Old deco mix

[abowie]

Both my stage cylinders have been full with EAN60 since December 2003 when they didn't get used after a dive was called off. I plan to use them in 3 weeks.

Reanalysing them today both have dropped a bit of pressure (about 20 bar but it's probably 15*C colder today too) but one has also decreased the O2 concentration from 60.4% to 58.2% (same analyser). The other one is EXACTLY the same.

Is there a GOOD reason why I still shouldn't use the one where the O2 has dropped? I can't see why O2 should preferentially leak out of a cylinder which suggests that oxidation of SOMETHING may be the cause.

 

[wedivebc]

Both my stage cylinders have been full with EAN60 since December 2003 when they didn't get used after a dive was called off. I plan to use them in 3 weeks.

Reanalysing them today both have dropped a bit of pressure (about 20 bar but it's probably 15*C colder today too) but one has also decreased the O2 concentration from 60.4% to 58.2% (same analyser). The other one is EXACTLY the same.

Is there a GOOD reason why I still shouldn't use the one where the O2 has dropped? I can't see why O2 should preferentially leak out of a cylinder which suggests that oxidation of SOMETHING may be the cause.

Steel cylinders? Metal uses up O2 when it corrodes (oxydizes)
I would dump it and refill. I know a guy who got seriously nauseated after breathing 2 year old air off a steel 72.
He won't do that again.

cheers,

 

[ninjamuzo]

I'd get the gas changed. You also would be very close to needing a tank inspection and probably a pressure test.

 

[icediver]

Shaking the cylinder and reanalysing may get you a different result. I have seen where o2 has settled in a tank.

 

[wedivebc]

Shaking the cylinder and reanalysing may get you a different result. I have seen where o2 has settled in a tank.

Really? Is that why there are tornadoes, so the O2 gets mixed up in our atmosphere? :06:
I don't think there is any scientific support for that theory.

cheers,

 

[miketsp]

Shaking the cylinder and reanalysing may get you a different result. I have seen where o2 has settled in a tank.

Given Dalton's law of partial pressures I find it hard to imagine O2 settling at 120bar (60% x 200bar pp)!
Over 2% loss of O2 is a awful lot of O2. Which means serious oxidation.
Let's see if I remember how to do the calculation.
I assumed 7litres at 200bar since you didn't specify.
So 200 x 60.4 =120.8bar before
200 x 58.2 = 116.4bar after.
Loss of 4.4bar O2pp.
n=PV/RT
So n= (4.4 x 7)/(0.083145 x 293) = 1.264289 mols
assuming 20C = 293K
Then at 32g / mol we have lost over 40g of O2. (= 1.43oz)
If your measurement is right that's a lot of corrosion.

Get your cylinder checked. :11: