Does air go bad?

[bleeb]

I don't feel like doing the exact calculation, but I can assure you that this would require a very large percentage of the tank (possibly all) to be oxidized.

Let's see...

LP85 (approx 34 lb empty)
85 cu.ft.
= 2407 liters
= 107 moles of air
= 23 moles of O2
= 1.3 kg Fe (assuming FeO2)
= 2.8 lb Fe

Seems like quite a bit, but not impossible, particularly given a salty wet fill and a warm year or three which could catalyze enough rust for someone to pass out.

 

[spt29970]

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 the tank (possibly all) 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.

The reason there is no link to the "actual data" is that this is a reference to an journal article published in the era before the web. People actually did research and published on something called "paper." Just because it isn't on line doesn't mean that it doesn't exist!

As far as the chemistry is concerned, the calculation can be done on the back of an envelope. A typical tanks holds about 5lbs of air. The molar mass of O2 (16) is nearly that of N2 (14) and since air is 21% O2, there is 5lb * (16/14) * .21 = 1lb of O2 in the tank. Each O2 molecule binds 1 iron since the corrosion is

O2 + Fe -> FeO2.

The molar mass of Iron is 26, which implies that each lb of O2 binds 26/16 = 1.625 lb of Fe. However, you don't have to react all of the O2 in the tank before it becomes dangerous. Lets say anything below 10% O2 is fatal. Then you only need to react 0.5lb of O2 (probably less). This means you only need to oxidize around 1.6* 0.5 = 0.8lb of the iron in the tank. This is a lot, but hardly a "very large percentage" of a 30lb tank!

 

[jefffalcone]

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

Could you please provide a reference to a dive accident involving tank corrosion leading to a hypoxic mix?

I did a search and I couldn't come up with anything. I believe that it is hypothetically possible, but has it ever actually happened?

 

[jefffalcone]

Let's see...

LP85 (approx 34 lb empty)
85 cu.ft.
= 2407 liters
= 107 moles of air
= 23 moles of O2
= 1.3 kg Fe (assuming FeO2)
= 2.8 lb Fe

Seems like quite a bit, but not impossible, particularly given a salty wet fill and a warm year or three which could catalyze enough rust for someone to pass out.

2.8 lb Fe would be enough to oxidize all of the O2, and I don't believe this is possible. The oxidation reaction can only occur on the exposed surface. I don't think there is anywher close to this much Fe on the surface. Anyway hypoxia can occur with significantly less reaction. I would imagine 10% O2 could easily cause loss of consciousness. That would only take 1.4 lbs Fe. Still I would feel safe diving the 3 year old air.

 

[jefffalcone]

The reason there is no link to the "actual data" is that this is a reference to an journal article published in the era before the web. People actually did research and published on something called "paper." Just because it isn't on line doesn't mean that it doesn't exist!

As far as the chemistry is concerned, the calculation can be done on the back of an envelope. A typical tanks holds about 5lbs of air. The molar mass of O2 (16) is nearly that of N2 (14) and since air is 21% O2, there is 5lb * (16/14) * .21 = 1lb of O2 in the tank. Each O2 molecule binds 1 iron since the corrosion is

O2 + Fe -> FeO2.

The molar mass of Iron is 26, which implies that each lb of O2 binds 26/16 = 1.625 lb of Fe. However, you don't have to react all of the O2 in the tank before it becomes dangerous. Lets say anything below 10% O2 is fatal. Then you only need to react 0.5lb of O2 (probably less). This means you only need to oxidize around 1.6* 0.5 = 0.8lb of the iron in the tank. This is a lot, but hardly a "very large percentage" of a 30lb tank!

You missed my edit. I said a very large percentage of the surface area. Oxidation can only occur on the surface.

Also I believe you have that reaction wrong. I looked it up twice. It is:

O2 + Fe -> 2Fe0, so the weight of iron is actually twice what you are saying.

Not to mention that the steel tank is not made from pure iron. Anyway we're just splitting hairs at this point. Do you honestly believe that over a pound of rust can accumulate on the inside of a tank?

 

[spt29970]

Let me make one correction to my previous post. Rust is a mixture of many oxides of iron. The most common is Fe2O3, which means that the molar binding ratio is 2*26:3*8 = 52:24 = 2.2. So to consume 0.5lb of O2, you only need about 1lb of Fe.

