Does air go bad?

[EastcoastdiverLLC]

Hi All,

I know you're supposed to have a visual check on tanks each year.

My wife and I bought six new tanks three years ago. All are filled and they haven't been used in three years. Actually, two have never been used.

I'd like to use them now and hate to waste the fills. Does air go bad? Would it be okay to use the air and then get the visual check done on the tanks when I have them filled? Am I being penny wise and pound foolish?

Thanks.

Does/should the air go bad? Short answer, no.

Longer answer: If any moisture was introduced to your cylinders when they were initially filled, corrosion could begin to deplete the oxygen content of the gas within. Three years is a long time for the corrosion to work on your tanks (rust in steel or aluminum oxide in aluminum). I could go into details of dive accidents involving this senario, but thats not necessary.

Conclusion: Air is cheap. Get them VIP'd and refilled.

Gary

 

[vicdiver656]

As far as I know, air does not go bad, but if there is any contaminating material in the tank, even a small amount, it can taint the air.

 

[miketsp]

According to all major medical gas suppliers, the shelf life of their products is indefinite.
This assumes of course that they have quality control over the cleanliness of their cylinders.

The only thing you'd have to check with an air or nitrox mix is the O2 % as some corrosion may have ocurred if there was humidity in the mix.

Interestingly if it was a 100% O2 cylinder then it would still be 100%.

 

[divechilly]

Good advice! Thanks to all for the suggestions. These are steel tanks. I think that I'll have them inspected and refilled. As many of you point out, it's not worth the risk of diving with air that has possibly been contaminated.

Thanks.

 

[jefffalcone]

I guess I'm a risk taker, because I would dive the tanks w/o any hesitation. I'm sure they're fine. For the air to have become anoxic, you would need to have a heck of a lot of rust in there. I be willing to wager my life that this hasn't happened.

 

[Fireguy18]

I don't know the exact answer to the question of whether it goes bad or not, but I do know from previous experience that its gets pretty stinky! Our SCBA cylinders at our fire station have a 3 months stale date. Every cylinder gets purged and refilled if it was filled longer than 3 months ago. Someone is checking on the exact reasoning for this and I'll post when I get it.

 

[spt29970]

I guess I'm a risk taker, because I would dive the tanks w/o any hesitation. I'm sure they're fine. For the air to have become anoxic, you would need to have a heck of a lot of rust in there. I be willing to wager my life that this hasn't happened.

Willing to bet your life that this hasn't happened? I refer you to:

NCJRS Abstract - National Criminal Justice Reference Service

NCJ Number: NCJ 027183
Title: SCUBA (SELF-CONTAINED UNDERWATER BREATHING APPARATUS) TANK CORROSION AS A CAUSE OF DEATH
Journal: JOURNAL OF FORENSIC SCIENCES Volume:20 Issue:3 DatedJULY 1975) Pages:571-575
Author(s): J D TEMPLE ; R T BOSSHARDT ; J H DAVIS
Corporate Author: American Soc for Testing and Materials
Promotions Manager
United States
Publication Date: 1975
Pages: 5
Origin: United States
Language: English
Annotation: CORRODED SCUBA TANKS MAY RESULT IN A DEPLETION OF OXYGEN BELOW THE NORMAL 20 PERCENT AS LOW AS TWO OR THREE PERCENT, WHICH MAY RESULT IN DEATH.
Index Term(s): Fatalities ; Evidence identificatn and analysis/

To cite this abstract, use the following link:
NCJRS Abstract - National Criminal Justice Reference Service

 

[jefffalcone]

Willing to bet your life that this hasn't happened? I refer you to:

NCJRS Abstract - National Criminal Justice Reference Service

NCJ Number: NCJ 027183
Title: SCUBA (SELF-CONTAINED UNDERWATER BREATHING APPARATUS) TANK CORROSION AS A CAUSE OF DEATH
Journal: JOURNAL OF FORENSIC SCIENCES Volume:20 Issue:3 DatedJULY 1975) Pages:571-575
Author(s): J D TEMPLE ; R T BOSSHARDT ; J H DAVIS
Corporate Author: American Soc for Testing and Materials
Promotions Manager
United States
Publication Date: 1975
Pages: 5
Origin: United States
Language: English
Annotation: CORRODED SCUBA TANKS MAY RESULT IN A DEPLETION OF OXYGEN BELOW THE NORMAL 20 PERCENT AS LOW AS TWO OR THREE PERCENT, WHICH MAY RESULT IN DEATH.
Index Term(s): Fatalities ; Evidence identificatn and analysis/

To cite this abstract, use the following link:
NCJRS Abstract - National Criminal Justice Reference Service

Wow, ton of information there. There's no link to the actual data. Doesn't really say much without it. Might as well be saying driving is unsafe because worn brakepads my cause a fatal crash.

I was a chemistry major. I don't feel like doing the exact calculation, but I can assure you that this would require a very large percentage of surface of the tank to be oxidized. This is basically a hypothetical situation which could thoeretically occur, but I am quite sure has not. Yes I would bet my life on this. If this thread is still kicking tomorrow night when I get back from jury duty I will show you some calculations to prove my point.

 

[jefffalcone]

alright I can't sleep so here goes:

to reduce the oxygen content from 21% to 18% (Breathing 18% oxygen is still highly ulikely to cause death. mountain climbers routinely breath less than this at altitude w/o any ill effects) you would need to use about 15% of the total oxygen in the cylendar.

100 cubic feet = 2832 liters/22.4 moles per liter= about 19 moles *.21= about 4 moles O2.

.15*4moles=0.6 moles O2 need to react.

Oxidation of iron happens by the following reaction

2Fe + O2 = 2FeO

so 1.2 moles of Fe would need to react producing 1.2 moles of FeO


Rust (FeO) weighs 34 grams per mole, so you would need to produce 41 grams of rust.

Now for the rest of this we'll have to use our imagination a little. Have you ever scraped rust off of something. It is pretty light. Try to imagine what 41 grams of it would look like. I think this would be more than enough to coat the entire surface area of the inside of a tank. Once the surface area was completely oxidized, the reaction would no longer occur, since the molecules need to come in contact with each other in order to react.

Well, that's the best I can do right now. I know I haven't proven anything, since I've simply stated that 41 grams of rust would be more than enough to completely cover the surface area of the inside of a tank w/o providing any evidence. Anyone who has ever scraped rust off of anything can tell you that it is pretty light. You would need quite a bit to weigh 41 grams. Hey I'm convinced that at least it takes a heck of a lot of rust to reduce the oxygen content to 18%. Now I know that I have breathed 18% oxygen while carrying a 50 pound pack up a very steep hill and I didn't loose consciousness. I'd be very surprised if I passed out from floating lazily near the surface while breathing 18% O2. Once you get down to 30 FSW the partial pressure of O2 will double making 18% perfectly safe to breath.

Like I said, I'll take my chances that the 3 year old air has produced signigicantly less than 41 grams of rust and even if it has It probably won't kill me.

If none of this makes sens, forgive me. It's 1 am, I only slept 2 hours last night and I haven't done any chemistry in about 6 years.

 

[jefffalcone]

I suppose that the easier solution would be to just analize the mix in the tank. If it's below 18% I'll eat my hat.