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California Diver — October 21, 2022
An unusual dive accident claimed the life of a scuba diving instructor in Amstelveen, a suburb of Amsterdam, on Sunday, October 16.
The fatality occurred Sunday morning at the De Meerkamp pool complex. The victim, a man from the Nemo diving club, was conducting a lesson in an indoor swimming pool when a scuba tank was believed to have exploded. Upon further investigation by the Netherlands Labour Inspectorate, it was discovered that the tank didn’t actually explode; the valve came loose of the diving cylinder on his back, fatally hitting the man.
The cause: A cylinder with internal thread G3/4 was used was fitted with a tap with external thread M25x2. The screw thread of the tap was slightly smaller than that of the screw thread in the diving cylinder, leading to the valve coming loose from the cylinder forcibly at high pressure. The force exerted at the base of the valve of an ‘average’ SCUBA cylinder is in the order of several thousand pounds.
Numbers are written on diving cylinders indicating the type of screw thread and also on the taps. If they match and are properly mounted, they are safe to use. Unfortunately, the size difference is small and just because they are mismatched does not make it impossible to fit together.
When being fitted, valves should screw in smoothly without binding or feeling loose. However, a 25-mm valve stem, when inserted into a G3/4 inch cylinder neck, will feel very loose until the valve is screwed in about halfway. At this point the threads begin to bind up and more force is needed to fully screw the valve in place. To the inexperienced person, the resulting connection would appear robust.
Thread gauges are available and should be used to check that threads on valves and cylinder necks are compatible with each other. It is also important that the valves are tightened to the torque specified in the manufacturer’s instructions. Too little torque can lead to the valve working loose, whereas excessive torque could damage the threads and weaken the integrity of the connection.
The pool at the De Meerkamp pool complex has since been reopened and has resumed normal operations.