Gas mixing physics

[cyklon_300]

Posters on the Nitrox Equip thread were discussing the rate at which high pressure gas mixtures reach a state of homogeneity. Some (including me) were under the impresssion that this occurred in a very short time period (approaching instantaneously) in a scuba cylinder.

Others stated that they allowed periods of time and/or physically agitated the cylinders to promote reaching equilibirium.

I just found the following statement concerning trimix which I thought was quite interesting:

"It is now evident that helium based mixes are extremely vulnerable to real gas mix laws and can be more than 30% in error and/or can take more than 12 hours to reach a homogenous state. This robust, reliable unit quickly determines exactly what gas mixing has occurred and is an inexpensive and mandatory component for every mixed gas application. I will never again use a mix, or allow a mix to be used by a student or customer without first testing it with the helium analyzer. To do so otherwise would be irresponsible and reckless.” - Bill Nadeau

Bill Nadeau is the Proprietor of Deep Fathoms in Nanaimo, British Columbia. Bill has authored many texts on Scuba Diving including the IANTD Gas Blending Manual and IANTD Mixed Gas Blending Training Slides. He is also a member of IANTD Board of Advisors and an IANTD Trimix and Trimix Blending Instructor Trainer Trainer (ITT). Some of his many distinguished achievements include:
British Columbia Safety Council on Diving Advisory Board Member
First Mixed Gas Cave Dive in Canada - 1998
Deepest Cave Dive in Canada - 262ffw - 1998

The statement of "12 hours to reach a homogenous state" is what caught my eye.

Any physicists or gas blenders care to comment?

 

[Drew Sailbum]

or better yet, blend your mix the day before the dive. This is particularly so if you did it right and filled very slowly.

The problem is in confirming the contents of the cylinder through analysis. Theory is nice, but in reality mix does not reach a homogeneous state immediately. Same is true for nitrox as for trimix. Blending immediately before the dive is not good.

Where this really becomes an issue is when people top off their tanks after using a portion of their gas on a first dive. Analysis of the resulting blend should wait as long as possible in such a case. Physical shaking or rolling of the cylinder accelerates the mixing of the blend.

 

[SimonN]

I wonder if Bill Nadeau sells a He analyser?

Many people have done many dives (probably including most if not all important exploration dives) without using a He analyser.

I know what that says to me.

 

[Waterlover]

Do you have any references to back up your statements regarding gas not mixing for a given amount of time and rolling and shaking it to mix it? I am interested in learning more about gas mixing and the info would help me......

 

[Uncle Pug]

What exactly are the "real gas mix laws" refered to in the first post?

 

[Drew Sailbum]

Waterlover once bubbled...
Do you have any references to back up your statements regarding gas not mixing for a given amount of time and rolling and shaking it to mix it? I am interested in learning more about gas mixing and the info would help me......

Rolling the tanks is an old gas blender's trick. Heck, even the young gas blenders can use it too.

In Recreational Nitrox Gas Blending Manual by Bart Bjorkman (Best Publishing Company) the author specifically notes the need for waiting six hours before analyzing a nitrox mix. A homogeneous state is achieved ultimately through molecular migration. (I'll look through my other manuals tomorrow for more detailed references.)

As to the "real" gas laws... this term is used to distinguish the real world properties of gases, as opposed to theoretical "ideal" gases. Ideal gas law proposes that all "ideal" gases behave the same when compressed. In the real world, gas blenders know that the compression properties of helium are quite different than that of oxygen or nitrogen.

 

[MikeFerrara]

I have argued this a number of times. Gas does not mix immediately. I have seen this time and time again. If I mix say 32% (not much extra O2) and add the air on top fairly fast I can analyze immediately and read 32%. The next day it will still be 32%. Adding alot of air (if it's fast enough) mixed things up. On the other hand if I mix 50% or greater and add all the air throught the hyper filter (very slowly) I may read as low as 38%-42%. If I roll the tank ot let it sit a while I will read the target mix.

Some have suggested that the problem is gas in the tube or valve that was not mixed and all one needed to do is bleed some gas from the tank. I have tried this with no change in results.

 

[Uncle Pug]

Drew Sailbum once bubbled...
Rolling the tanks is an old gas blender's trick.

As to the "real" gas laws...

So when Cyk quoted BillN he was just adding a word: "real gas mix laws"

Kinda like me saying:
"Rolling the tanks is an old wives gas blender's trick."

Relax... just having fun at your expense... the point of my rhetorical question was that there is no such thing as "real gas mix laws." Just anecdotes that have lead to the *scientific* rolling of tanks around on the foor....

 

[cyklon_300]

If mixing via molecular migration occurs as a function of time. then the mixing rate should vary with temperature as that would increase molecular motion. Yes, no?

Also, how sensistive are the various measurement sensors to temperature? If blenders see a more stable (indication of homogenous state?) reading over time could this be attributable to the fact that the gas has cooled to a temperature more likely to be within the potentially temperature-dependent calibration range of the sensor?

Agree with PUG, rolling tanks might be good exercise, but is unlikely to change the mixing rate of the contents.

 

[Big-t-2538]

Not that I'm a blender, but wouldn't rolling have at least some effect on mixing contents. Much like dropping food coloring into a glass of water, wouldn't friction (all be it a very small coeficent in a gas medium) have some help with molecular movement?