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Mismatched threads caused Dutch tank death

Mismatched
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Professional divers and dive-centres should check the valve fittings on all their dive cylinders – that’s the urgent advice of the Netherlands Labour Inspectorate, following the event that killed an instructor in Amsterdam last week (16 October). The advice also applies to recreational divers who possess their own tanks.

The inspectorate (Inspectie SZW), which was charged with investigating the workplace incident, was quick to identify mismatched tank/valve threads as the fault that led to the diver’s death.

The instructor from the Nemo diving club was conducting a course at the De Meerkamp swimming pool complex, as reported on Divernet. One of his students later reported on social media that he had seen the man “launched into the air” by the force of a blast. He was resuscitated by staff at the site and taken to hospital, but died later from his injuries. 

The inspectorate has now stated that it was not the tank exploding but a valve ejected with explosive force and hitting the diver that caused his injuries. The cylinder neck had a G3/4 internal thread but had been fitted with a valve with an M25 x 2 external thread. This is very close in size but marginally smaller, which had allowed it to come loose under pressure when filled and later moved.

“Despite various international publications about the prevention of such incidents, such situations unfortunately continue to occur,” the inspectorate has stated. “Also, in previous incidents, the main cause was always that the diving cylinder and the mounted valve were incompatible.

“In such a case, the screw-thread of the tap is slightly smaller than that of the screw-thread in the diving cylinder. Result: an increased risk that the valve will come loose from the cylinder forcibly at high pressure.”

Tank-thread markings (Inspectie SZW)
Tank-thread markings (Inspectie SZW)
Valve-thread markings (Inspectie SZW)
Valve-thread markings (Inspectie SZW)

The screw-thread size on a cylinder is stamped on its neck, and that of a valve close to the thread. Divers finding these to be incompatible on an already mounted valve should not refill the tank but instead make it safe by slowly depressurising it – without leaning over it. Divers in any doubt about how to proceed should consult an IDEST-certified cylinder technician.

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