Nitrox PPO2 blending waiting period for homogenisation of mix

[cool_hardware52]

Anyway another interesting factor for your "fudge" the gas in the core of the cylinder may not be the same temperature as the cylinder it self, it maybe much hotter. I have done tests where the pressure drop of the cylinder is much greater than the temperature of the cylinder it self predicted.

Given that it's virtually impossible to measure the actual gas temp as you note, experience and "fudge factors" have served me pretty well.

(and a booster to fix any "misses" )

Tobin

 

[JBRES1]

OK , getting back on track.
After reading the 6 pages of this thread, have we come up with an answer to the mix question ?
How long does it take before the mix is stable ?

All I know is that when I fill my doubles I start off with the isolator open during o2 filling and closed during top off with air.(I fill the air off of the left and right post ) This works for me, the same % of o2 is in each cylinder, and close to the same amount of air. I let each cylinder sit before I re-open the isolator valve.

Jim Breslin

 

[Anti-Hero]

Interesting ... I've never had my isolator closed during a EAN fill. My mix has always been withing a couple tenths of a percent from the initial reading.

 

[cool_hardware52]

OK , getting back on track.
After reading the 6 pages of this thread, have we come up with an answer to the mix question ?
How long does it take before the mix is stable ?

It's not possible to supply one simple answer, i.e. "wait one hour" or "overnight" or "10 minutes"

It's largely dependent on the filling conditions. If the air top is introduced at a high rate, i.e. fast fill, the mixing is almost immediate. This of course requires banked air at a pressure considerably higher than the final fill pressure.

If the air top is slow, as is typical for small home compressors, mixing can take quite a while.

Tobin

 

[Rick Murchison]

... the isolator ... closed during top off...

Why in the world would you do that? Once filled with a closed isolator, they won't mix at all between tanks, you know... If you want to assure the same mix in both tanks, you need to leave that isolator open during all stages of the fill.
Or, looking at it another way, why is the isolator there? It ain't for filling the tanks.
Rick

 

[DA Aquamaster]

I'm a chemist by training, and I have a difficult time believing that gas in a scuba cylinder is not homogenized by the filling process.

However, I've had a lot of fill station operators tell me that there are real-world significant differences in gas analysis that is performed immediately after filling and gas analysis that is performed later. I doubt that "rolling" cylinders make any difference, the gas has got to equilibrate on its own. I am unable to explain the differences in gas analysis.

Back when I took deco procedures and Advanced nitrox, we flew in rented 40's for the deco gas and had them PP blended for 50%. The shop filled several of them for the class using PP blending and very slow fill rates while we attended the morning's lecture. After lunch most of the tanks were significantly off. After rolling them on the floor for a couple minutes they all analyized at almost exactly 50%. If I had not seen it, I would not have believed it my self.

In doing my own PP blending, I developed the habit of blowing the last 100 psi in at closer to a normal rate maybe 400-500 psi per minute. In my experience, I never blew myself up and the mixes always immeidately analyzed accurately with no rolling or wiating required.

...and no two gauges are alike, so if someone brings in a tank with O2 (or O2+HE) to top, who knows what the gauge differences will be.

I've done it so many time at the shop I use (they have me top all mine and my buddies tanks) that I do know. But it's taken quite a few fills to learn it.

My experience has been the same. For a given mix in a given situation overtime you learn exactly how much to over or under shoot the ideal O2 and other gas pressures based on tempertures, fill rates, where you started and how much if any you boosted and the need to make any correction becomes much less common.

The other variable here is the 2 sensor as most are at best only accurate +/- .5% so any attempt to get closer than that may make you feel good, but it doesn't really mean anything.

 

[Rick Murchison]

In doing my own PP blending, I developed the habit of blowing the last 100 psi in at closer to a normal rate maybe 400-500 psi per minute.

Another reason to make the top gas "other than oxygen" - don't want any adiabats flapping around in a valve full of oxygen, you know
Rick

 

[Peter Guy]

Just late to the party and was reading about the fills being +/- .1% or whatever. Which leads me to the question, other than because you might have Obsessive/Compulsive Disorder, why does it REALLY matter? How close are you really diving to the MOD on your backgas?

 

[Rick Inman]

Just late to the party and was reading about the fills being +/- .1% or whatever. Which leads me to the question, other than because you might have Obsessive/Compulsive Disorder, why does it REALLY matter? How close are you really diving to the MOD on your backgas?

See my quote at the bottom that I stole from Rick that he stole from someone else? I think it applies to mixing, too. It's fun to get as close as you can and then just go diving!

 

[fppf]

Just late to the party and was reading about the fills being +/- .1% or whatever. Which leads me to the question, other than because you might have Obsessive/Compulsive Disorder, why does it REALLY matter? How close are you really diving to the MOD on your backgas?

Yeah, a bit of OCD. But if you can, why not right?