When your cylinder undergoes a visual inspection and hydrostatic test the following will happen. It is picked up and taken to a cylinder test station.
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.
All visual & testing is carried out to Australian Standard 2030.1 & 2337.1
An external inspection is carried out looking for damage or alterations that may include bulges, dents, digs, cuts, pits & line corrosions.
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.
An internal inspection is then carried out looking for rust, scale, corrosion or oil or any other surface contaminated.
Older aluminium 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. Aluminium 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.
In particular to scuba cylinders, 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 no more than 50% of test pressure then released, this is carried out multiple times to check accuracy of the panel and remove air bubbles from the system.
The cylinder is then pressurised to its test pressure (normally 22mpa to 36mpa) and held there for up to 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 kilogrammes divided by five thousand (1/5000).
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.
If the operator was not sure of the expansion he will carry out a tare mass test, or a water capacity mass test.
This is when the cylinder is weighed and the weight compared to the original weight stamped on the neck of the cylinder.
This is when the cylinder is filled with water and weighed and the weight compared to the original water capacity weight stamped on the neck of the cylinder. This is when we can see if the cylinder has received any expansion.
Some Cylinders are high pressure vessels and as such should be handled carefully.
In particular Scuba Cylinders 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. Other cylinders are as per chart below.
Cylinder Hydrostatic pressure testing is carried out in accordance with Australian Standard 2030.1 & 2337.1 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.
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.