The dangers on your back (or side)
The hidden danger on your back (or side) Due to a number of serious incidents involving diving cylinders exploding over recent years the HSE and recreational diving bodies are repeating their safety advice about the potential hazards of internal corrosion of diving cylinders if they are not used properly. http://www.hse.gov.uk/pubns/dvis10.pdf In the last few years there have now been five incidents involving cylinders, which fortunately were not fatal but could very easily have cost a life. The problems appear to arise due to moisture getting into the cylinder. The high pressure environment combines with this water and allows very high rates of corrosion to occur. Such rates can narrow to the wall of a cylinder to a dangerous extent within six months (well inside routine inspection periods (2.5 and five years for visual and hydrostatic respectively). Currently, the industry view is that providing cylinders are operated correctly there is little risk of water entry and so there are no immediate plans to alter the current testing regimes. However, it is in divers’ own interests to safeguard themselves and those who may fill their cylinders to take account of any misuse that could lead to water contaminating the inside of a cylinder.
Normal usage
Cylinders should be filled from a properly maintained and operated compressor to ensure that dry air is used for filling and moisture is not introduced in that way. In normal usage where a reasonable reserve is maintained (around 50 bar) then there is little if any change of water getting inside the cylinder and consequently little risk of excessive internal corrosion. If you need to store a cylinder for any length of time you should never completely empty it but retain at least 30 bar pressure to protect against moisture being sucked in from the atmosphere.
Out of gas incidents
If a cylinder is emptied of gas whilst submerged, water can be drawn into the cylinder from the surrounding environment without the user realising it. In an out of gas situation, the priority, rightly, is to restore a breathable gas supply and safely exit the water. Alternatively, some divers are taking the decision to breathe down completely their decompression stages when they are nearing the need to be refilled. In both instances where this occurs in the water, there is a real and significant risk of water ingress and consequent accelerated corrosion.
If you ever completely empty a cylinder and the cylinder remains in contact with water, you should have it visually inspected before refilling and putting it back into service.
If in doubt
If you are in any doubt whether a cylinder has been contaminated in this way an internal check is the only way to be sure. It doesn’t require so much water that you can hear it sloshing around inside the cylinder (but if you do, empty the cylinder and get it checked immediately). And if you are the Dive Manager when a diver reports an out of gas, make sure they are advised to get their cylinder checked.
Think SAFE – Dive SAFE
Jim Watson
BSAC Safety and Development manager
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