Anyone ever hear of tank rolling after mixed gas fill??

[BarryNL]

The person who filled your tank was either pulling your leg or is stupid. If they were actual serious then find another shop to fill your tanks. Do they role a normal air fill, after all it also has "Mixed Gas." Nitox just has a different percentage of what a normal air fill would contain.

Actually, they are more or less correct. Once you compress gases to 200 bar they behave more like liquids than gases. Rolling tanks will indeed mix the gases faster, though you'd probably have to do it enthusiastically for a few minutes, rather than give it a quick roll.

Has anyone actually analyzed a partial-pressure blended tank immediately after filling and then again a little while later to see if maybe this does actually make a difference?

Yes, and yes.

 

[Debay777]

again, i know the pressures arent quite the same, but deep sea divers who have to spend days and weeks in compression chambers dont look like they are "swimming" in thick air. i just dont see it. maybe if the tanks sat there for a few months the mix might seperate.might. but as soon as you pick it up, drive to the boat, or dive shop, get to the dive site, ready your gear, and splash in, im sure it would be mixed quit well.

 

[Crazy Fingers]

I am a chemical engineer whose career is based on analyzing gas mixtures, so I am quite confident when I answer this question.

Non condensible gases that can be considered roughly ideal (nitrogen, oxygen, argon, helium, hydrogen, neon, etc.) will completely mix very, very quickly. Their molecular velocities are high enough so that, on average, they are bouncing back and forth in that tank literally hundreds of times a second. Not only is this theoretically true, but definitely is supported by my personal observations. At work we have NIST traceable gas standard mixtures in cylinders, and they have certified contents stating the concentration of each component. One had a label stating it was produced in 1985 and probably hadn't moved in 10 years. I analyzed it by cross-referencing it to another, newer NIST-traceable gas cylinder using a mass spectrometer (the most accurate method possible for analyzing gases). The concentration was exactly the same as what was on the label, within experimental error. Actually, the simple fact that NIST will certify a primary standard gas mixture in the first place is evidence that it does not stratify.

Now, highly non-ideal gases, such as water vapor, freons, sulfur dioxide, etc. will tend to stick to the walls of the cylinder and could possibly stratify or have concentration gradients within the cylinder. This is due to their affinity for each other or for the metal of the cylinder. However, these gases are either not present in breathing mixtures (as in the case of sulfur dioxide), or their concentration is very low and relatively unimportant (water vapor).

As others have stated, the reason you stir other mixtures, such as sugar in kool-aid, is because you are trying to transfer mass from one phase to another! You are trying to make solid sugar dissolve into water. This happens more effectively when the concentration gradient is greatest. Thus you mix it up, otherwise the water "layer" right next to the sugar crystal saturates and mass transfer is slowed greatly. But in a gas cylinder, everything is in the same phase (and the most mobile phase!) so this isn't an issue.

As for those who have tested your mixture before and after shaking, I think it's due to temperature variations or random error in measurement. I will admit that I am not familiar with commercial oxygen analysers used in the scuba industry, so can't fully speculate why the error is occuring. My first suggestion would be to take 25 measurements of the exact same mixture and determine your 95% confidence interval. Do this by taking the standard deviation (use excel formula) and multiply it by 1.96. This number is the error in measurement, plus or minus. You now have a range of your analyser where you can be 95% certain it is right (provided there is no systematic errors!).

Maybe after I get my nitrox cert I will analyze a cylinder with one of the mass spec systems!

 

[victor]

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.

What an interesting thread.
I started out as a non roller, was converted into a roller, and now back to a non roller.

The argument that it was stagnent gas in the dip tube that had not mixed has some validity in my mind. However the action of releasing some gas from the tank will, and I am guessing, cause significant amounts of turbulance in the tank and that might improve the mix.

As getting an accurate reading can be very important I cannot see any harm in giving the tank a roll around before measuring, better to wait 24 hours though.

 

[miketsp]

I am a chemical engineer whose career is based on analyzing gas mixtures, so I am quite confident when I answer this question.

Non condensible gases that can be considered roughly ideal (nitrogen, oxygen, argon, helium, hydrogen, neon, etc.) will completely mix very, very quickly. Their molecular velocities are high enough so that, on average, they are bouncing back and forth in that tank literally hundreds of times a second.
..snip..

I found this statement a little strange.
The first time I saw a thread on tank rolling I thought "What a load of ****, of course the gases are going to mix quickly."
Then after reading opinions from people actually doing it I decided to investigate the theory and started working out some diffusion velocities for the components of air at 200bar and was quite shocked to get results of the order of cm/hr. My calculations were validated by a few other qualified posters.
In other words the theory backed up the rolling requirement because air at 200bar is actually quite viscous.
Now you as a qualified chemical engineer are telling me that my calculations based on established references (see PS below) were off by a factor of about 18000000?
After running through the calculations I find it hard to believe that the molecules are zinging from wall to wall - the number of impacts they have while crossing the tank is just too high, there's almost no free space between molecules. The molecule may be moving at high speed but it makes almost no progress.
I think you're confusing molecular speed and diffusion speed.

PS I just found the thread where we discussed diffusion rates. See posts #23 on...

http://www.scubaboard.com/showthread.php?t=102307

 

[Ice9]

I always thought rolling a tank to mix the gas inside was like shaking a chicken-bone at it to keep the evil spirits away...

 

[Crazy Fingers]

I found this statement a little strange.
After running through the calculations I find it hard to believe that the molecules are zinging from wall to wall - the number of impacts they have while crossing the tank is just too high, there's almost no free space between molecules. The molecule may be moving at high speed but it makes almost no progress.
I think you're confusing molecular speed and diffusion speed.

You're right about the collisions. I ran some quick numbers on a simulator and got ~1500 molecular diameters as a mean free path. That's pretty small, much smaller than I was expecting. So you're right, the molecule does encounter other molecules quite often. Originally I assumed that the mean free path would be larger than the diameter of the cylinder and thus the molecular velocity would give an idea of the number of impacts which would occur. However this definitely does not appear to be the case.

However, I still strongly believe that the diffusion will happen quickly. I'm betting on less than a minute. But I will call a couple people I know in the specialty gas industry and see if they can shed some light on this (they actually make primary standard mixes) If anyone knows, they will. I'll also try to calculate some diffusion numbers and read the thread tomorrow when I have more time (like at work :14: ) Hope to get back with their info soon.

As for stratification, I maintain that there's no way this will happen over time with breathing gases.

 

[dumpsterDiver]

I also witnessed the exact same situation regarding rolling tanks. The nitox tank had just been filled and the customer immediately tested it, and the mix was reading like 7% lower than it was supposed to be..

The fill guy said to him, just roll the tank around for a few minutes and re-test. I heard this interaction go down, and their was no doubt in my mind that the shop monkey was full of crap. How in the world could the air not mix with the oxygen?

Well he rolled it for only 2-3 minutes and retested and the mix was exactly right. We repeated the little experiment on both his tanks and got similar results. The shop guy says it only happens with the HP tanks (3500 psi).

I have never had to roll around my aluminum or my LP steels to get the mix to test right, even testing them immediately after the fill.

There is probably no way that I would have believed this, if I had not seen it with my own eyes.