EANx Shaken or Stirred?

[lamont]

i don't understand where the oil/water analogy comes from physically. these are gases, they've got an average velocity and mean free path which are going to govern how they diffuse through a container and they should mix very quickly, even starting from a fully partitioned container with no mixing caused by the fill whip...

 

[miketsp]

Ah, yes, but the question is "how quickly?"
My experience is that I need to wait a couple of hours after finishing a partial pressure blend to get a good final reading. Some folks say rolling the tanks around will speed the process; I don't see why it wouldn't (the close density makes rolling the tanks more effective than the oil/water example previously cited).
That said, since a two hour wait is convenient for me, I haven't explored the half-hour to just under two hour window to see if I could reasonably wait a shorter time - if anyone has, let us know. But I do know that in my experience, readings within a few minutes of a PP fill are unreliable.
Rick

At 200bar there is a significant deviation from the ideal gas laws so the PPs will change according to the temperature if you have a hot cylinder.
By moving the cylinders around you're not mixing the gases (remember Dalton) but you will accelerate the cooling process.
However you'll get faster results just by blowing them with a fan. Less tiring.

 

[Rick Murchison]

I should mention that an acquaintance of mine swears by the "final slam" method. He fills very slowly until the last 50 psi or so, then slams that in. Minimal heating and a reliable mix analysis right away. As I'm accustomed to waiting anyway I can't recommend this one way or another.
Rick

 

[Diesel298]

heres my throry.
as we all know pressure and temp. are directly related.
PP blending meand varying pressures of different gas.... so when they are introduced into the cylinder. they need to mix..
but i think that would happen rather quickly...
i have notioced a small wate will get you a slightley different reading....

 

[MXGratefulDiver]

Easy enough to find out for yourself ... measure your EAN mix right after filling ... measure it again a few hours later ... note the difference.

Come back and let us know what you get for results ...

... Bob (Grateful Diver)

 

[jonnythan]

Bob, how about you let those of us without analyzers know how this WOULD go?

 

[DA Aquamaster]

My thought has always been that brownian motion of the particles involved should result in the gases uniformly mixing even if they were introduced into the tank with minimal turbulence. (Brownian motion is what is demonstrated with the drop of food coloring in a jar of water experiment.)

Howver on a trip last fall I analyzed my deco gas (50%) when picking up the tanks and noted the O2 content was much lower than predicted. The person at the shop then proceded to roll my deco bottles around on the floor for a couple minutes and they then analzyed at 52%. They also analyzed at 52% again the following morning.

So, rolling the tanks seems to work even if the need for it is counter intuitive. In this case both the 02 and air used to achieve the mix were banked and the shop claimed (nd appeared) to be religeous about using very slow fill rates so I am not sure where temperature differences in the gases would have come into play. But either way, based on this experience it would appear that rolling accellerates the process of thoroughly mixing the contents of the tank and/or equalizing the temp of the gas.

With regard to Bob's suggestion to measure EAN mixes right after mixing and several hours later, when I mix my own (banked air on top of transfilled O2.) deco gases I measure them right after the fill is complete to confirm/double check the math and normally check them again just before the dive. I never see any differences in excess of possible error of measurement of the sensor itself (less than 1%).

 

[Diesel298]

so what about gas thyats banked. like a premixed 32?
there not rolling the banks

 

[Mike Nebel]

If you partial pressure blend at room temperature (say 70 degrees), and analyze your gas immediately and again after 24 hours, you will see a difference (not consistantly the same from tank to tank) in the O2 percentage. The rolling of tanks to blend the gases accelerates the mixing of gas particles. If you just wait 24 hours without rolling tanks, you should get a reliable analysis, as the gases will blend through molecular movement.

 

[jonnythan]

Sounds to me like a heck of an argument to own your own analyzer.