UK law treats a diving cylinder as a pressure vessel — and like any pressure vessel, it has to be inspected and tested on a schedule by a competent person. Here's what that actually involves, how often, and what makes a cylinder pass or fail.
UK testing follows the framework used by BSAC and IDEST-registered stations. Two distinct checks run on different clocks:
The valve is removed and the cylinder is examined inside and out using calibrated lighting and a borescope. The inspector looks for internal and external corrosion, pitting, dents, cracks, heat damage and thread wear, gauges the neck threads, services or replaces the valve, and applies a current inspection stamp.
The cylinder is filled with water, all air removed, and pressurised above its working pressure inside a water-jacketed test chamber. The inspector measures how much the cylinder expands under pressure and how much it stays expanded afterwards (permanent set). Too much, and the cylinder fails. It's then dried, the valve reinstalled, stamped and certified.
Many aluminium cylinders additionally require periodic eddy-current testing of the neck threads to check for sustained-load cracking. Steel and aluminium cylinders also have different end-of-life considerations, so the testing station treats them differently.
If a cylinder fails, a reputable station will explain why and either return it to you or — for an additional fee — render it permanently unusable in line with industry guidance, so a condemned cylinder can't quietly go back into service.
Any cylinder used with oxygen-enriched gas above 23.5% O₂ must be oxygen-clean: degreased and free of oils and particulate that could ignite in an oxygen-rich environment. Cylinders moving from air to nitrox service need full oxygen cleaning and re-certification, and oxygen-service valves are cleaned to the appropriate standard. It's a small job that prevents a serious one.
To turn a test around quickly, bring the cylinder with a note of its gas service (air / nitrox / oxygen), and flag any history of water ingress or damage. Commercial operators and dive shops can usually arrange collection rather than dropping off individually.
We provide hydrostatic testing, visual inspection, valve servicing and oxygen cleaning for scuba, surface-supplied and industrial cylinders — with collection across Cumbria, Lancashire and the North West.
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