Article by Peter Fear,
The Scuba Doctor,
first published in Dive Log, December 2005.
Scuba Cylinders are high pressure vessels and as such should be handled carefully. They should be washed after use and the valves serviced every twelve months the same as your other valuable gear.
In Australia all scuba cylinders are required to be hydrostatically tested every twelve months.
This is carried out in accordance with Australian Standard 2030.1 by a Certified Test Station.
The cylinder is then stamped around the neck area indicating the month and year it was tested along with the test station number.
Aluminum cylinders manufactured from 6351 aluminum are required to have an ultrasonic or eddy current neck test every two years to inspect for neck cracks that may have developed over time relating to that grade of aluminum. This is indicated with E stamped as part of the test stamp. It is sometimes called a "Visual Plus" test after the instrument used to conduct the test.
If the cylinder does not have a current test stamp, it will not, and should not, be filled.
The cylinder is in test for twelve months from the date of the test stamp.
What Actually Happens?
When your scuba cylinder undergoes a visual instection and hydrostatic test the following will happen.
It is picked up from the Dive Shop by a cylinder test station. Most shops do not have their own test unit due to the economics involved in setup, annual license fees, running costs, staff training along with annual inspections by Quality Assurance.
Details of the cylinder are entered on a test certificate: i.e. the cylinder number, the standard to which it was manufactured, working pressure, test pressure and the owner's details.
An external inspection is carried out looking for damage or alterations that may include gouges, dents or corrosion. Special attention is paid to repainted cylinders as they may have been heat treated or had dents and gouges filled. To carry out this inspection thoroughly the cylinder boot and mesh are removed. Dents, gouges, corrosion or heat treating can all effect the integrity of this high pressure vessel and they can fail test as a result.
The cylinder is then drained and the valve removed,. If in the opinion of the operator the valve requires to be serviced it will usually be serviced at an additional cost. The neck thread and internal space is checked for corrosion, wear, pitting or rust.
Older aluminum cylinders often fail in this area as the neck thread may have corroded or worn beyond the allowed tolerance, this is an automatic failure under the standard. Aluminum cylinders can also be badly corroded internally especially if salt water has entered the cylinder.
Steel cylinders as we all know can be subject to rust, some brands more than others due to the amount of carbon used in the steel when manufactured.
It needs to be pointed out that rust in steel cylinders is usually a result of carelessness by its user or your fill station. If your cylinder has been filled with moist air from a station that has not been maintaining its filters or compressor adequately rust will occur, oil may also be found if this is the case.
Cylinders found to be rusty or corroded internally and are not beyond repair are brushed using various forms of wire brush attached to a long shaft driven by an electric drill. The residue is cleaned out and rumbling chips inserted into the cylinder, it is then placed on a rumbler that rotates the cylinder at about 90 to 100 rpm for up to eight hours depending on the severity of the corrosion. The cylinder is then filled with water and inverted to assist in removing the chips. It is then cleaned, dried and internally inspected.
This time consuming process removes remaining corrosion and polishes the internal cylinder wall and is an additional cost.
If the cylinder passes all the previous inspections it is ready to be hydrostatically tested, when it is filled with water and connected to the test panel. The cylinder is pressurised to fill pressure twice to check accuracy of the panel and remove air bubbles from the system.
The cylinder is then pressurised to its test pressure and held there for one minute. The pressure is slowly released and the expansion measured on a manometer tube. The maximum allowed expansion is the water capacity of the cylinder in kilograms divided by five.
The cylinder is disconnected from the test panel and inverted to drain after which it is dried using warm air. After a final internal inspection to ensure proper drying the neck "o"ring is replaced and the valve installed.
The cylinder is then stamped, filled, leak tested at the neck and the test certificate completed.
Failed cylinders are destroyed in accordance with the standard.
The price for a standard hydro or cylinder test varies from shop to shop and test station. It generally ranges between $25 and $55. Considering the process involved and the fact that the cylinder is returned full the value is exceptional.
Charges for ultrasonic neck inspections, rumbling and valve servicing can also vary.
- Never buy a second hand cylinder without seeking good advice first.
- Your valve may not be serviced unless you ask for it unless it is damaged, in poor condition and noticed by the station operator.
- Put your cylinder in for test at the end of the month due on your test stamp, to be tested and stamped with the next months date.
- If you have your own compressor don't neglect to have your cylinders tested annually. I recently had an old aluminum cylinder in for test that hadn't been tested for a number of years from a diver with his own compressor. The size of the neck crack in that cylinder scared the living daylights out of him and me both.
The Scuba Doctor is a specialised cylinder testing and scuba gear service laboratory located at 1/49 Peninsula Avenue, Rye, Industrial Estate on the Mornington Peninsula. They are open weekends and public holidays for divers convenience. More about: Cylinder Testing