Stratification of Nitrox Mixes

[ianr33]

so many fancy answers....

How many of you actually mix gases rather than just "know" gas "laws"?

Hey Wreckwriter, I dont mix gases but I absoluteley believe your observation of the O2 % changing with time,question is why? I find it really hard to believe it is due to incomplete mixing. The change in temperature seems a likely answer to me.

 

[Genesis]

don't play favorites.

Nor do they get suspended because you think they are.

When you find an experimental result that conflicts with the laws of physics, you need to investigate why before just shooting off and declaring the law invalid.

Next time you have such a "suspect" tank, make one change to your analyzer setup - run the gas through a good length of wide (2" or so) copper pipe so that the gas comes to ambient temperature before it gets to your sensor. Do the same when you analyze it later. THEN compare notes.

I'd be willing to bet your "anomaly" disappears - what you're measuring is a sensor non-linearity caused by the gas temperature, rather than an O2 percentage difference.

 

[roakey]

Problem is, hox mix makes the O2 read high, not low.

I've seen this when my Nitrox cylinder got put out in the sun and my reference cylinder of air was kept in the shade.

Roak

 

[Genesis]

But in practice my assumption would also not be that the gas laws suddenly decided not to apply to that tank

I'd go looking for the cause of the deviation from expected results - there are a number of ways to do this, including transfilling a few hundred PSI from the "reads low" tank into an empty tank, then measuring THAT after both have reached equilibrium.

(Since the amount drawn off would by definition be from that first exhausted, the difference should be "captured" by this procedure. If its not.....)

 

[spaz]

Do the experiment!!!...Do many...I want answers...I want answers now!!!!
Thanks

Sorry, it is the manager in me talking.

 

[TheDecoStop]

in response to "Problem is, hot mix makes the O2 read high, not low."

I just took a quick look at some older sensor calibration charts that give the numbers to use as constants while calibrating a sensor at a specific temperature and humidity. The charts show the O2 reading decreasing with an increase in temperature at the same humidity. They state the main reason for this as a gas's ability to hold moisture at different temperatures. They then proceed to state the fact that humidity greatly affects the sensors readings.

This would lean towards the cooling effect of rolling or sitting and then analyzing as being the reason for the varied readings vs. gas mixing or stratification. I have done tests after mixing that have shown me that my readings are more along the lines of what I expect if I roll a tank a little and then analyze, or just let it sit for a bit and come back and analyze it later. I haven't spent too much time trying to figure out why, because I only view the act of analyzing as a confirmation of what I already know I put in the tank... as long as I did the math right!

Sorry for the long winded post...

Rick

 

[MikeFerrara]

I mix alot of gas by PP blending. As far as nitrox goes I mix more 50% than anything. I won't pretend to be a physics expert but her is what I see several times a week. I can duplicate it at will and in fact will do it tonight for students.

Whjen I mix something like 32% I can mix and analyze and over time there will be very little change. When I can I add the O2 let cool, check the presure and top as needed then top with air.

If I am mixing 50% I use the same procedure but I am adding less air. The whip from the hyper filter is flow restricted and so is the hyperfilter. This gives a super slow fill rate and little heating. If I analyze immediately I can read as low as thirty something percent. If I let the mix sit for a couple of hours it will read 50%. If I roll the tank it will read 50%. As slow as I fill this much of a variation is not due to temp.

BTW if I don't use the hyper filter (flow restricted) and add air much faster I can analyze immediately and hit it right on and it will be on 24 hours later as well. This is totally repeatable.

Alot of air fast gives good numbers that stay good vs a little air slow slow requires time or further agitation.

In order to do a mix like this and analyze right away we must add the last bit of air fast, let the mix sit or roll the tank.

Once mixed (stable reading) the gas stays mixed.

I don't know what gas laws to apply but I don't believe you will get a homogenous mix instantly without some agitation. Anybody who says different hasen't done much PP mixing.

 

[MikeFerrara]

I did a little research and there seems to be much written on just how to get gasses to mix. There seems to be some products on the market also. If gasses mix so fast by themselves I couldn't really see the point. Last night I was finishing up a blender class where two of my three students work at Purdue. One is faculty and the other is staff in one of the engineering departments. BTW the third student is a Lawyer who started life as a biologist.

As I stated in my last post the class witnessed the fact that the initial reading after topping with air VERY SLOWLY was way way low (like 20% low).

We talked this over and none of them was surprised. None could present equations or specific laws that explained it though. One student did point out what happens when a smoker exhales. The idea was to use a visable gas as an example. Anyway the student who is on staff in the engineering department says the people who pretty much wrote the book on gas mixing are in his department. He has promised to try to get us a quote.

 

[ianr33]

MikeFerrara once bubbled...

We talked this over and none of them was surprised. None could present equations or specific laws that explained it though. One student did point out what happens when a smoker exhales. The idea was to use a visable gas as an example. Anyway the student who is on staff in the engineering department says the people who pretty much wrote the book on gas mixing are in his department. He has promised to try to get us a quote. [/B]

I look forward to the answers to this,it really does seem just wrong to me,yet it is obviously observed. Might have to change my belief system yet



And just to be awkward,the smoke you see from a cigarette is a finely divided solid,so will be MUCH heavier than a gas and so is not a fair analogy

 

[Bottomfeeder]

MikeFerrara once bubbled...


Alot of air fast gives good numbers that stay good vs a little air slow slow requires time or further agitation.
<snip>
I don't know what gas laws to apply but I don't believe you will get a homogenous mix instantly without some agitation. Anybody who says different hasen't done much PP mixing.

I mix nitrox and trimix every week, and that is my observation also.

I often use a Haskel booster that is very slow. The most dramatic differences come when I adjust a mix by boosting O2 or He on top of already mixed gas. My setup is particularly slow, as I have to boost, wait for the drive compressor to catch up, boost, wait, etc. If I boost 200 psi of gas on top of say 3400 PSI, the tanks do not get even a little warmer. If I boosted He and expected 14% O2, the immediate reading will be something like 2 - 4%. When I check it the next morning, it will be maybe 10%. I have had it take up to 2 days to settle out. When I boost O2, I get ridiculously high readings for a day afterward.

If I boost and then put the tanks in the truck and drive 5 hours to dive, the readings are what I expect and stable when I get there.

If I mix normally and blow air on top of O2 or heliox the analysis is correct immediately.

All of this is both repeatable and reproducible