not sure we've given any facts, lots of speculation, some based on a study 30 years old comparing apples to oranges.
how long air can be safe inside the tank
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captain
Contributor
Exactly, 30 years ago the predominant tank was the steel 72 and compressor filtration technology was not where it is today. Also the most likely scenario for water getting in tanks was from wet fill whips from filling tanks in a water tank. Not a common practice today.
If my tanks were sitting for three months unused, I would be checking the oxygen content of them, or just refilling them before use.
Oxidization uses oxygen to form rust, so just a little condensation could change the oxygen level over time.
Oxidization uses oxygen to form rust, so just a little condensation could change the oxygen level over time.
captain
Contributor
My former work place had steel and aluminum SCBA cylinders for emergency escape use. If not used in an emergency or leaked down they stayed full the entire 5 year hydro cycle.
In Australia, they would have been checked every year for hydro, mandatory.
and exterior visual inspection every month depending where they were. not mandatory but good OH & S practice.
and exterior visual inspection every month depending where they were. not mandatory but good OH & S practice.
Proper storage is debatable. There has been very little research, and what little research has been done is old not well designed. There has NOT been any definitive research at all. There probably never will be.
Nonetheless, industry experts have storage recommendations for "best practice" based on the old data. It is important to understand the underlying history and research so as to make up your own mind.
Aluminum Cylinders
Modern aluminum cylinders are pretty robust. As long as you keep up with annual visual inspections there really isn't much that can go wrong. In the old days there wasn't an annual visual inspection program and significant corrosion was a problem. The old 6351 aluminum cylinders were cracking, but most of those cylinders have been retired and many dive shops won't fill them anymore (even with a valid eddy current examination). No one has ever documented a case of air going bad in an aluminum cylinder, even with severe corrosion, so you can probably store air in aluminum indefintely.
The limiting factor here is cylinder inspection. Some studies have suggested that aluminum corrosion can weaken the cylinder wall, but most of these cylinders were very badly corroded and hadn't been inspected in many years. I believe that there is only one documented case of a properly-maintained 8081 aluminum cylinder exploding during the filling process, and that was from a manufacturing defect. All of the other aluminum cylinder explosions were the old 6351 alloy.
So after a year of storage, check the oxygen content of you gas (just to be sure) and then the cylinder will need to be visually inspected before it is filled again.
Steel Cylinders
Steel cylinders require more care. They are more prone to rusting and have very thin walls, so rust pits are more significant. Again, old studies have shown that rust can significantly weaken steel cylinders walls in a very short period of time. More of the studies were conducted with a relatively large volume of water inside of the cylinder, but we can nonetheless draw some important conclusions.
You never know how much water is inside of your cylinder, so you've always got to assume that there is some water from the filling process. Salt water is much more corrosive than fresh water. Since the sidewalls are so thin, even small pits can weaken or condemn the cylinder. Best to store steel upright so corrosion is focused on the thicker base than the thinner sidewalls. A year is a long time for even small amounts of water to corrode the sidewalls to a significant degree.
Unlike aluminum, the gas inside of a steel cylinder can change significantly with corrosion. Research and real-life events have proven this beyond doubt. Rust consumes oxygen and, due to the specific alloy, produces a small amount of carbon monoxide. The question is to what degree will this happen in your cylinder that has been sitting for a year.
Again, since you never know how much water is inside of your cylinder, so you've always got to assume that there is some water from the filling process. People always say that there can't be water in their cylinder, but there can be and often is water in the cylinder. The question is, how much?
The "best practice" is to check you breathing gas for oxygen content after a cylinder has been stored for a long period of time. If the oxygen content is stable, then it is clear that you do not have enough moisture and rust in the cylinder to lower oxygen content. But that says nothing about corrosion pits.
Again, "best practice" suggests that you have the cylinder visually inspected before it is refilled to check for corrosion pits.
Properly maintained and properly filled, you shouldn't have any problems with modern 3AL and 3AA scuba cylinders.
Nonetheless, industry experts have storage recommendations for "best practice" based on the old data. It is important to understand the underlying history and research so as to make up your own mind.
Aluminum Cylinders
Modern aluminum cylinders are pretty robust. As long as you keep up with annual visual inspections there really isn't much that can go wrong. In the old days there wasn't an annual visual inspection program and significant corrosion was a problem. The old 6351 aluminum cylinders were cracking, but most of those cylinders have been retired and many dive shops won't fill them anymore (even with a valid eddy current examination). No one has ever documented a case of air going bad in an aluminum cylinder, even with severe corrosion, so you can probably store air in aluminum indefintely.
