Anyone ever hear of tank rolling after mixed gas fill??
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Sideband
Guest
Rec Diver:
Sure it does, and there have been many. Lighten up. I highly doubt that his post is going to keep anyone from learning the answer.
Joe
Who STILL likes his shaken and not stirred.
WarmWaterDiver
Contributor
Orlando Eric:
Sorry, somehow I double posted when editing - see below.
WarmWaterDiver
Contributor
The key element is the batch process, or at best, semi-batch process involving a closed container for the blending under discussion here. One gas (or gas mix, to describe 'air') at a time is batched into the tank.
There's a whole lot more empty space between gas molecules, than the gas molecules themselves occupy, for the gases under discussion, even at 3500 PSI. It takes a combination of some time and / or external force to get the mixing. Mixing requires collisions between molecules and / or the container. And, as some have noted, the viscosity of gases increase with temperature, the opposite of the behavior of liquids, due exactly to the increased number of collisions - so more friction resistance to flow, instead of less. But, that doesn't really weigh in as much as the relative amount of occupied space vs. void space for the gases.
See the discussion / definitions for diffusion & effusion - this might help clarify.
http://www.molecularsoft.com/data/help/Gas_Laws-Effusion_Diffusion.htm
For liquid molecules, that's not the case (not a lot of void space between molecules in liquids). And, one can have miscibility issues between liquids (traditional 'oil and water' analogy, for example), but gases and vapors are not immiscible in each other.
Anyone who wants to hijack into a discussion of dense phase fluids / supercritical regimes can PM me instead if such discussion's warranted, rather than sow additional confusion.
For those of you gas blenders who use continuous blend or membrane systems, for EAN production, have you experienced stratification in tanks filled in that manner? My bet is 'no', beyond the mix being pumped may be slightly different than the 'heel' of the previous mix still in the tank, if not fully depressured before refilling. The homogenous mix is obtained before entering the closed vessel (tank), except for any difference between the mix being pumped and the 'heel' gas remaining from the last fill.
There's a whole lot more empty space between gas molecules, than the gas molecules themselves occupy, for the gases under discussion, even at 3500 PSI. It takes a combination of some time and / or external force to get the mixing. Mixing requires collisions between molecules and / or the container. And, as some have noted, the viscosity of gases increase with temperature, the opposite of the behavior of liquids, due exactly to the increased number of collisions - so more friction resistance to flow, instead of less. But, that doesn't really weigh in as much as the relative amount of occupied space vs. void space for the gases.
See the discussion / definitions for diffusion & effusion - this might help clarify.
http://www.molecularsoft.com/data/help/Gas_Laws-Effusion_Diffusion.htm
For liquid molecules, that's not the case (not a lot of void space between molecules in liquids). And, one can have miscibility issues between liquids (traditional 'oil and water' analogy, for example), but gases and vapors are not immiscible in each other.
Anyone who wants to hijack into a discussion of dense phase fluids / supercritical regimes can PM me instead if such discussion's warranted, rather than sow additional confusion.
For those of you gas blenders who use continuous blend or membrane systems, for EAN production, have you experienced stratification in tanks filled in that manner? My bet is 'no', beyond the mix being pumped may be slightly different than the 'heel' of the previous mix still in the tank, if not fully depressured before refilling. The homogenous mix is obtained before entering the closed vessel (tank), except for any difference between the mix being pumped and the 'heel' gas remaining from the last fill.
Randy43068
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Hey Joe, how about "rolled?"Sideband:
Hope to see you again at Gilboa or someplace yet this summer for some diving!
Well, here are the weights for the 3 relevant gasses;
O=15.9994
N=14.0067
He=4.0026
The molecular weights of nitrogen and oxygen are pretty close, therefore any stratification is likely to occur very, very slowly. For example, the earh's atmosphere is not stratified - if it were we'd be breathing pure oxygen.
Helium however is a different matter. Helium is found in natural gas wells as a byproduct of radioactive decay of, primarily, uranium and thorium. The reason why there is so little in the earth's atmosphere (about 5.2 parts per million) is because helium is so light ( about a quarter of the gas that surrounds it) that it actually escapes from the atmosphere into space.
Producers of natural gas don't want helium in their gas because it lowers the BTU (caloric) content of their product and use fractional distillation to get rid of it (and then sell it to us).
Sooooo... it seems to me that it theoretically possible that trimix stratifies. However, the molecules are so small that they even pass through the cylinder walls, so my guess is that if you waited long enough for a tank of trimix to stratify the helium would have leaked out through the tank wall. You'd have nitrox...
the only circumstance I can see where stratification would be an issue is if one were to chill the cylinder to 77K/-195C/-320F where both oxygen and nitrogen are liquids, and only helium is still a gas. Perhaps if you then were to warm up the tank very, very slowly you could have some measure of stratification, at least between the Helium and the nitrox mixture. I wonder what that temperature would do to your lubricants and o-ring integrity...
Therefore, for all intends of purposes it seems unlike that stratification is an issue to worry about. (but I've been wrong many, many times before and all the stuff I wrote is theory, not what happens in practicality).
