Old air

[bjpell]

I have a few lp120's with 40% in them. The air is about 2-2.5 years old. I don't remember for sure, but isn't it for some reason bad to breath old air? I don't remember why though, or how long it was good for? I cant think of a reason why the air would be bad unless something was growing in the tank like bacteria etc. Anyone know what I'm talking about or remember something about this form the fundamentals class.

 

[Scared Silly]

If the gas in them is 2+ years old then that means they have not had an inspection in at least that long of time period. Probably time to drain them and have someone take a peak inside.

Gas should not go bad but if there was water in there you could have lots of corrosion. Again probably time to drain them and have someone take a peak inside.

I just drained two old lp72s that had been sitting for 30 years. They looked great inside.

 

[snowflake4727]

If there was any moisture in them the high PPO2 could accelerate corrosion, this would also lower the O2 content.

 

[DA Aquamaster]

If the gas in them is 2+ years old then that means they have not had an inspection in at least that long of time period. Probably time to drain them and have someone take a peak inside.

Gas should not go bad but if there was water in there you could have lots of corrosion. Again probably time to drain them and have someone take a peak inside.

I just drained two old lp72s that had been sitting for 30 years. They looked great inside.

I agre with you that 2 years is long enough and a VIP is a good idea and cheap insurance. But if the air analyzes ok and smells fine, diving them is a great way to "drain" the tanls for the VIP.

 

[Doc Harry]

The University of Rhode Island did some studies back in the 1970s regarding corrosion and both aluminum and steel cylinders. (Cichy, Francis, Hilbert Schenk, and John J. McAniff. Corrosion of Steel and Aluminum Scuba Tanks, University of Rhode Island Technical Report 62, 1978)

To get to the point -- in one study, steel cylinders were filled with 500 ml of salt water and incubated at 100 degrees F for 90 days. The amount of water was probably not realistic but the temperature and duration were realistic - like putting the cylinders in a shed over the summer in Florida or Texas. My guess is that the researchers were trying to accelerate the pace of corrosion to shorten the study duration.

Anyway, the steel cylinder suffered such bad corrosion that there were sheets of corrosion hanging from the internal walls. More concerning was the fact that wall thickness had been reduced to only 1/3 of the original wall thickness. The cylinders were truly in danger of exploding. There were also concerns that corrosion products would plug the cylinder valve.

The gas was analyzed. Oxygen was significantly reduced (down to 15.0%), carbon monoxide was elevated (10 ppm). Carbon dioxide was normal (0.01%).

In 1974 there was one documented case of a death that was caused by breathing a corrosion-induced hypoxic mixture. (Schench, Hilbert V., and McAniff, John J. United States Underwater Fatality Statistics-1974. NOAA Report URI-SSR-75-10)

In this case, the diver took a steel tank to a depth of 12 feet to search for an outboard motor. The victim had last used this tank three months previously without any problems and it only had 300 psig remaining. Five minutes into the dive his bubbles were noted to cease and his body was later recovered.

Analysis of this accident revealed severe corrosion of the tank with large amounts of rust. The oxygen content of the gas was measured to be only 2% to 3%.

The steel tank in this accident had neither a current hydro nor a current visual inspection.

So.... based on these old studies and other similar studies ("best evidence"), the industry recommends that a steel cylinder that is stored for more than 90 days should have the gas analyzed for oxygen and carbon monoxide content, or better yet the gas should be dumped and the cylinder refilled with fresh gas.

The industry also recommends storing steel cylinders at a low pressure to minimize pO2 (since corrosion was demonstrated to be proportional to pO2).

For a complete discussion of all of these studies, see:
Cylinder Storage: Fact and Fiction

 

[rstofer]

What isn't clear to me is the manner in which moisture enters the tank. In the referenced link, a test is described where a tank with regulator was submerged to a few feet and the regulator purge button pressed several times. What isn't described is whether there was pressure in the tank. This missing fact seems a little critical to the conclusions. If the tank was empty, of course the pressure at depth would push water into the tank. But we never let our tanks get empty so what's the point? Now, if the experiment was conducted with 500 psi in the tank, the conclusions would be earth shattering. Hm...

