( 
)Seeing much bad information on used tanks prompted me to write this guide. We are a DOT hydrotester for 6 years and have over 20 years in the scuba industry.
Old steel cylinders
There is nothing wrong with buying old steel cylinders IF they are in good condition. It is best to have the cylinder inspected before buying or selling so all parties know the condition of the cylinder. Steel scuba cylinders from the 1970's are ok as long as internal and external rust and pits are within limits. I have seen industrial steel cylinders from the 1940's that were pristine inside and passed all inspections and hydro retest.
Old aluminum cylinders
Luxfer, Walter Kiddie, Alcan, US Divers, and Dacor cylinders made in January 1988 and before were all made out of a different alloy aluminum called T6351-T6 that has proven to exhibit sustained load cracking (SLC) over time. The tanks may be safe to use but require annual visual inspection and testing with special equipment called an eddy current tester to test the thread area for developing cracks. This drives the cost to own beyond what is economical and the eddy current tester will not find cracks in the crown area of the tank. The Department of Transportation has mandated this testing along with visual inspections for cracks along with requirements that precautions must be taken when filling the tanks. This implies the use of a blast cabinet to fill which most dive shops do not have. I would not recommend buying aluminum made in 01/88 or before. Do not store a full old aluminum tank. Google 6351 for more information about this subject.
Original Hydro Date
The date the cylinder was manufactured is stamped on every tank and is called the original hydro date. It will be the oldest date stamped on the tanks and is expressed as MO M YR for month manufacturer year. Retest on scuba tanks is done every 5 years. If the tank is not in use for many years, drain it to about 100 psi and have it retested before it will be reused. Annual inspection is not required for a tank not being used but must be done before it can be refilled.
Old air?
Air does not go bad if it was oil and moisture free. Take a wiff to see how it smells. Any oder is a sign there is moisture in the tank and it should be drained and inspected. Never ship a tank with air in it although it is ok to ship with the valve installed IF it can be properly protected with a heavy stiff tube around it.
Old steel cylinders
There is nothing wrong with buying old steel cylinders IF they are in good condition. It is best to have the cylinder inspected before buying or selling so all parties know the condition of the cylinder. Steel scuba cylinders from the 1970's are ok as long as internal and external rust and pits are within limits. I have seen industrial steel cylinders from the 1940's that were pristine inside and passed all inspections and hydro retest.
Old aluminum cylinders
Luxfer, Walter Kiddie, Alcan, US Divers, and Dacor cylinders made in January 1988 and before were all made out of a different alloy aluminum called T6351-T6 that has proven to exhibit sustained load cracking (SLC) over time. The tanks may be safe to use but require annual visual inspection and testing with special equipment called an eddy current tester to test the thread area for developing cracks. This drives the cost to own beyond what is economical and the eddy current tester will not find cracks in the crown area of the tank. The Department of Transportation has mandated this testing along with visual inspections for cracks along with requirements that precautions must be taken when filling the tanks. This implies the use of a blast cabinet to fill which most dive shops do not have. I would not recommend buying aluminum made in 01/88 or before. Do not store a full old aluminum tank. Google 6351 for more information about this subject.
Original Hydro Date
The date the cylinder was manufactured is stamped on every tank and is called the original hydro date. It will be the oldest date stamped on the tanks and is expressed as MO M YR for month manufacturer year. Retest on scuba tanks is done every 5 years. If the tank is not in use for many years, drain it to about 100 psi and have it retested before it will be reused. Annual inspection is not required for a tank not being used but must be done before it can be refilled.
Old air?
Air does not go bad if it was oil and moisture free. Take a wiff to see how it smells. Any oder is a sign there is moisture in the tank and it should be drained and inspected. Never ship a tank with air in it although it is ok to ship with the valve installed IF it can be properly protected with a heavy stiff tube around it.
Guide created: 12/09/07 (updated 06/27/09)
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