420 Dive Log Australasia OCTOBER 2026

get ready. He had seen him checking that their air was on as they were seated, waiting for their turn to move to the entry area. Jim had fully opened his cylinder valve before putting his BCD on, as was his usual practice, and he guessed that, at some point, the helpful crewmember must have passed behind him to check if his air was on and had mistakenly closed Jim’s valve, thinking he was opening it. He had then twisted it back a quarter of a turn, re opening it slightly. This meant that Jim could still breathe properly when he entered the water and began to descend. It was only when he went deeper that he began to have a problem, as the slightly open valve could not supply enough air to keep the regulator working. If the crewmember had closed the valve completely and not turned the handle back a little, Jim would have discovered the problem when he was still in the shallows, possibly while he was still on the surface waiting to descend, as he used up the small quantity of air in his regulator hoses. However, because the crewmember opened the valve a little after closing it, Jim didn’t “run out of air” until he was at depth. When they first learn to dive, most people are taught to turn their cylinder valve all the way open and then twist it back a quarter or a half turn. It was this practice (as well as the crewmember’s inexperience) that led to Jim’s problem. The “turn-back” advice dates from the middle of the last century when scuba diving was in its infancy, and it was feared that cylinder valves could get stuck or be damaged if they were left open all the way.

With modern valves, this is completely unnecessary, so forget what you were told about quarter-turns or even half-turns in your training. Your cylinder valve should either be fully open when you are diving or fully closed when you are not diving. Nothing in between. By the way, note how well Jim and Michelle responded to this unexpected development. They didn’t panic. Instead, one helped the other without hesitation, and the other reacted with common sense once he had recovered from his initial shock. This was partly because they are good, well-trained divers and calm people, but it was also because they often dive together and, in the past, they have often thought about and practised dealing with possible underwater emergencies. So, when the unexpected happened, they were ready.

Simon Pridmore is the author of Technically Speaking, a short series of books about technical diving.

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DIVE LOG Australasia #420 October ‘ 26

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