Is the implication that a 3442 psi tank, fully charged, will gain moisture just because a purge button is pressed?

Or, is it more likely that the moisture gets in the tank from poor compressor system design? In the semiconductor business, we routinely made clean dry air with a dewpoint of -100 deg F. In other words, really, really dry. I have no idea what the dewpoint of scuba air might be. An interesting topic for research.

From the referenced link, it is impossible to tell whether the steel tanks had internal galvanizing. There is certaiinly anecdotal evidence that LP steel tanks can be serviceable over a very long period of time. There is probably a little less history on HP tanks but I'm not sure that any of the cited research applies to a modern HP tank with internal galvanizing. I'm not sure it doesn't, either...

As to storing air in a steel tank for more than 3 months? This needs more research. I find it staggering that a modern steel tank can corrode to the extent that the ppO2 is diminished beyond life support while sitting in my garage for such a short period of time. There just has to be anecdotal evidence that tanks can be stored for years and years without such an occurrence.

I would like to read more about the incident where a diver died of hypoxia while diving a tank that was used 3 months prior. Unfortunately, NTIS wants $48 to print on demand United States Underwater Fatality Statistics - 1973. Ten years between inspections isn't uncommon in the used cylinder market.

I'm in the process of buying a used LP 72 that hasn't been hydroed in 10 years so the air is at least 5 years old. The seller suggested diving the air before sending the tank for hydro, VIP and oxygen cleaning (yes, I plan to use if for Nitrox). I decided that the value of an air fill was insignificant so I sent it out without diving it. Probably a good choice.

I really need to buy an O2 analyzer because it is a given that my tanks are stored full and most of them have EAN36. They will almost certainly be stored longer than 3 months at a time. I had an Al 80 stored full for 10 years without incident.

I should probably do my own visual inspection on the tanks before I send them in. All I get back is a binary pass/fail without the analog of 'Man there was a ton of rust, it took forever to tumble it clean'. I guess I would like to have more information and I should probably do it myself. I'm the one diving the tanks.

Richard

 

[Fishpie]

Doc H
....and the Aluminium tanks????

 

[snowflake4727]

If the tank is filled in a wet tank and water is in the end of the fill whip it could get forced into the tank. While this amount is minimal, it is still minimal. Think of all the salty tanks put in a water bath to fill, salt is in the water.

 

[Graeme Tolton]

The gas was analyzed. Oxygen was significantly reduced (down to 15.0%), carbon monoxide was elevated (10 ppm). Carbon dioxide was normal (0.01%).


Can anyone tell me what rust formation has to do with carbon monoxide??

 

[DA Aquamaster]

Rstofer is correct. The only way for water to get into a tank that is pressurized above ambient pressure is through water droplets in the valve or fill whip during a fill (always blow the gas out of both the valve and the whip prior to connecting them to ensure they are dry with no water droplets inside) or through a compresor with an in operative or innefficent moisture separator and filter system.

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In the gas analysed above, I'd bet my next paycheck that both the moisture in the tank (the stuff that caused the rust) and the CO came from a faulty compressor/bad fill.

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Elvated PO2 would provide more O2 for oxidation in a steel tank and PO2 depends on both pressure and FO2 so it sounds great to say steel tanks shoudl be stored at low pressure. But in the real world, a quality fill is very dry and if you have no moisture, you will have no oxidation. There are after all steel o2 clyinders that have been in constant use since before WWI. I have seen one made in 1911 that not only passed hydro test, but qualified for a + rating.

So the suggestion that steel tanks be stored at low pressure is well intended but misguided. The truth is that you need to be very careful to get quality fills from a quality source with suitably dry gas using fill procedures that eliminate the potential to introduce water into the tank because it is a lack of moisture not a low PO2 that will prevent oxidation in the tank as whether it is air, nitrox or O2, O2 is always present.

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When preparing a steel tank for O2 service, I liek to tumble the tank to remove any flash rust and then use a rust inhibitor to prevent any rsut while the cleaned tank dries so that there is no rust in the tank. Rust tends to attract moisture and concentrates even a minimal amount in one area, so removing any flash rust prevents more rust as well as removing a potential fuel source.