Some research has brought up the following for the UK.
The article makes good reading not just the testing periods.
http://www.ukdivers.net/equipment/cylinders.htm
Hydraulic Testing
The hydraulic 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 hydraulic test.
The new standard doesn't extend the test expiry for cylinders with a current test-due date. In other words, cylinders tested before the new standards came into force, and with either no test-due sticker (see below) or a 2-year sticker, will still need their next test at the expiry date of the old 2-year interval test/inspection.
Don't forget that as before, the hydraulic test period is not affected by the date of the visual inspection. For example, say you don't use your cylinder for a year or so then have it visually inspected 3½ years after it's hydraulic test. The cylinder will only then be in test for a further 18 months. This is because you will need another hydraulic test five years after the last, regardless of the visual inspection date.
In this test the cylinder is filled with water and then placed inside a water-filled, high-pressure chamber. The water pressure inside the cylinder is then increased to five thirds of it's maximum working pressure. This is sufficient to cause the cylinder to expand slightly. This expansion causes the water outside the tank to be displaced and this is channeled into marked collection tubes that allow it to be measured. If the tank's expansion is within acceptable limits (< 5%), it successfully passes the test. If not, the tank may not be refilled.
