Pressurization Levels During Testing
The tank's working pressure is the pressure to which it is normally considered full. The hydrostatic test fills the tank to 1 2/3 of the working pressure. The reason for this is to ensure the tank is capable of holding gas pressures far greater than it is normally required to. To give an example, during hydrostatic testing an oxygen tank with a designated working pressure of 300 bar will be pressurized to 500 bar. Hydrostatic rules dictate that the pressure must be held for at least 30 seconds to allow the tank walls to fully expand, before it is depressurized.
An acceptable expansion of the tank following depressurization is to within 10 percent of the reading taken when the tank is pressurized. For example: A 10-liter tank expands by 1 liter when pressurized during the hydrostatic test. To pass the test the tank expansion must not be greater than 0.1 liters following depressurization.
All oxygen cylinders are required to have the date of the last hydrostatic test on the neck of the tank. This is to show users of the tank when the next due test date is and to verify the tank is safe to use.
Period Between Testing
Oxygen cylinders must be tested every five years. Testing must occur sooner if the cylinder is dropped from a height, exposed to heat damage or painted with an inappropriate substance.