Just... 50... More... Bar... (SCUBA)

[Oz]

Wouldn't have thought of this. And I'm thinking I shouldn't post it because I know a group of you will pipe up and talk about seals, etc. It's been a non-issue the times I've tried it and the guy that talked to me about it has done it much more than me again without any seal issues. So, use it at your own risk and don't come crying to me if your house blows up and your dog dies.

If your SCUBA tank is getting a little low and you find yourself in need of a few extra bar in your cylinder, freeze it.

Fill it up with what you have, throw the cylinder in the freezer for an hour. Fill it up again. The needle should drop significantly after chilled. Freeze it one more time and fill it up again.

Once it returns to room temperature, it appears to be good for about 30-50 more bar than your SCUBA tank could do in one shot. YMMV.

Let the safety zealots flame away about freezing polyurethane based o-rings! Oh wait... polyurethane doesn't freeze (change properties) until you get close to -200 or so... It will be interesting to see where peoples safety concerns are on this one.

Oz

[Mike M.]

PV=RT

Reduce temperature, pressure goes down.

[Oz]

I was thinking of the other angle the safety police might talk about... making the metal more brittle? I dunno. 0-10 degrees F shouldn't do much to aluminum/aluminium structure.

If you were going to try this with liquid nitrogen, no doubt you might be changing aluminum/aluminium's crystalline structure and freezing the polyurethane o-rings.

I would think it a good idea to _NOT_ submerge your air cylinders in liquid nitrogen before you fill them.

Oz

[Oz]

PV=RT

Reduce temperature, pressure goes down.

A law of physics that I never thought of applying to shooting until recently!

[RobinC]

As you will be shooting on a firing point at least a continent away from me I have no objection!
You did'nt work at NASA 20 years ago did you!!
Best regards
Robin

[Quest1]

I might have to worry about moisture collecting in the cylinder when you freeze the cylinder. There might also be a problem with the seals drying out prematurely by freezing the cylinder.

[Anschutz]

why not just go and get the scuba tank refilled, seems a lot less hassle !. Colin

[Steve Swartz as Guest]

Two minor nits to pick:

1. PV=nRT (don't forget the gas constant!)
2. It isn't the cold temperature itself that damages the o-rings. Its the contaminants (like water) and surrounding materials (like metal) that damage the o-ring when they expand/contract that damage the o-rings

Typically, the operation of equipment outside its design envelope is not recommended; particularly under field conditions. Dramatic failures can occur limited only by the amount of energy inherent in the system when it fails. 80 cubic feet at 3200 psi contains a tremendous amount of energy.

Steve Swartz

[Mike M. as guest]

I certainly would not recommend putting a tank into a cryostat. But you could reasonably put it in the refrigerator, or even freezer. Take it down to -5C or so (about 25 degrees F).

[RobinC]

Why not just get the scuba tank refilled!!!!!!!!!!!!!!!!!!!

[Guest]

And why not let him do it that way? Doesn't matter to you, does it? Personally, I think it's an interesting idea, even if only from a viewpoint of applied physics. And you never know when you have 130 bar left in the bottle and need air for a 60+ match, could come in handy.

[Oz]

Yeah, the thought behind this was really to help get by in a pinch. For a lot of people, getting to a SCUBA fill location is not a trivial proposition. This would be used to "get by" for an extra week or two and still provide 70-80 shots in a cylinder.

For me, the dive shop is 5 minutes away and costs $5. So it was obviously just a curiosity; But I was very surprised to see how effective it is and wanted share.

Oz

[Bob-Riegl]

To Mike----FYI----that's PV=nRT for the Ideal gas equation according to Van derWaals. What did they know back in the 1700's? Chem Prof a/k/a "Doc"

[robf]

the reason cylinders are checked is because the moisture in them condenses and vapourises, causing microscopic pitting and weak points... and you want to add freezing and thermal expansion into the mix as well?... how fast does it heat up to beyond room temp from frozen once filled?

one reason airgun cylinders get an easier life than their sub surface brothers is that they rarely drop to a low pressure and get filled from this, they tend to always have a residual pressure in them before filling.

airplanes that do short haul and get pressured/depressured more than long haul flights of the same mileage can also show more pressure stress wear.

i'd just get the thing filled and give it an easier life...

doesn't seem the cleverest advice to me... but then it's your cylinder/money/life.