Stratification of Nitrox Mixes
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That wasn't stratification. Stratification (of gases) refers to a mix of gases that separate into layers when undisturbed. This was a massive (hundreds of thousands of tons) single volume of CO2 that suffocated people and livestock before it had a chance to mix with the atmosphere.
edit - And why bump a 22 year old thread for this?
Agro
Contributor
I mix alot of gas by PP blending. So do I. As far as nitrox goes I mix more 50% than anything. I won't pretend to be a physics expert but her is what I see several times a week. I can duplicate it at will and in fact will do it tonight for students.
Whjen I mix something like 32% I can mix and analyze and over time there will be very little change. Sometimes I have differences of 4-5% When I can I add the O2 let cool, check the presure and top as needed then top with air.
If I am mixing 50% I use the same procedure but I am adding less air. The whip from the hyper filter is flow restricted and so is the hyperfilter. This gives a super slow fill rate and little heating. If I analyze immediately I can read as low as thirty something percent. So do I. If I let the mix sit for a couple of hours it will read 50%. I do not everytime. Sometimes yes, sometimes no. If I roll the tank it will read 50%. Yes, rolling always gives me a perfect result. As slow as I fill this much of a variation is not due to temp.
BTW if I don't use the hyper filter (flow restricted) and add air much faster I can analyze immediately and hit it right on and it will be on 24 hours later as well. This is totally repeatable. In my case it is not. A faster fill does not guarantie a good mix. Perhaps my "fast" is slower than yours. Perhaps a queston of valve's layout? I do not know why but even after fast fill I must wait for a good mix.
Alot of air fast gives good numbers that stay good vs a little air slow slow requires time or further agitation. My experiences: generally spoken yes but the conrete situation my be different.
In order to do a mix like this and analyze right away we must add the last bit of air fast, let the mix sit or roll the tank. This is not my experience. Letting the mix sit does not always(!) work, sometimes reading is wrong even after 24h. Rolling perfectly works. Doubles are difficult to roll so I just lay them down for 24 h. No rolling, no moving at all. This perfectly works. Knowing this I do not roll single tanks anymore, just lay them down for 24 h.
Once mixed (stable reading) the gas stays mixed. Yes. Even on trimix I neever ever had any stratification. No matter how much He is in there, no matter how many years(!) tank was sitting around.
I don't know what gas laws to apply but I don't believe you will get a homogenous mix instantly without some agitation. Anybody who says different hasen't done much PP mixing. Fully agree. A fast fill MAY give a perfect mix but may also give a vodoo mix.
Whjen I mix something like 32% I can mix and analyze and over time there will be very little change. Sometimes I have differences of 4-5% When I can I add the O2 let cool, check the presure and top as needed then top with air.
If I am mixing 50% I use the same procedure but I am adding less air. The whip from the hyper filter is flow restricted and so is the hyperfilter. This gives a super slow fill rate and little heating. If I analyze immediately I can read as low as thirty something percent. So do I. If I let the mix sit for a couple of hours it will read 50%. I do not everytime. Sometimes yes, sometimes no. If I roll the tank it will read 50%. Yes, rolling always gives me a perfect result. As slow as I fill this much of a variation is not due to temp.
BTW if I don't use the hyper filter (flow restricted) and add air much faster I can analyze immediately and hit it right on and it will be on 24 hours later as well. This is totally repeatable. In my case it is not. A faster fill does not guarantie a good mix. Perhaps my "fast" is slower than yours. Perhaps a queston of valve's layout? I do not know why but even after fast fill I must wait for a good mix.
Alot of air fast gives good numbers that stay good vs a little air slow slow requires time or further agitation. My experiences: generally spoken yes but the conrete situation my be different.
In order to do a mix like this and analyze right away we must add the last bit of air fast, let the mix sit or roll the tank. This is not my experience. Letting the mix sit does not always(!) work, sometimes reading is wrong even after 24h. Rolling perfectly works. Doubles are difficult to roll so I just lay them down for 24 h. No rolling, no moving at all. This perfectly works. Knowing this I do not roll single tanks anymore, just lay them down for 24 h.
Once mixed (stable reading) the gas stays mixed. Yes. Even on trimix I neever ever had any stratification. No matter how much He is in there, no matter how many years(!) tank was sitting around.
I don't know what gas laws to apply but I don't believe you will get a homogenous mix instantly without some agitation. Anybody who says different hasen't done much PP mixing. Fully agree. A fast fill MAY give a perfect mix but may also give a vodoo mix.
It is related to his work in another thread. It is his belief that the air in the lungs stratifies quickly, with heavier gases going to the bottom and lighter gases going to the top. (He does not explain how the alveoli affect this stratification.) As a consequence, a diver exhaling while doing a controlled emergency ascent will eliminate oxygen before CO2. Thus, as the diver ascends, the air remaining in the lungs will have a high concentration of CO2, creating a CO2 buildup and leading to panic. That is why, he believes, someone with free diving experience is vital in scuba, because in an emergency, free divers know how to manage their air.
At least, that is how I understood the argument.
