Nitrox PPO2 blending waiting period for homogenisation of mix

[Doc Harry]

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.

 

[RJP]

I am unable to explain the differences in gas analysis.

I think Rick may have explained it in the post right above yours:

Particularly with doubles, at 200+ ATA there is plenty of the "last gas in" left in the manifold and dip tubes to skew an initial analysis, and this can last a long time.

Try releasing about "one full breath" before the analysis and I think you'll find this problem goes away.

 

[RJP]

I have to say that I have analyzed tanks right after an O2 topoff...

By the way, just as a basic procedure, I put any oxygen required in first. If I need to do any topping it's gonna be with 'anything but oxygen.'

Have to agree with Rick. I wouldn't be really comfortable working with 02 above 3000psi, and even then only into a tank that is designated for 100% 02 use.

If I wanted to "top off" a tank to a richer mix than what was in it (for instance a tank filled with 21% that I now want to have filled with 32%) mathematically I could drain them down to ~3000 and then top off with 100% to give me 3442psi of 32%

However, in practice I would drain them down to ~2000, then put in 450psi of 02 to bring the tank to 2450 and then top with air to 3442psi to get a target mix of 32%. This way I'm working with 02 nicely below 3000psi.

I suppose if you were dealing with LP tanks this might be a bit different, in that you could boost 02 without going over 3000...but I'd still take the same approach I outlined above. "Air" is free, so I'd happily bleed a little more of it for a margin of safety.

 

[JBRES1]

I start filling the cylinder with 100% o2 then top off with air. Sometimes I start from 0 psi, other times I start with the last mix at about 350 to 500 psi.
One reason I start with the 100% o2 first is that most gas suppliers will give you a cylinder that is a 2250 or 2400 rated cylinder. The other is I don't care to be working with high pressure o2 even if I could get it.

Jim Breslin

 

[Sabado]

Gases don't stratify...they diffuse and equalize very quickly. The internal turbulence that filling a cylinder creates is enough to have an adequate mixture. I have seen people shake the cylinder up after filling. I think it is like kissing your sister...maybe fun for a bit, but it doesn't do you any good!

I once heard someone say that he uses Nitrox because the extra O2 helps his SAC rate...these people have gross conceptual errors...can't blame them much, though. Someone made them believe that and they did no research for themselves.

happy turkey day.

 

[Rick Murchison]

Here is a thread from a few years ago y'all might find interesting from a gas mixing perspective...
Rick

 

[rstofer]

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.

Doesn't Graham's Law of Diffusion come into play here? There are many Google hits but I didn't tumble to one that had specific numbers. I don't know if we're dealing with microseconds or fortnights.

Richard

 

[iztok]

I suppose if you were dealing with LP tanks this might be a bit different, in that you could boost 02 without going over 3000...but I'd still take the same approach I outlined above. "Air" is free, so I'd happily bleed a little more of it for a margin of safety.

LDS I usually get my EAN from requires me to bleed tank completely empty before they will do PP blend. They start with O2 then add air.

 

[fdog]

LDS I usually get my EAN from requires me to bleed tank completely empty before they will do PP blend. They start with O2 then add air.

This is just plain lazy.


All the best, James

 

[cool_hardware52]

LDS I usually get my EAN from requires me to bleed tank completely empty before they will do PP blend. They start with O2 then add air.

This is just plain lazy.


All the best, James

They may lack a booster. If they are cascading O2 starting with an empty scuba tank makes some sense.

Dumping leftover nitrox is cheap, dumping left over Trimix is another matter.

Tobin