Dive Cylinder - Its Your Life Support.
Information about Types of cylinders
Steel cylinders are made by drawing and spinning and are 4-5mm thick. They are usually galvanised prior to painting.
Aluminium alloy is made by extrusion and forming. They are not as strong as steel and therefore require an 11mm wall thickness. They are anodised prior to painting and do not corrode as much as steel. Aluminium cylinders are easily identified by their flat bottom. They are not popularly used in Ireland.
European Standards
BS EN 1968:2002 for Steel Gas Containers
BS EN 1802:2002 for Aluminium Gas Containers.
Labeling and Marking
Cylinders must be either correctly labeled with their contents OR painted in the approved scheme (i.e. for compressed air and nitrox this means the black and white quarters on the shoulder).
Stamps Marking
The test or inspection date stamp punched into the cylinder will from now on show the year in full, followed by the month (as opposed to month then year as now). In addition to stamping the cylinder with the test date, it will also be mandatory to mark the cylinder with punch-out stickers to make it easy to see when the cylinder is next due for test.
Contents Labels
According to the new standard EN 1089, cylinders must now also have stickers showing their intended contents, e.g., Breathing Air, Nitrox or Trimix.
Hydrstatic Testing
The hydro test period has now been changed from 4 to 5 years. This has been done mainly to bring it into line with standards for testing other industrial compressed gas cylinders. Similarly visual inspection periods have changed from 2 years to 2½ years after a hydro test.
Visual Inspection (every 2.5 years)
Essentially this test is a detailed visual inspection, both internal and external with the following failure criteria:
- Bulge - all fail
- Dent - fail if any greater than 2 mm
- Wear - reduction of wall thickness greater than 25%
- General Corrosion - reduction of wall thickness greater than 20% OR where original surface is not visible
- Area Corrosion - (i.e., where corrosion is limited to less than 20% of surface) a reduction of wall thickness greater than 25%
- Isolated pitting - Reduction of wall thickness greater than 40%
- Thread damage - all physical damage or imperfections (recutting of threads can be achieved up to a maximum tolerance )
- Weight Check - Weight of cylinder ( minus valve ) less than 95% of the tare weight stamped on the cylinder
Any cylinder that has been subject to impact damage should be pressure tested before use. If in the opinion of the inspector, any of the above criteria are borderline, then hydro testing may also be required.
Test procedures
All testing is undertaken by qualified and experienced technicians. The procedure is as follows:
- Removal of the tank boot, bands and valve.
- The exterior of the tank is then inspected for impact damage and corrosion.
- The interior of the cylinder is examined using a special endoscope.
- The valve is checked for smooth operation and its threads lubricated to help prevent galvanic action between the dissimilar metals of the tank and valve.
- Valves may also require periodic overhauls, just like scuba regulators. Included in the service by Lambay Diving.
- If no corrosion or damage is detected, the cylinder is reassembled and filled.
- A sticker is then placed on the tank which identifies the facility providing the inspection and the month and year in which the inspection was done.
- If damage or corrosion is detected, the technician will decide upon an appropriate course of action.
- Extensive oxidation or deep pitting is treated by degrees of sand blasting.
Cylinder valve
Most are Balanced type which means that high pressure air is acting on both sides of the valve assembly to prevent any stiffness while turning the valve. The cylinder valve is fitted into the neck of the cylinder and is provided with an 'O' ring seal so that the diving regulator can be connected to it. The valve is screwed into the cylinder neck with either a taper thread seal or a parallel thread seal and an 'O' ring. It is not recommended that divers attempt to service their own cylinder valves, unless they possess specialist skills. Provided that the cylinder valve is washed in fresh water after use and is kept clean, there should be no need for it to be serviced between the intervals of cylinder test.
The UK Heath and Safety Executive (HSE) have produced a document entitled The safe use of gas cylinders. Available from this link in a pdf format.
Nationwide delivery & collection available.
Why test a Dive Cylinder
Opposite we go into the legal requirement for testing, these standards are implemented for the safety of the diver using the cylinder.
But why should we test the cylinder, a newer dive cylinders operate between 232 bar and 300 bar pressure - if we compare this to a car tyre, it is between 100 and 150 times greater. If a weakeness appears in a car tyre we get a blow out, if this happens in a dive cylinder the amount of energy released is over 100 times greater, so we need to make sure that the cylinder is safe for us to use. So testing is a good idea.
Another reason to test is when filling a cylinder we have to use a compressor, if this has not been maintained correctly impurities can enter the tank and contaminate the air in it. When the cylinder is tested oil and rust build up can be seen and cleaned out by the technician.
The cylinder valve over time can become clogged by salt build, so at Lambay Diving Services in every inspection we include cylinder valve cleaning as part of the service process. Not all test stations offer this service as standard.
Care and Maintenance
Care and maintenance of dive cylinders is relatively straightforward, after every dive wash down the cylinder, including the boot area and valve, with fresh water. Any rust spots that appear on the cylinder should be treated immediately with an appropriate paint. Never leave a cylinder standing in an area where it may get knocked over, as dents can cause weakness in the cylinder and may cause the cylinder to fail the test. If storing for long periods, store the cylinder standing up with 20 to 30 bar pressure. The thickest part of the cylinder is the base so you want any possible corrosion to be here.
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