[HIJACK]
@Goonzodiver,
What make and model is the oval mask pictured in your avatar, (I'm thinking it is a Beuchat Super Compensator Rubber Mask), and are you satisfied diving it? TIA.
rx7diver
[/HIJACK]
Goonzodiver
Registered
The stratification I proposed would only happen because in my "hypothesis" the vocalization by emitting "aaah" during CESA is not an efficient means of eliminating CO2.
The stratification would happen not only in the direction of top->bottom (due to gravity), but also especially "inside lung"->"surface of the lung" due to the expansion of the air in the center of the lungs that tends to crush the CO2 near the lung wall, hindering its diffusion in the rest of the volume.
We are talking about a situation in which the diver on the ascent has around him a pressure P-dP (CO2 would be emitted at that pressure), while inside the lungs there is still air at pressure P.
During a CESA, no real exhalation movement is made. It is believed that vocalization is sufficient to eliminate CO2, I am very doubtful.
If this mechanism is true, thinking of doing a 50 meter CESA (as someone proposed) as you do a 9 meter CESA (the limit for which PADI considers the CESA safe) could be an large underestimation of the difficulty that you face.
@rx7diver yes, I am satisfied diver with it
.
Goonzodiver
Registered
Maybe we are using different definitions of stratification., bu it seems to me that the mechanism is very similar (creation of unmixed regions of gas due to the effect of external forces, such as gravity).
"Since carbon dioxide is 1.5 times the density of air, the cloud hugged the ground and moved down the valleys, where there were various villages."
The effect of gravity seems clear to me.
There should be two phenomena counteracting in a Scuba cylinder.
Stratification due to gravitational forces and diffusion of gasses.
Let's assume a perfect tube with O2 on the "bottom" with density of 1.43 grams/Liter and N2 on the "top" with density of 1.25 g/L
(Top and Bottom refer to direction of gravity.)
We also assume a perfect membrane separating them at time t=0 and a magic wand removes the membrane perfectly without stirring, friction...etc.
The gases will tend to stay "stratified" where the N2 will "float" on the O2 (1.43/1.25 = 1.14 => O2 is 1.14 times denser than N2) like a thermocline where colder water is denser than warm water, halocline where salt water is denser than fresh water, atmospheric gases...etc.
This is caused by gravity. This tube in a "perfect" centrifuge will cause the gases to stay stratified because the force of gravity in a centrifuge is much much larger than the force of diffusion.
However, at 1G gravitational force, the force of diffusion will cause the gasses to mix and because their densities are relatively close (for comparison: CO2 is 1.96 g/L or about 1.37 times denser than N2 and 1.57 times denser than O2)
Full diffusion should take about 15-20 minutes with 200 Bars/3,000 psi at room temperature.
Helium He (NOT He2) is much much lighter at 0.18 g/L so the stratification should occur but the diffusion rate of He is also much faster at 1G.
Regardless, it is very difficult to counteract diffusion after mixing has occurred in O2, N2 and He without a centrifuge or gravity assist. And any kinetic energy (movement) adds to the mixing and diffusion occurs faster.
It is much easier for Chlorine gas (3.2 g/L), CO2, Rn Radon (9.73 g/L), and Hydrogen Sulfide HS2 (1.54 g/L) to "sink" to the bottom of caves or tunnels or other places without a kinetic mixing force. Carbon monoxide CO and Methane CH4 can get trapped in the top of caves. Running water (At the bottom of the cave) will absorb CO2 and "clean" the air inside the cave with some varying effectiveness.
Experiment: Take two identical tanks with same starting pressure and mix
Top off with only O2 slowly and analyze both immediately. You should get the same reading
Roll one tank and re-analyze then wait 15 minutes and analyze both
Disclaimer: I concede to being very stupid and you should probably NOT be listening to idiots on the internet.
The CO2 was mixing with air from the moment the gases emerged from the lake. Unfortunately the amount and speed of the addition of the CO2 meant that the mixing didn't happen quickly enough to prevent loss of life.
But that's not stratification. If you add two different gases together, there's always going to be a period where certain areas will have different concentrations of each gas. Stratification occurs when those differences increase over time. If those concentrations lessen over time, the mixture is not stratifying.
Atmospheric gases do not stratify at 1 g. Nitrogen, Oxygen, and Helium molecules all zip around at well over Mach 1. The speed is inversely proportional to the mass of the gas, so Helium atoms are especially zippy with an average velocity of nearly 2,400 mph (4,000 kmh). These molecules flying around in random directions is what initially drives the volume of gas towards a homogeneous mixture and then keeps it there once it is achieved.
You need a lot of energy to constrain these speedy molecules and gravity is famously incredibly weak. So weak that the gravitational force of the entire Earth exerts less than 200 pounds of force on the average person.
So how much gravity do you need to stratify atmospheric gases to a noticeable amount?
I'll refer you to the second answer on this Physics Stackexchange thread Is it possible to split up air into its parts by something like centrifuging? for the math. But the final result is that you need to centrifuge a sample of plain old air to about 5000 g to roughly double the concentration of CO2 in the outer band of the sample.
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