“Back a Quarter Turn”
Among the very first things most instructors tell their students is that, when assembling scuba gear, “You turn the air all the way on, then back a quarter (or half) a turn.” If you ask them why, most instructors will tell you, “It’s so the divemaster can check to see whether your tank is turned on or not.” In other words, most dive instructors have no clue as to why or where this procedure came about in the first place.
The first widely distributed scuba valves in the USA were the US Divers models “J” and “K” from the early 1950s. If what I’ve been told by a number of people is correct, these valves contained dissimilar metals. If you left either of these valves turned all the way on, these dissimilar metals would come in contact with one another while possibly being exposed to salt water. Given sufficient time, such a valve could theoretically become stuck in the open position. To prevent this from happening, US Divers told customers to open their valves all the way, then turn them back a partial turn.
Obviously, the practice has persisted — even though it’s been unnecessary for half a century. Okay, is there anything wrong with opening a valve all the way, then turning it back a partial turn?
Famed cave explorer Sheck Exley (along with many others) found out the hard way why this may not be such a good idea. Early in his cave diving career, he made the mistake of turning a valve all the way off, then opening it a partial turn.
Guess what? A partially open valve may be able to give you enough air at the surface; however, should you suddenly begin to breathe had at depth, you may find yourself starved for air.
Sheck said this mistake almost killed him. It’s why, for the past several decades, cave divers have been taught to turn their air either all the way on, or all the way off.
Over the years, I’ve talked to numerous divers (and read more than one story) that report similar incidents. Most of these incidents resulted in a close call; however, I’ve been told (but have been unable to verify) that there was at least one incident in which a divemaster accidentally turned a diver’s air all the way off, then open a partial turn — which resulted in a fatality. (It’s bad enough when you do this to yourself; the thought that a so-called dive “professional” might do it to you is bone chilling.)
Ideally, we’d all wake up to the fact this practice isn’t safe and abandon it overnight. The problem is, this myth has been perpetuated for so long, most divemasters and instructors don’t know any better. The typical divemaster, when confronted with a valve that is already turned completely on, might try to force it open further. This could result in valve damage.
So, what to do? Until the major training organizations wake up to the danger inherent in this practice, it’s unlikely you, I or anyone else can singlehandedly eliminate it. This means that, despite the fact you’ve turned your own valve all the way on, some dive boat monkey may still try to forcibly yank it open further (or shut it down without your knowledge).
One possible solution is to open your valve all the way and turn it back only one eighth to one sixteenth of a turn (for the benefit of the dive monkey). This way, if what you have done is accidentally closed the valve, you are much more likely to know it.
By the way, it’s a good idea after turning on your air (or the dive monkey has “checked” it for you) to take several deep breaths from your regulator while looking at your SPG.
- If you (or anyone else) has accidentally turned the air off, the needle will drop with each breath.
- If the air was turned all the way off, but then opened a partial turn, the needle will most likely bounce with each breath.
Either of these signs will warn you of the need to turn your air all the way on before entering the water.
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