Actually it does matter. As a PSI inspector, for extended periods I advocate storing both steel and aluminum cylinders "nearly" empty for the reasons stated in the following article.
Note: This is merely a portion of the interview, the whole interview may be found at:
http://www.luxfercylinders.com/news/releases/19991028.shtml.
Regards,
Dennis
Aluminium Tanks - What Every Diver Should Know
An interview with Bill High, President of PSI Inc.
by Dave Moran
October 28, 1999
"
DM: How long has PSI been involved with cylinder testing and investigations?
BH: I formed PSI, Inc. (initially called Professional Scuba Inspectors) in 1982, following three years of research. I had completed four years as NAUI's president and was looking for another way to serve the diving community. Our first seminar was conducted in January 1983. We have trained over 10,000 inspectors for military, government, hydrostatic retesting, dive and fire industries in North America, and at numerous international locations. We have 65 instructors in ten countries conducting nearly 200 seminars each year.
. . . .
Does 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.
There is one additional consideration for aluminium cylinders. Because of its
low tolerance for heat, and where there is potential for the aluminium cylinder to be involved in a fire,
there is merit in storing the cylinder either full or nearly empty (about 20 psig). A 3000 psig aluminium cylinder containing less than about 1500 psig is at risk of exploding in a fire. The metal will lose its strength before the heat causes a rise in internal pressure sufficient to activate the pressure relief device (burst disc).
. . . "
PS: I admit that AL cylinders can be stored full also . . . but do we want to possibly risk our local Firefighter's lives on a burst disc?
