This is another one of those techniques that we’ve all taught as instructors because we’re just copying what our instructors told us. I’m not aware of it being mentioned in any of the PADI materials. I certainly used to do it myself under some vague comprehension that doing so meant that the valve wouldn’t stick open. This was despite having never seen a valve stuck open up to that point or since I started teaching students to open the valve all the way.
The turning point for me came when I started tec diving where you turn your valves all the way open or closed (there’s a story in there somewhere but not for this blog. Ask me down the pub). Anyway it occurred to me that the valves we used in tec were just the same as the ones in recreational so what was different? Had a magical valve pixie appeared and sprinkled his enchanted dust of non-valve stickiness over tanks that were used for tec? Or was it, as I suspected, complete bollocks?
I started to ask about a bit and discovered that most instructors teach this technique. In the UK, no one had seen a tank valve stick open. However overseas some instructors where aware of this happening but that it really just meant that you needed a bit of extra oomph to close it. It certainly wasn’t a servicing job.
So it seems to be a technique born out of experience based on either poor quality valves or ones where regular servicing may not always be so regular. The quality of valves on cylinders in the UK is usually very good and as I mentioned at the start I have never seen or heard from anyone else of one sticking open. Therefore I suggest that you teach your students to open the valve all the way.
I’ll explain why: Closing a valve a quarter or half turn back is potentially very dangerous. If you are about to enter the water and suddenly think that your air needs to be switched on, you might ask your buddy to do it for you (I’m aware there are ways to check these things but if everyone did every check then there wouldn’t be any accidents caused by missing them). If your buddy isn’t sure they might end up switching your air off and then opening the valve a half turn back. This has the effect of allowing air into the regulator and allows you to breathe. Your gas may then cut off at depth when the pressure in the tank drops below a point where it can hold the valve open.
Whilst this is unlikely, it’s completely avoidable by having gas all on or all off. That’s why we do it that way in tec so as to avoid any confusion.
So forget about all this quarter turn back business and teach your students to open the valve all the way. Then you’ll look like the cool new instructor on the block, swaggering in with your attitude and new fangled ways that the old hands will at first distrust but then in a series of unlikely events, mainly caused by someone doing that thing the old way, you’ll step in, save the day and in return earn the grudging respect of the towns folk. That may also be the plot of most 80s movies starring Michael J Fox.
Next time we’ll be tackling the thorny scuba related issue of how much of the music of Phil Collins is too much? (Preview answer: A lot).
Postscript: Since putting this up there’s been quite a few people taking issue. A mate of mine pointed me in the direction of this link which I think helps add a little weight to the argument. Thanks Paul Flower!