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

[sharpenu]

Those who believe that the gasses stratify need to get a refund from your instructor. Remember back in open water, when they told you that a liquid which is exposed to a gas will contain that gas dissolved in proportion to the pressure of the gas?

That is because pressure and temperature are measuring the same action- the energy of gas molecules knocking each other around. Put more molecules in the container- they collide more often- increasing pressure AND temperature. Heat (add energy) to the gas- the molecules move faster, they collide more often- increasing pressure and temperature.

Why is this important? Because air is really a blend of O2 and N2 (pluse traces). For all intents and purposes, air is two seperate gases- O2 at 3.07 psi (.209 ata) and N2 at 11.61 psi (.79 ata). The motion of the molecules blends the gas.

Remember a fluid has no definite shape. Liquids and gases are types of fluids.

A gas has no shape or volume- it will expand or contract so that it is spaced equally throughout the container.

A liquid has definite volume, but no definite shape. The density and weight of the molecules overcome the energy of the brownian motion at lower temps. That is why liquids seperate and gases do not.

 

[in_cavediver]

Those who believe that the gasses stratify need to get a refund from your instructor. Remember back in open water, when they told you that a liquid which is exposed to a gas will contain that gas dissolved in proportion to the pressure of the gas?

First, yes its possible. Its not a stable situation and usually requires very slow fill rates in odd shaped tanks, such as doubles, to occur. The gas will eventually mix, it may takes a bit of time though. Hence the analyze your mix before you dive.

That said, I doubt rolling tanks really does much other than burn calories.

 

[Night Diver]

My personal method consists of jumping up and down vigorously for at least a minute after strapping on my rig. Especially if I've left the tank sitting in the garage for more than a day or two. I do find some odd stares but hey, at least I know my gases aren't forming some diabolical gaseous black-and-tan in the bottom of my tank.

Sorry y'all I had to.

 

[CIBDiving]

First, yes its possible. Its not a stable situation and usually requires very slow fill rates in odd shaped tanks, such as doubles, to occur. The gas will eventually mix, it may takes a bit of time though. Hence the analyze your mix before you dive.
That said, I doubt rolling tanks really does much other than burn calories.

I'll agree that a problem could occur in doubles. If you were to close the isolation valve and fill one side with nitrogen, the other with Oxygen, it would take a considerable amount of time before the sides were equal once the Iso was opened. Even in this case kicking the rig around ( or even putting it in a "shaker" ) isn't going to make a noticable difference.

 

[MikeFerrara]

The experts speak out again huh? LOL

I've mixed a LOT of gas and here's what I've seen. When mixing "normal" recreational nitrox mixes, I mix analyze and dive. you're only putting in a little O2 and hitting it with a big air top off and it analyzes right on the money right away. Mixing trimix has always worked out just as well for me.

However, back when we used to dive "stroke" mixes like 80%, I would add LOTS of O2 and then top with air using my hyper filter. The hyper filter added a flow restrictor to the line and the mix controller already had one so that makes two and the air top off was SUPER slooooow. I could mix it and it would analyze at maybe 40 something percent. The first time I saw it I dumped it and started over and the same thing happened a second time. I let it sit for a couple hours while I did some other stuff and the reading didn't change. Out of frustration I laid the tank on the ground and kicked it around some. Guess what? When I picked it up and analyzed it I got 80%! I've also seen it to a lessor degree when using the hyper filter to mix 50%. If I skip using the hyper filter (I know very bad but I haven't blown up yet) or get the air in faster and even with the hyper filter the mix is on the money. I sold the hyper filter but with some flow restrictors I can repeat the results at will.

I've argued this with lots of people. I know the theory and the gas should mix almost instantly on it's own but under certain conditions it doesn't. I've repeated this little experiment a number of times just to prove it to folks who couldn't bring themselves to believe it. I asked the folks at Purdue for an explaination and their short answer was that in real life, the geometry of the container, flow rate into the container, pressure ect would all have an effect and what we sometimes see is absolutely possible. In fact, note that there is quit an insdustry built around the design and manufacture of gas mixing devices. Any one see a nitrox stick lately? If the gas mixes itself instantly then why bother? Once mixed I haven't seen gases seperate, except of course some methods of gas detection take advantage of the fact that when excelerated around a corner different gas molecules will take a different path and become seperated.

In most cases, there is a better solution than rolling tanks on the ground but if you don't think it can have an effect try my little test.

 

[mongoose]

I've seen a few posts that mentioned the mix changing as a result of temperature drop. Would that in fact matter?

It would seem intuitively, to me at least, that a 64/36 mix of N2/O2 would remain 64/36 no matter what the temperature of the gas is. The respective amounts of molecules of each gas in the container would not change as temperature drops, only their energy as they bang around.

I could maybe see it stratifying inside the container in the event of a HUGE drop, say from 100degC to -100DegC and being put in a centrifuge or something... but outside of some cryogenic environment, would we be likely to see such a change?

 

[DivesWithTurtles]

I've seen a few posts that mentioned the mix changing as a result of temperature drop. Would that in fact matter?

It would seem intuitively, to me at least, that a 64/36 mix of N2/O2 would remain 64/36 no matter what the temperature of the gas is. ...

Mongoose,
The gas mix doesn't change, but the reading that the analyzer returns is temperature and humidity dependent.

 

[CIBDiving]

The experts speak out again huh? LOL

I've mixed a LOT of gas

You Laugh at other's opinions far to easily.

I've notice that you mix a lot of gas - it seems to be mostly hot air but I believe I've detected a hint of methane too.

 

[nessum]

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 no... it's NOT supposed to work....

And yet... if you analyze it... roll it for a while & analyze it again it seems to make a difference.

Perhaps next weekend I'll blend up 2 tanks. Analyze them. Allow one to sit while I roll the other & then analyze them again. See if there's much difference between the 2.

 

[MikeFerrara]

You Laugh at other's opinions far to easily.

I've notice that you mix a lot of gas - it seems to be mostly hot air but I believe I've detected a hint of methane too.

To easily for what?

I laugh because I keep reading how somthing that I've seen many times isn't possible and this is a recuring subject that has been discussed and debated often. As far as your contention that I mostly mix hot air perhaps you'd care to be more specific? Or is your comment mostly hot air? If you're inclined to question my credability or sincertity, please do so but also please follow it through.