Nitrox gas separation

[Teamcasa]

..And I have friends who are absolutely convinced that the automakers are in bed with the oil companies who would shut off their access to plastic & rubber components if they made a purely electric car.

Don't we all!

 

[Charlie99]

Total BS. Gas won't separate like that.

Agreed. Unless your tank has some little Maxwell's Demon's running around inside the tank pushing the molecules off to their respective corners.

when people tell you they have to roll or mix a tank of nitrox after filling ( Partial Pressure Blending )

Disagree. There IS a set of circumstances that will lead to a stratified partial fill mix. It only happens with a very slow fill where the air topoff doesn't create turbulence. It has been observed by many reliable sources.

Here's a link to someone whose observations I trust: http://www.scubaboard.com/forums/196660-post27.html

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 here is what I see several times a week. I can duplicate it at will and in fact will do it tonight for students.

When 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.

Other posters have also noted that they have seen inconsistent results when analyzing partial pressure nitrox right after filling if the air topoff is very slow, but that if they just do a little burst of faster filling towards the end of the topoff that just that little burst of air will make the nitrox near homogenous. So if your partial pressure air is fast enough to generate a bit of turbulence in the tank, then you would never see a need for rolling tanks or waiting a while. That doesn't mean that the problem can't occur.

 

[Randy43068]

I've seen this phenomenon and there is an awful lot of anecdotal evidence, but I have a hard time believing any mixing is going on simply by rolling the tank. There is nothing in the tank to agitate the gases to cause any mixing.

??

I'll read what's at your link. Thanks!

 

[Roko]

I've seen this phenomenon and there is an awful lot of anecdotal evidence, but I have a hard time believing any mixing is going on simply by rolling the tank. There is nothing in the tank to agitate the gases to cause any mixing.

Except a good olde bucket of inertia.

 

[JeffG]

I have no doubt that gases mix and stay mixed, but gas at 200 bar also does not behave the same as gas at a few bar.

Please explain why the analysis changes over time.

If you analyze immediately after introducing a small amount of say He without mixing the sample will have a higher percentage of He and lower percentage of O2 than what is present in the tank.

If you either wait, or agitate the He percentage will fall and the O2 percentage will rise.

I can demonstrate this phenomena even after a quick blast of gas out of the tank to clear the valve and dip tube.

Tobin

I have also seen exactly this.

We were topping with He on a mix. (The O2 was to hot for the dive we were doing)

We would analyze the gas after the top and it would report 1-2% O2.

We flipped the doubles over a couple of times and the readings stabilized at 11% (Which is were we wanted it...or at least close enough for rock n roll )

 

[cool_hardware52]

Except a good olde bucket of inertia.

Yup. If it has mass it has inertia. 100 cuft of gas (air or nitrox) = ~7.8 lbs.

Tobin

 

[Teamcasa]

I've seen this phenomenon and there is an awful lot of anecdotal evidence, but I have a hard time believing any mixing is going on simply by rolling the tank. There is nothing in the tank to agitate the gases to cause any mixing.

.. if Nitrox sits in a tank for an extended period the nitrogen will separate from the oxygen.

Temperature alone will account for different readings directly after topping off or partial filling of different gases. Just as you can see CO2 “smoke” in differing stratus, the same will hold true in a tank. After the temps normalize or equalize the mix will not separate. Agitating, spinning or rolling it on the floor only servers to normalize the internal temperatures and it alone accounts for the changes in readings of your sensors.