Cylinder Testing: Pros, Cons and Thoughts.
In Australia it is required to hydrostatically test dive cylinders every 12 months for them to be filled; some think this is excessive when compared to other places around the world. To put this in perspective:
Australian standards require a hydrostatic test every 12 months, which also includes a visual inspection of both the exterior and interior of the cylinder.
New Zealand has similar requirements. Their visual inspection is required at the 12 month mark, followed by a hydrostatic test at the 24 month mark.
USA requires a visual inspection annually and a hydrostatic test every 5 years.
For those unaware of what a hydrostatic test entails, it is essentially a stretch test to check the elasticity of the cylinder. It ensures the cylinder will return to an acceptable size after being stretched. We start by conducting a visual inspection of the exterior and interior of the cylinder and inspect the valve for movement, binding, corrosion and its general condition. If everything is looking acceptable at this point, we fill a cylinder full of water, ensure there are no air bubbles trapped in the cylinder or testing equipment, and then perform a series of pressurisation tests. A standard hydro is 2 stretches: an initial stretch to 50-75% of the test pressure (I usually go to the working pressure if it falls within that margin) and a full test pressure stretch.
The working pressure and test pressures we go to are set by the cylinder manufacturer and are stamped into the neck area of the cylinder. For example, a standard Faber 12.2L cylinder has a working pressure of 232Bar and a test pressure of 348bar, and a Luxfer 11.1L ally rated to 207Bar has a test pressure of 345Bar. Most cylinder manufacturers (especially those working to the American specifications), work to a rule of approximately 5/3 working pressure for the test pressure. As you can see above, Faber tanks work to a slightly different ratio. However, that will change depending on the specifications to which the cylinder is designed and produced.
The majority of cylinders that are working to the 5/3 ratio are American specification, and are labeled with DOTXXX. Most ally tanks these days are DOT3AL followed by their working pressure in psi (2957/3000/3500). Some steel tanks may be DOT3AA, although these are not seen much in the Australian market as they often do not have Australian Design Approval. The majority of steel tanks in Australia will be BS5045 for older Fabers, or more recently ISO9809-1. The main complaint I hear about the frequency of cylinder testing in Australia is that stretching the cylinder as frequently as we do causes harm, and we're wearing out our cylinders prematurely by taking them to a higher pressure than their working pressure every year, instead of bi-yearly or longer. However, this is not correct. In my experience, the biggest cause of damage to a cylinder is corrosion, followed closely by heat. Throughout the last few years of testing cylinders, 9 out of 10 of the tanks I have failed have been due to their condition. They haven't even made it to the hydro stage of testing. Salt water diving affects all of our dive gear, but cylinders are generally not the focus during the end of dive clean up. Corrosion on both steel and aluminium cylinders can occur quickly if not cared for and maintained. Steel cylinders rust. This is pretty obvious when it happens and can be cleaned up with minimal effort if caught quickly. Aluminium pits, and instead of getting a large unsightly blemish, it eats into the material and can be overlooked very easily. The exterior will usually have a light layer of aluminium oxide which looks like a white-ish powder on the cylinder, but the real damage is happening underneath.
Heat on cylinders is the silent killer. Most tanks that I have failed were due to corrosion, but being a visual symptom, you can act on it before it's too late by inspecting your gear thoroughly each time it's used. Allowing a cylinder to get hot frequently actually changes the elasticity of the cylinder; meaning it will not return to its manufactured specification. This is thanks to the heat of a fast fill and subsequent cooling changing the temper of the material. Effectively work hardening the aluminium or steel.
By filling a cylinder either too fast or too far(or a combination of the two). The compression of gasses inside the cylinder will transfer that heat into the material of the cylinder itself. Now, depending on the manufacturer's specifications for each cylinder, the maximum operating temp can vary. However, personally I go by the rule: "if it's uncomfortable to hold in the palm of your hand, it is way too hot." In Australia, by testing cylinders every 12 months we are ensuring that the cylinders in use are as safe as possible. Not just from a corrosion point of view, but the material as well. A hydro every 12 months may take the cylinder above the working pressure, but it is performed with water as the testing medium, so there is no compression, therefore no heat. So in the case of a Faber 12.2L steel cylinder, its working pressure is 232Bar and the test pressure is 348Bar. An air fill to 232 will create heat regardless of fill speed. A hydro does not, and the pressure it is tested to is specified by the manufacturer and is stamped into the cylinder. Meaning, the cylinders are designed to be taken to those pressures when done correctly. For me, the main pro to hydro testing every 12 months is that we get a baseline for the cylinder - especially if it is tested by the same station each time. You end up with a log of how it scored, and each year, if it's holding similar scores, the health of the cylinder is holding stable. If one year it is hot filled a few times it can show in the hydro test, giving an idea that the cylinder may be on the way out. Seeing as though Australia has a fairly hot temperature for a large portion of the year, it's not a bad thing to know the health of your cylinders.
If you test cylinders less frequently, you can see if the material is being damaged by corrosion internally and externally. However, it could be almost 5 years later before you realise it is on its last legs.
I hope this has answered a few questions that may be out there(maybe even created a few too!). Please feel free to get in touch if you have any questions, queries or comments. Thank you for reading.