
a pair of fixed volume tanks/plenum connected by an open manifold will always seek equilibrium of pressure, and you'll draw off both in equal quantities (*assuming matched tank capacities) unless the flow of the isolation manifold is significantly less than the demand load, which its not.
this is not dismissing the claim that botching a partial pressure fill can lead to lethal breathing gas mixes, because it obviously can. also not claiming that 'mix on the fly' of dissimilar mixes in a twinset is a viable process. just that if you have a twinset with dissimilar mixes, at the same pressure, and you measure the mix at each post with the isolator open, you're going to get the same result at both, and its going to be 50/50 between the two mixes measured as a time weighted average. fluid dynamics are rarely that simple, and the instantaneous mix at either post will vary slightly due to inertia / restrictions / gas stratification in the tanks / thermal changes / multitude of other variables, but its going to be in the ballpark.
KevinNM
Contributor
Again, it was reported that in one such experiment it took a week or more for two different gases to mix completely:
Last edited:
TSandM said: ↑
Rick Murchison did this experiment. It ran weeks, as I recall.
Viscya said
"As I recall it ran for weeks, and still came nowhere close to homogeneous. I believe it just mixed the gas in the valve itself . The tanks were relatively unaffected."
Perhaps I need more coffee but in your link, significant mixing never happened. On further reflection (sleep) I'm revising my estimate to >10 years. Will anyone run the test? Will the tank pressure make a difference? Will any of the current participants of this thread still be active members of Scubaboard when the results are in if my guess is right about it being >10 years? How many more posts before I find a way to turn a physics question into one of philosophy? <1?
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Yes. See post #3 and post #7 in the thread that @Lorenzoid linked to. Don't ask me to quantify the difference, though.
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@AlexL remember how the two are tied together and remember the gas density difference between them. What you are saying is literally that you can fill o2 in one tank, he in the other to equal pressures, and when you connect a regulator you get the same mix out of either post and that that mix is going to be 50:50. We know this to be definitively untrue since people have died from it.
Go fill a set of doubles with unequal mixes and tell me what they show up at. What you are saying, that the mix is going to be the same, implies that they are mixing evenly through the manifold. Since the manifold is a long and tortuous path for one gas, and not for the other, there is going to be a difference. Since the gas densities are not the same, the He is going to have an easier time moving through it than the O2, so the ratio isn't going to be the same .The ratios I gave above were obviously exaggerated to prove a point.
Go fill a set of doubles with unequal mixes and tell me what they show up at. What you are saying, that the mix is going to be the same, implies that they are mixing evenly through the manifold. Since the manifold is a long and tortuous path for one gas, and not for the other, there is going to be a difference. Since the gas densities are not the same, the He is going to have an easier time moving through it than the O2, so the ratio isn't going to be the same .The ratios I gave above were obviously exaggerated to prove a point.
OK, I'm going to propose that pressure affects the time scale because it changes the ratio of gas volume to orifice size. Then I'm going to add a zero to my estimate and say >100 years at 3000 psi. Then I'm going to ponder whether the calculation would even need to be accurate to within two orders of magnitude and then I'll ask who has a twinset that they are willing to test it with for say one year before drawing a sample. With the data supplied by the testing we can model it to within one order of magnitude. Sound reasonable?
Is data one or two orders of magnitude more convincing than theory? Is that a philosophical question?
packrat12
Contributor
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.
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.
