Scuba Tank Storage

[Gilldiver]

The military did some extreme tests with scuba tanks. They put water in pressurized cylinders and put them in wet, hot conditions. From those extreme conditions we can draw some conclusions about how to store scuba cylinders:

1. A clyinder expands everytime it is filled and contracts when emptied. Repeated expansion and contraction fatigues the metal and eventually it will fail. (This is why we do hydro testing every five years, to detect cylinders that no longer tolerate repeated expansion/retraction.) So, it is best to store cylinders nearly empty to reduce metal fatigue.

2. Small amounts of mositure inside of the cylinder cause corrosion. If the tank is stored on its side, the corrosion is spread across the thinner sidewall. If the tank is stored upright, the corrosion is focused on the thicker base. Thus cylinders should be stored upright.

3. Oxidation of the metal cylinder is directly proportional to oxygen content, or the partial pressure of oxygen. Therefore cylinders should be stored almost empty to minimize the partial pressure of oxygen and corrosion.

4. In steel cylinders, oxidation of the steel consumes oxygen in the breathing gas and has been shown to dramatically reduce the oxygen content of the gas in certain situations. Therefore, steel clyinders should be stored nearly empty so that must be filled with fresh gas before use. Alternatively, the gas in a steel cylinder must be tested for oxygen content after long-term storage.

I will try to remember all of this the next time I am looking at the steel O2 storage tanks in my cascade. The ones at full pressure and with the original Hydro dates going back to 1909.

What you have stated has no backup data, it is just regurgatated BS.

 

[Doc Harry]

As a side note, the danger point is not at depth. A hypoxic mix is most dangerous on the surface.

Oops! Yes, thank you for the correction, I had it backwards.

I will need time to find the primary source of data.

 

[Doc Harry]

deleted duplicate post

 

[String]

1. A clyinder expands everytime it is filled and contracts when emptied. Repeated expansion and contraction fatigues the metal and eventually it will fail. (This is why we do hydro testing every five years, to detect cylinders that no longer tolerate repeated expansion/retraction.) So, it is best to store cylinders nearly empty to reduce metal fatigue.

So why would metal fatigue be an issue when its full? its the cycles that cause the stress, keeping it in one state or another is fine. Reference for this is where?

3. Oxidation of the metal cylinder is directly proportional to oxygen content, or the partial pressure of oxygen. Therefore cylinders should be stored almost empty to minimize the partial pressure of oxygen and corrosion.

Id love to see the thermodynamic calculations for this that show any real world difference.

4. In steel cylinders, oxidation of the steel consumes oxygen in the breathing gas and has been shown to dramatically reduce the oxygen content of the gas in certain situations.

Where is your reference for this? It goes against every single test and proper physical calculation. The amount you're talking about here is absolutely minuscule. Nothing you could detect without a very well equipped lab.

 

[String]

4. Steel cylinders corrode quickly.

They do ?! Define quickly. Tanks in use here are 20+ years old and still pay hydro and visual. Aluminium has a shorter life span but that is still many years.
Where is your reference for this?

5. Personally I am concerned about the amount of oxygen in my tank, especially when I am breathing gas under pressure. Take an air fill, reduce the oxygen content through oxidation over long-term storage, take it depth, and you may have a hypoxic mixture.

People store scuba and other compressed air tanks for years sometimes decades. After that time period the gas content is unsurprisingly the same as it was when filled.
Again, lets see your references.

 

[Doc Harry]

Here is some data from the Luxfer web site regarding actual data and most of the concerns y'all have raised. Below are a few quotes, there is much more information in the entire press release.

Luxfer: Press Releases

"What are the main reasons for an aluminium cylinder being condemned and withdrawn from service?

"Aluminium cylinders have received a great deal of attention in the USA since an explosion in Riviera Beach, Florida in February 1998. This was widely discussed on the Internet. Before that event, the occasional explosion of either steel or aluminium cylinders was largely ignored by the dive industry. Internal and external corrosion are the main causes of steel cylinders failing inspection or test. Aluminium cylinders fail less often, but when they do the cause is either excessive corrosion, often in the thread area or beneath the boot, or from a crack discovered during a visual inspection.

"Storing a full cylinder for long periods - several weeks or months - have a detrimental effect on its structural strength?

"There are several considerations when making this decision. In a clean dry cylinder, clean dry air will store quite nicely for long periods, certainly a year or more. Poorly filtered air containing various hydrocarbons or other contaminants could deteriorate in quality over weeks or months. The cylinder itself is designed for a sustained pressure load during its entire life. If moisture is left in the cylinder, then the increased oxygen in a full cylinder will contribute to more rapid corrosion that in turn could weaken the cylinder dramatically over a matter of months.

"More importantly, internal corrosion activity will reduce the oxygen content of the air. In the USA we have two examples of oxygen depletion in steel cylinders (corrosion is more rapid in steel). Following three months of storage, a corroded cylinder contained only 15% oxygen, a near-deadly deficiency. Analysis of a dead diver's air supply in a heavily corroded steel cylinder revealed a deadly 6% oxygen content. The University of Rhode Island's study of cylinder internal corrosion reveals that under equal conditions of moisture in cylinders, aluminium cylinders fare much better, so air stored in aluminium cylinders may retain its quality longer."

Is that good enough, or do you want me to dig up the actual case reports? I am more than happy to do the research if you desire.

 

[Charlie99]

It would be nice to see something about the actual cases, rather than just the statements posted on a manufacturer of ALUMINUM cylinders.

I went to the PSI site and couldn't find anything along this line, even though the stuff on the Luxfer site is supposedly an interview with Bill High of PSI. I'm betting that the cases were truly extreme corrosion, with more than a pound of rust in the tanks.

 

[String]

It certainly goes against health and safety official guidelines and those of faber and the other big makers of cylinders.

More worryingly, for the above to be true it would have to go against thermodynamics which isnt usually negotiable.

Can't comment on aluminium cylinders as i dont use the things due to them storing less gas, being heavier, physically bigger and more expensive to service than steel.

 

[Doc Harry]

It would be nice to see something about the actual cases, rather than just the statements posted on a manufacturer of ALUMINUM cylinders.

I'll get right on it Monday morning, actual case reports and actual cylinder inspection reports.

 

[Charlie99]

I'll get right on it Monday morning, actual case reports and actual cylinder inspection reports.

The only other reference I could find was another scuba forum, Steel Tank Corrosion.... - Northwest Diver Forum

This also leads back to Bill High of PSI as being the source of info at a talk in 2002 and the supposed incident was on the upper East Coast "many years ago".

The U of RI test is "Cinchy, F. et al. CORROSION OF STEEL AND ALUMINUM TANKS, U of Rhode Island, Narragansett, RI. 1978, 20 pages.", but I wasn't able to located a copy.

It would be nice to put this into either the "confirmed" category or the "urban legend" category.