I don’t trust the integrity of aluminum scuba cylinders… at least, not enough to:
- overfill any aluminum cylinder (in fact I often under-fill aluminum stages and decompression bottles keeping below the manufacturer’s suggestions for working pressure);
- keep them in service more than a year or two after their first hydrostatic test cycle (which is every five years where I live);
- wander very far from a very conservative approach to the frequency of formal visual inspections, choosing instead to follow the manufacturer’s suggestions for cylinders in Heavy Service;
- miss Eddy Current testing as part of the VIP procedure (EVEN WITH BRAND NEW CYLINDERS!);
- be trusting of loners and rentals, especially those with the look of being in service since, and taking direct hits during, the Gulf War.
My reasons for being a “mother hen” are based on a professional ‘cover everybody’s arse’ strategy to risk management. And a certain knowledge that high-pressure vessels have an enormous potential to harm. I’ve witnessed the aftermath of two separate aluminum tank failures and have a very strong mental image of the chaos each caused. I read somewhere that the amount of energy stored in a “recreational scuba cylinder,” which one can take to mean an aluminum 80, is about the same as two WWII British military hand grenades. A sobering thought.
Of course, one should be equally cautious with steel cylinders, which have a similarly dangerous potential. However, aluminum cylinders more easily carry the scars of mild to moderate abuse in typical everyday service. Couple this with their inherently different reaction to repeated filling and emptying – aluminum’s fatigue limit – and the dramatic reduction of an aluminum cylinder’s endurance limit from several hundred thousand fills to perhaps hundreds when it is over-filled – and its potential for failure is increased.
Of course, an easy out would be to avoid using aluminum cylinders altogether, but the buoyancy characteristics of aluminum makes 80s and 40s excellent stages, bailout, and decompression bottles. Besides, avoiding their use would be a dramatic over-reaction.
Working within manufacturer’s limits and the handling guidelines they supply us, aluminum is safe for many, many more fills than any of us is likely to ask it to endure.
But we do need to be mindful of those limits and guidelines.
Luxfer, the manufacturer of a popular brand of aluminum scuba cylinders of all sizes including the ubiquitous aluminum 80 writes the following about safety and its products… all great advice!
“If the cylinder is used in heavy service then it should be inspected every four months.
“Heavy service” means any one or more of the following:
- Cylinders being filled or “topped off” five or more times per week;
- Rental cylinders in use during the ‘season’ and ‘off-season’ times;
- Cylinders used wherever damage is more likely than in normal use or where the
- care and/or maintenance is slightly below recommended care.
If the cylinder is known to have had any unusual treatment or condition, it should be immediately visually inspected, prior to its next use.
“Unusual treatment or condition” means if the cylinder:
- Dropped, fell, was struck, was in an accident, or when the care and maintenance of the cylinder is obviously poor;
- Was stored improperly, and shows signs of damage;
- Has obvious corrosion since the last visual inspection;
- Has a gouge, dent, scrape, cut, dig or, in any way, has been damaged since the last
- visual inspection;
- Was stored with water, material or matter inside the cylinder;
- Shows signs of exposure to fire or high heat, including any one or more of the
- Charring or blistering of the paint or other protective coating;
- Melting or charring of the metal;
- Distortion of the cylinder and/or any cylinder accessory;
- Melting of fuse plugs, valve handwheel, valve protector, and/or any other
- valve component or cylinder accessory;
- Has been partially or fully repainted or treated to hide damage and/or
- fire damage;
- Is known or suspected to be leaking; or,
- Is known or suspected of having a crack.”
Dive Safe… be careful out there.