Roll, Roll, Roll your tank

[flyandive]

It works.. the mix will change after rolling.. I would have bet money this was not true.. until I saw it with my own eyes.. more than once,,

I would be willing to bet the more likely reason the mix changed was because of the cylinder cooling than the tumbling. Actual gas diffusion within a mixture is very quick. The analyzer actually measures the partial pressure of O2, not the percentage. As such it is affected by numerous other factors including temperature, humidity, atmospheric pressure, air density, etc etc. Which is why you are supposed to calibrate it on an air tank before analyzing. In a perfect world if you calibrated one at sea level then hooked it to an air tank at 6,000' it would read about 18% (10.5% at 18,000').

Also keep in mind sport analyzers only give an approximate reading. If you read the manual for many analyzers they can be quite far off, in order to be considered accurate. The MaxTech MaxO2, for example, manual claims an accuracy of +\-3%! Analox claims +\-1%.

My point is that there is no point. I would be amazed if tumbling or any other tricks will alter a mixture beyond the accuracy of a typical analyzer. Simply changing analyzers will have a greater effect on the reading from a tank at room temperature.

 

[dumpsterDiver]

Nope the rolling changes the mix more than 1 % and it happens quickly. I doubt the air inside is cooling..

 

[flyandive]

Nope the rolling changes the mix more than 1 % and it happens quickly. I doubt the air inside is cooling..

Huh, then I guess all those chemistry classes in highschool and college must have been wrong then. If I may:

http://lmgtfy.com/?q=Combined+Gas+Law

and even more revelent:

http://lmgtfy.com/?q=graham's+law+of+diffusion

 

[Wookie]

Once again, stubborn belief trumps science. The dinosaurs were roaming outside the Garden of Eden too. 4000 years ago. Rolling scuba tanks around.

 

[Mike2Fst]

I often do this with Trimix fills, especially if I don't have time to let them sit. Diving sidemount, I've had the opportunity to fill both at the same time, roll one and let the other one sit and then compare, makes a big difference on the analysis. I prefer to fill the night before, let then sit and analyse in the AM but sometimes the choice of gasses is last minute based on where the charter is going that day. Do what gives you piece of mind on getting an accurate gas analysis, whether I choose roll or sit depends on when I get my tanks filled. As far as rolling doubles goes, can be done, very ugly and entertaining for those who are not diving doubles

 

[Wookie]

I often do this with Trimix fills, especially if I don't have time to let them sit. Diving sidemount, I've had the opportunity to fill both at the same time, roll one and let the other one sit and then compare, makes a big difference on the analysis. I prefer to fill the night before, let then sit and analyse in the AM but sometimes the choice of gasses is last minute based on where the charter is going that day. Do what gives you piece of mind on getting an accurate gas analysis, whether I choose roll or sit depends on when I get my tanks filled. As far as rolling doubles goes, can be done, very ugly and entertaining for those who are not diving doubles

Again, if you get the opportunity, fill one on it's side and then stand it up without rolling. Fill another one upright and roll it around until you get tired. The one you fill on it's side will read what you think it should first.

This assumes you're topping with a compressor. If you can top with HP banked gas, it's all moot. Blasting air in from a bank gives you all of the helium (oxygen) mixing you ever wanted, then some.

 

[HowardE]

Once again, stubborn belief trums science. The dinosaurs were roaming outside the Garden of Eden too. 4000 years ago. Rolling scuba tanks around.

I agree with the Wookie here.

The people who roll tanks swear by it. Even though, it's a ludicrous concept. How can a smooth walled cylinder MIX anything?

Take a bottle of water, and put some rocks in it. Roll it around... Where are the rocks?

Now, gas and rocks aren't the same... because the gas mixes immediately, and the rocks never will... but how can a smooth wall being rolled around make anything mix at all... if this worked, then when you buy paint, they'd roll the cans instead of shaking them.

 

[miketsp]

stupidest thing Ive ever heard

This is a well discussed topic on the forum. If you search for "diffusion" you'll find a lot of threads circa 2007 discussing this.

It's simply that at 200 bar gases start to get viscous and diffusion velocities are very (surprisingly) slow so there is a long spontaneous mixing time. Various posters ran calculations on this.
By rolling them you really do speed up the mixing (diffusion) process.

Some people talking about smooth walls not mixing anything. Once the gas gets viscous at 200Bar there will be a lot of drag on the walls.

 

[vicp]

To all you "science " dudes (I am a PhD one), miketsp is correct.

I never noticed this with Nitrox, but while taking a Trimix blending class recently, we noticed that analysis for He (& O2) - using a Divesoft analyzer - did not match the proportions we put in at first. Only after waiting some time and agitating the cylinders did the numbers begin settling to expected values. Doing a trimix fill in an AL40, which is easy to handle, I shook, rolled and inverted the tank for a minute or two and the analysis was right on. The folks with doubles had to lie them down and then still wait for the numbers to come up.

The bottom line is that freshly filled tanks, depending on method of fill, may not immediatlely show correct analysis, especially for trimix. Agitating the tanks and/or waiting (overnight?) will give a much better analysis. I also plan to start checking my nitrox fills for O2% before use - not just after fills as I have been doing.

 

[Wookie]

To all you "science " dudes (I am a PhD one), miketsp is correct.

I never noticed this with Nitrox, but while taking a Trimix blending class recently, we noticed that analysis for He (& O2) - using a Divesoft analyzer - did not match the proportions we put in at first. Only after waiting some time and agitating the cylinders did the numbers begin settling to expected values. Doing a trimix fill in an AL40, which is easy to handle, I shook, rolled and inverted the tank for a minute or two and the analysis was right on. The folks with doubles had to lie them down and then still wait for the numbers to come up.

The bottom line is that freshly filled tanks, depending on method of fill, may not immediatlely show correct analysis, especially for trimix. Agitating the tanks and/or waiting (overnight?) will give a much better analysis. I also plan to start checking my nitrox fills for O2% before use - not just after fills as I have been doing.

I bank trimix into 444 cu ft cylinders. They are bolted to the wall. The gas mixes just fine after an hour or 2 after blending. No rolling, no shaking, no witch doctor dance. What mixes gas is time, not agitation. But agitation doesn't hurt anything. Go ahead and shake, rattle, and roll until you're as tired as you want to be. If it makes you feel better about yourself, go ahead and have a ball.