They were on a boat called Nemo chartered by Absolute Scuba but had brought tanks filled at the club, Underwater Research Group of Queensland.
While at 22m, some 10 minutes into their second dive, Thwaites signalled a stomach problem and a need to surface, though Masterman said she saw no signs of panic. They started to ascend but she lost sight of him while still at about 17m.
Divers on the boat saw Thwaites surface briefly on the anchor-line before he disappeared again, and assumed that he had dived again. His body was recovered from the seabed by police divers the following day.
Brisbane Coroner Christine Clements found that Thwaites had drowned after falling unconscious as a result of carbon monoxide poisoning. His air tank contained 2366 parts per million of CO – nearly 500 times the accepted limit.
At depth the toxicity of the gas would have been multiplied three-fold, to the extent that even if medical assistance had been immediately available it might have been too late. There were also elevated levels of carbon dioxide in his tank, and the air in Masterman’s tank for the second dive was found to contain around 160 times the accepted level of CO.
The tanks had been filled from an ageing compressor operated and maintained by Underwater Research Group of Queensland. It had been repaired a few weeks before the fatal dive, when it was noticed that it was providing incomplete fills.
Significant changes in the law were needed to regulate the diving industry and improve the maintenance and assessment of air compressors and other diving equipment, concluded the Coroner.
“The most important issue identified following Mr Thwaites’ tragic death was the need for education to alert divers of the risk of contamination when filling their cylinders,” she stated. “The risk of carbon monoxide contamination is a lethal risk, and a ‘sniff’ test will do nothing to alert a diver to the odourless gas.
“Divers should recognise that contamination can come from the external environment, but also from an internal ignition and combustion within an air compressor. This can occur if the compressor is not properly configured, filtered, cooled, and maintained and repaired.
“A hand-held testing device used every time a tank is filled should be part of every diver’s equipment and routine. A diver’s life depends on the reliability of their air supply under water.”