The limiting factor here is cylinder inspection. Some studies have suggested that aluminum corrosion can weaken the cylinder wall, but most of these cylinders were very badly corroded and hadn't been inspected in many years. I believe that there is only one documented case of a properly-maintained 8081 aluminum cylinder exploding during the filling process, and that was from a manufacturing defect. All of the other aluminum cylinder explosions were the old 6351 alloy.
So after a year of storage, check the oxygen content of you gas (just to be sure) and then the cylinder will need to be visually inspected before it is filled again.
Steel Cylinders
Steel cylinders require more care. They are more prone to rusting and have very thin walls, so rust pits are more significant. Again, old studies have shown that rust can significantly weaken steel cylinders walls in a very short period of time. More of the studies were conducted with a relatively large volume of water inside of the cylinder, but we can nonetheless draw some important conclusions.
You never know how much water is inside of your cylinder, so you've always got to assume that there is some water from the filling process. Salt water is much more corrosive than fresh water. Since the sidewalls are so thin, even small pits can weaken or condemn the cylinder. Best to store steel upright so corrosion is focused on the thicker base than the thinner sidewalls. A year is a long time for even small amounts of water to corrode the sidewalls to a significant degree.
Unlike aluminum, the gas inside of a steel cylinder can change significantly with corrosion. Research and real-life events have proven this beyond doubt. Rust consumes oxygen and, due to the specific alloy, produces a small amount of carbon monoxide. The question is to what degree will this happen in your cylinder that has been sitting for a year.
Again, since you never know how much water is inside of your cylinder, so you've always got to assume that there is some water from the filling process. People always say that there can't be water in their cylinder, but there can be and often is water in the cylinder. The question is, how much?
The "best practice" is to check you breathing gas for oxygen content after a cylinder has been stored for a long period of time. If the oxygen content is stable, then it is clear that you do not have enough moisture and rust in the cylinder to lower oxygen content. But that says nothing about corrosion pits.
Again, "best practice" suggests that you have the cylinder visually inspected before it is refilled to check for corrosion pits.
Properly maintained and properly filled, you shouldn't have any problems with modern 3AL and 3AA scuba cylinders.
Case in point: I bought my Worthington steel HP120's new in early 2008, manufacturing dates & original hydro were late 2007. I've been putting about 100 dives a year on them since (i.e. about 50 each), I take very good care of them, and I've had the annual visual inspection done twice now.
The first year was clean, but this year one of the tanks had a light layer of rust inside - not even enough for a tumbling, just a good whipping out. Now, I never drain my tanks and nearly always have them filled together at the same shop, usually the same or similar Nitrox, and all of these are up-to-date Florida filling stations, and none of them do "wet fills" anymore. Yet, somehow, water had probably gotten into one of the cylinders, but not the other. I have no idea how.
So, water gets in. Probably not saltwater, but some gets in somehow, and according to those studies, just a little water - especially saltwater - in a steel cylinder with a lot of oxygen-rich gas under pressure makes for startlingly fast corrosion.
But yes, the worry isn't so great with aluminums, but the OP didn't say which he had. Like Doc said, it's probably wise to at least test the O2 before breathing gas that has been stored for 3+ months, better yet would be to drain & refill, and best would be a full viz. But your lungs are worth at least the O2 check.
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GrumpyOldGuy
Contributor
So you are saying with steel tanks, do a visual inspection every 3 months?
Did you read the results of those studies, what happened in the steel tanks after just 100 days? Made me paranoid about my steels. If they are stored with a high-pressure, oxygen-rich mix, then a viz might indeed be in order, because with just a bit of water that's obviously an ideal situation for rapid corrosion. What are your conclusions, based on the study data?
GrumpyOldGuy
Contributor
Yes, I did. My thought was the amount of saltwater they used was crazy, as was storing the cylinder on the side/inverted. I would expect you could hear the water sloshing in those babies. If you think this is realistic, then even 90 days between inspections is too long, maybe every 30 days to be extra safe?
Seriously though, the study does demonstrate there could be a problem with storing tanks with bad fills and we should not ignore the issue, but does nothing to scale it down to reasonable values that one could expect (small amounts of condensate) . As I stated above, I stick with a year since is meshes with VIP's anyway. And if you you are going to store a tank for 9 months, do the VIP BEFORE you store it. That way if you have an issue, it gets cleaned up before it damages your tank.
Seriously though, the study does demonstrate there could be a problem with storing tanks with bad fills and we should not ignore the issue, but does nothing to scale it down to reasonable values that one could expect (small amounts of condensate) . As I stated above, I stick with a year since is meshes with VIP's anyway. And if you you are going to store a tank for 9 months, do the VIP BEFORE you store it. That way if you have an issue, it gets cleaned up before it damages your tank.
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