A hydro static or hydro test is performed once every 5 years on diving cylinders. Before the Hydro test the cylinder will need to be visually inspected. You may want to read the post on the visual test
Step 1 – Fill with water
The cylinder is filled with water. Liquid is used as it does not compress greatly under pressure. So if a cylinder does fail there is not a great mass of expanding gas to cause damage.
Step 2 – Fit test adapter
A hydraulic hose is fitted into the cylinder with a connector to attach to the hydraulic pump
Step 3 – Connect to Hydraulic pump
The cylinder is dropped into the water jacket and the hose is connected to the hydraulic pump. This is done by a fitting in the side of the water jacket.
The lid of the water jacket is then then fitted which has a water tight seal. The jacket is then purged of air with water. The level of the water is set on a graduated tube attached to the jacket.
Step 4 – The Hydro Test
The cylinder is then pumped to the test pressure. This is usually 1.5x the working pressure so 348Bar for a standard 232Bar cylinder. The pressure is then held for 30 seconds.
As the pressure increases the cylinder will stretch. As it does so the water in the jacket is displaced and the amount of displacement is measured on the graduated tube. When the pressure releases the cylinder relaxes and the water level drops below the initial level. The amount of water displaced during the pressure cycle and after is used to calculate the permanent set of the cylinder.
The cylinder is then removed from the jacket and emptied of water.
Step 5 – Drying
Water inside a cylinder is not good so it must be dried. If left in the cylinder will rust, even a short exposure to the atmophere can cause flash rusting. Once empty the cylinder is inverted over a steam jet. This heats the cylinder and with the addition of chemicals the cylinder can be hot oxygen cleaned. When hot the cylinder is dryed with an air jet and again inspected internally to ensure all the water is removed. The main danger of cold oxygen cleaning of cylinders is the fact that it is almost impossible to dry the cylinder adequately or fast enough.
Step 6 – Stamping the cylinder
This is done with a set of punches. The year and month of the test is put on the cylinder along with the test stations unique identification stamp. A “V” is stamped at the end if the cylinder has just had a mid-term inspection and not a full hyro test.
Step 7 – Refitting the valve
The valve is re-inserted into the cylinder with a new O’ring. A tool is inserted into the valve outlet and a torque wrench is fitted so the valve is fitted to the correct setting. If this is done the cylinder does not leak. It is not acceptable to fit the cylinder valve with a spanner and a hammer.