Emptying a tank ...

[Sharky1948]

dewpoint RISES as the pressure increases

Yep, my bad in the post. (I'll correct it...again! I had it right the first time I posted! LOL) I was thinking, as pressure increase the likelihood of condensation increases, but had my head on backwards when I stated the dew point consideration. Thanks for the correction!

And, for the record, I'm still baffled by those who believe the ppO2 in a pressurized cylinder is .21 This whole thread is a prime example of why no one should rely on blogs, discussion forums, etc. as the sole input to important decisions.

 

[pescador775]

Tanker, digger and sharky are on the right side of it. Personally, I would be interested in comments from a "trusted" LDS. TIA

No problemo, sharky. I knew what you meant.

 

[Jeremy Bouwman]

y bother with the end of a dive season? get a warmer exposure suit and get out there!!!

 

[paulwlee]

You have done your last dive of the season and realized that the good Samaritans (elves) at your LDS have filled your tank.

I personally don't bother emptying the tanks. I've never had rust or oxidation problems with tanks left full for several months. Maybe the tanks will fail hydro in 20 years instead of 30, which makes no difference to me.

 

[jwalko]

Actually pressure has everything to do with ppO2. ppO2 = fO2 X ATA At one ATA, ppO2 = the fraction of O2 or .21. At 3000 psi, roughly 204 ATA, the ppO2 is 42.8 Assuming you've taken an EAN class, remember the ppO2 calcs and realize that (in theory) the regulator would deliver 3000psi at about 6699 fsw.

I.e., as you said, ppO2 goes up when gas is more dense, which is exactly what pressure is all about.

Ok...I think I see where my logic went wrong. I was thinking of the air you are breathing and the PPO of the air after it comes out of the tank. At sea level, it is .21, which we all agree on. I see how this is different than the air inside the tank, at the higher pressure (Partial Pressure of gas=fraction of that gas x total pressure).

Now I understand what you are saying.

John

 

[paulwlee]

Make sense??

No.

(because PPO is the pressure of a particular gas within the mixture relative to the total pressure). At 10 meters (2 ATA), the density (not the pressure) of the gas increases to twice what it was at the surface, so the O2 percentage is double on each breath (PPO2 of.42)...what am I missing here.

You are missing that PP is the pressure of a particular gas within the mixture period, not relative to the total pressure.
Also, you are confusing the pressure in the tank with the pressure of the gas once it is out of the tank through a regulator.

Example: A full 3000psi EAN50 tank for simplicity.
INSIDE the tank, total pressure is 3000psi(~200atm), PPO2 is 3000psi*0.5 = 1500psi ( ~100atm)
PPN2 is likewise 3000*0.5 = 1500 psi.

Now if you BREATH from it, the gas will come out and be the same as ambient pressure, then, at the surface PPO2 = 14.7 * 0.5 = 7.35psi (0.5atm)
PPN2 = 14.7 * 0.5 = 7.35psi (0.5atm)

In both cases, sum of partial pressure is the total pressure.
3000psi (~200atm), and 14.7psi(1atm) respectively.

 

[nadwidny]

If you bleed off a cylinder quickly you run a risk of condensation forming inside the cylinder as the pressure drops rapidly

This is nonsense as others have pointed out. I just dumped 4 cylinders of gas (2XAL80s, and 2XLP104s) for hydros. They were at 2000-2500. As per my usual practice I dumped each one in under 5 minutes (including breaks to rewarm my hand which was holding the towel/muffler on). Lots of frost on the outside, nothing on the inside.

 

[do it easy]

As per my usual practice I dumped each one in under 5 minutes (including breaks to rewarm my hand which was holding the towel/muffler on).

Another use for neoprene hoods- wrap the towel around the valve and put the hood over the whole thing. Then grab a beer!

 

[Packhorse]

Better still put a couple of beers under the towel and hood and you have a beer chiller!!


I have had this discussion on another forum and was quite suprised by a couple of posters who said they have drain half a cup of water from a fast drained tank!!
I sugested this was more the fault of a dodgy comperssor.

This was my OP on the subjects

Myth or fact?

I have heard this one a few times before and never really belived it.
Anyway the topic came up on another forum and I did a bit of research.
A dive compressor should remove most moisture from the air. Infact the standard says it should have a dew point of -54deg C. In other words the air must be -54 deg before condensation forms.
At higher pressure the dew point will be higher. It works out like this

200bar -3deg
100bar -11deg
1bar -54deg

So as long as ice does not form while there is over 200 bar in the tank (assuming the air is dry enough (if its not your dive shop has a problem with their compressor)) then no condensation will form inside.
Below 200 bar Ice can form without internal condensation forming. I very much doubt the tank will reach -11 C especially if drained while in a water bucket (like many shops use to fill tanks).

This is not to say draining a tank fast as is a good idea. The noise alone can do damage and the extream cold may damage the O rings.
But I very much doubt condensation will form inside.

Anyone care to do a myth busters test and open up a tank after drain it fast??

 

[nadwidny]

Better still put a couple of beers under the towel and hood and you have a beer chiller!!

Someone give this man a medal.

[Comic Book Guy]Best Suggestion Ever.[/Comic Book Guy]