I don't know where you got the idea that corrosion is self sealing. Rust is very porous. It certainly doesn't make an effective barrier to oxygen. Yes, we do seem to be posting past each other, but to answer your question, yes I believe that you can corrode a pound of iron if a tank which has moisture in it sits for several years. I have seen old pieces of metal corrode away entirely. Besides, if the tank is under pressure it will accelerate the corrosion.

Let me summarize where we agree and where we disagree.

1. We agree that it doesn't take most or all of the tank to corrode before it becomes unsafe.
2. We agree that clean tank will likely have no problem for long periods of storage.
3. We agree that for a tank to become unsafe, it would have to be misused pretty badly.

Where we seem to disagree is that for a $5 air fill, I would toss air that is over a year old, whereas you would take your chances. Why don't we leave it as we agree to disagree. Besides, the next time I am at a university library I will try to look up the article in the "JOURNAL OF FORENSIC SCIENCES" which appears to describe an actual fatality.

 

[jefffalcone]

The reason there is no link to the "actual data" is that this is a reference to an journal article published in the era before the web. People actually did research and published on something called "paper." Just because it isn't on line doesn't mean that it doesn't exist!

quote]

I did not mean to imply that it doesn't exist. All I was saying is that the link you provided wasn't evidence of anything. It is hardly more than a headline. Neither of us has any idea what is in the actual report.

The link you provide, does however, only claim that oxidation can reduce the oxygen content by 2 to 3 percent, not the 11% that you have stated could happen.

 

[jefffalcone]

Let me make one correction to my previous post. Rust is a mixture of many oxides of iron. The most common is Fe2O3, which means that the molar binding ratio is 2*26:3*8 = 52:24 = 2.2. So to consume 0.5lb of O2, you only need about 1lb of Fe.

I don't know where you got the idea that corrosion is self sealing. Rust is very porous. It certainly doesn't make an effective barrier to oxygen. Yes, we do seem to be posting past each other. But to answer your question, yes I believe that you can corrode a pound of iron if a tank which has moisture in it sits for several years. I have seen old pieces of metal corrode away entirely. Besides, if the tank is under pressure it will accelerate the corrosion.

Ok, let's just say I'm way off base. I know that oxidation requires contact between the molecules. I assumed that this would only happen on the inside surface, but I am no expert on metals, and you seem quite knowlegable. I dropped out during my juniour year and I havn't thought about chemistry since. We can both agree that you would need in the neighborhood of 1 pound of rust to be in danger. Do you really think that this much rust is in those tanks?

 

[jefffalcone]

Let me make one correction to my previous post. Rust is a mixture of many oxides of iron. The most common is Fe2O3, which means that the molar binding ratio is 2*26:3*8 = 52:24 = 2.2. So to consume 0.5lb of O2, you only need about 1lb of Fe.

I don't know where you got the idea that corrosion is self sealing. Rust is very porous. It certainly doesn't make an effective barrier to oxygen. Yes, we do seem to be posting past each other. But to answer your question, yes I believe that you can corrode a pound of iron if a tank which has moisture in it sits for several years. I have seen old pieces of metal corrode away entirely. Besides, if the tank is under pressure it will accelerate the corrosion.

Let me summarize where we agree and where we disagree.

1. We agree that it doesn't take most or all of the tank to corrode before it becomes unsafe.
2. We agree that clean tank will likely have no problem for long periods of storage.
3. We agree that for a tank to become unsafe, it would have to be misused pretty badly.

Where we seem to disagree is that for a 5$ air fill, I would toss air that is over a year old, whereas you would take your chances. Why don't we leave it as we agree to disagree. Besides, the next time I am at a university library I will try to look up the article in the "JOURNAL OF FORENSIC SCIENCES" which appears to describe an actual fatality.

Honestly I'm just enjoying the debate. I firmly believe that it is safe to breathe this air, but I also agree with you that $5 for piece of mind ain't that expensive.

I'd love to see the article if you do find it. I always enjoy learning something, and I don't mind being proven wrong.

Thanks for entertaining me while I battle insomnia.

 

[spt29970]

The original poster asked if it was okay to dive old fills because he wanted to save $5 each. I agree that it is very unlikely that there is anything wrong with the gas, but it isn't guaranteed. The simplest, and most prudent course of action is to have them visually inspected, which is standard practice in the industry.