Rgds
WeekendDiver
O=15.9994
N=14.0067
He=4.0026
The molecular weights of nitrogen and oxygen are pretty close, therefore any stratification is likely to occur very, very slowly. For example, the earh's atmosphere is not stratified - if it were we'd be breathing pure oxygen.
Helium however is a different matter. Helium is found in natural gas wells as a byproduct of radioactive decay of, primarily, uranium and thorium. The reason why there is so little in the earth's atmosphere (about 5.2 parts per million) is because helium is so light ( about a quarter of the gas that surrounds it) that it actually escapes from the atmosphere into space.
Producers of natural gas don't want helium in their gas because it lowers the BTU (caloric) content of their product and use fractional distillation to get rid of it (and then sell it to us).
Sooooo... it seems to me that it theoretically possible that trimix stratifies. However, the molecules are so small that they even pass through the cylinder walls, so my guess is that if you waited long enough for a tank of trimix to stratify the helium would have leaked out through the tank wall. You'd have nitrox...
the only circumstance I can see where stratification would be an issue is if one were to chill the cylinder to 77K/-195C/-320F where both oxygen and nitrogen are liquids, and only helium is still a gas. Perhaps if you then were to warm up the tank very, very slowly you could have some measure of stratification, at least between the Helium and the nitrox mixture. I wonder what that temperature would do to your lubricants and o-ring integrity...
Therefore, for all intends of purposes it seems unlike that stratification is an issue to worry about. (but I've been wrong many, many times before and all the stuff I wrote is theory, not what happens in practicality).
Rgds
WeekendDiver
Debay777
Guest
- take 35% oxygen and place in a blender.
- slice other trace gases into managable clumps and be sure to include a pinch nitrogen. - place the various slices into the blender with the oxygen.
- mix on puree for 30 secs. make sure there are no lumps.
- pour into an air compressor and pump directly into an air tank.
LOL. I dont really know if the gases seperate over time or right after the tank is filled. I think that i would probably shake, roll, or manipultate my tank just to make sure. but after its filled, your gonna move it to your car or boat. it will be bumped or moved repeatedly from that point until you get wet. then you are subjected to a splash, swimming and then moving under water. if that doesnt mix it what will? compression chambers can subject people to tremendous pressure. in every video/documentary about diving where divers had to spend days and even weeks in the chambers not once did the air appear to be thick as syrup. i dont know if the air was up to 3000 psi or not but even so, the pressure was high and i saw no difference in the air thickness. the people in the tanks didnt look like they were swimming in thick air. they just looked bored.
so, does rolling the tanks help? who really knows. aside from making an acrylic tank with air sensors inside to verify the mix changes/settleing, well never really know. does it hurt to roll them? no. in diving safety is first. why not roll/shake/spin the tanks? it certainly wont hurt. especialy if your the one who finds out at 80 feet that it does.just MHO
- slice other trace gases into managable clumps and be sure to include a pinch nitrogen. - place the various slices into the blender with the oxygen.
- mix on puree for 30 secs. make sure there are no lumps.
- pour into an air compressor and pump directly into an air tank.
LOL. I dont really know if the gases seperate over time or right after the tank is filled. I think that i would probably shake, roll, or manipultate my tank just to make sure. but after its filled, your gonna move it to your car or boat. it will be bumped or moved repeatedly from that point until you get wet. then you are subjected to a splash, swimming and then moving under water. if that doesnt mix it what will? compression chambers can subject people to tremendous pressure. in every video/documentary about diving where divers had to spend days and even weeks in the chambers not once did the air appear to be thick as syrup. i dont know if the air was up to 3000 psi or not but even so, the pressure was high and i saw no difference in the air thickness. the people in the tanks didnt look like they were swimming in thick air. they just looked bored.
so, does rolling the tanks help? who really knows. aside from making an acrylic tank with air sensors inside to verify the mix changes/settleing, well never really know. does it hurt to roll them? no. in diving safety is first. why not roll/shake/spin the tanks? it certainly wont hurt. especialy if your the one who finds out at 80 feet that it does.just MHO
PerroneFord
Contributor
MikeFerrara:
LOL,
Mike the guy I work with has been blending over 30 years. Next to the blending station is a wooden platform where we stage tanks. Whenever we pp mix a single, we roll the tanks. When I asked why (after my adv nitrox course) he had me do that little experiement. I don't have to bother telling you the results.
When I mix doubles, I hit the air top off HARD to get things moving in the tanks. It's also why I'll have to load oxygen first when I start blending trimix and heliox. I want that top gas to go in hard and mix well.
Maybe it doesn't make a lot of difference. But it's sure not hurting anything.
Since this thread was first posted I made a startling discovery. When PP blending the contents in the tank are perfectly mixed however the gas in the dip tube is not. Due to the small size it seems to isolate itself from the rest of the mix. If you roll it, or just wait a while it will eventually mix as well. What I also found is if you blow a little gas out the valve right after PP blending your mix will come out fine. The stagnant aitr in the dip tube is blown out and the homogenous mix in the tank will fill the dip tube.
Debay777:
the pressure in a recompression chamber is no where near 3000 psi. at about 300fsw ambient pressure is 10 atm or about 150 psi. equivalent depth to get an ambient pressure of 3000psi would be about 6600 fsw. there might be a bit of an issue with ox tox there.
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