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it helps to insure that the valve will not get stuck in the open position. some people open them so far that after a dive, even more in cold water IMO, it will tend to get stuck or freezr in the open position. i have witnessed this once at the quarry and it took a wrench to get the valve closed.
also it helps to keep pressure off the internal o rings to help prolong their working life.
there might be some other reasons behind it, but these are the two reasons that i have found why it is done
Why, when opening a cylinder, is it recommended to do half a turn back?
Another reason to turn it back half a turn, at least on boat dives, is the dive master will generally check your cylinder before you go in the water. If it is already cranked, and they crank it further, it could very well get stuck.
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ParzDiver
"....with the crust cause we're divers, it's not a freaking tea party!!!" - ScubaSam
I've never heard to turn it back half a turn....I've only ever heard to turn it back a quarter of the way. I certainly wouldn't be turning it half way back as it may not deliver air at depth (deeper than X feet). Personally, I don't turn it back a quarter of the way either and have never had freezing issues....though I also don't dive in super cold water.
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"Worrying about buddies on a full DIR boat is like worrying about where you're going to score weed at Burning Man." -Rainer
Turning it back half a turn would have no affect on the flow of air, at least that anyone would notice. Also when it is checked and it moves then the checker knows it is open. If it doesn't move then it is suspect to be closed.
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Think of how stupid the average person is, and realize half of them are stupider than that.
Why, when opening a cylinder, is it recommended to do half a turn back?
Depends on who you ask, as many folks do NOT recommend turning the knob back at all.
If you ask ME I suggest you open it all the way and leave it open all the way. Don't CRANK it open to the point that you can't now close it, but make sure it is fully opened.
The idea of leaving it "a little closed" so that if someone plays with it they will not crank it open too much, etc is just plain silliness in my mind. A valve is either OPEN or it is CLOSED, this way there is no doubt as to which way to turn the valve in order to get it to the other state. (eg from OPEN to CLOSED or from CLOSED to OPEN.) Though any divemaster or instructor who doesn't know which way to turn a valve needs to have every C-Card in their possession revoked. (Even my 5-year old can keep "lefty-loosey, righty-tighty" straight.)
FWIW, I read that the old valves (circa 1950-1960) had a habit of locking up if you opened them all the way. The article then stated that this is not an issue with current valves. Info was taken from one of the NSSCDS manuals.
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Never do anything you can't undo until you understand all the things you can't do once you do it.
Why, when opening a cylinder, is it recommended to do half a turn back?
Actually the usual recommendation is 1/4 turn back. But the amount of "turn back" isn't really important. All you are doing is making sure the valve isn't jammed all the way open. This is bad for the valve. All you really need to do is 1/4 inch back. Just don't have it jammed open.
I've never heard to turn it back half a turn....I've only ever heard to turn it back a quarter of the way. I certainly wouldn't be turning it half way back as it may not deliver air at depth (deeper than X feet). Personally, I don't turn it back a quarter of the way either and have never had freezing issues....though I also don't dive in super cold water.
half a turn not half way back... this will never affect air flow, but keeps you in the safe side in case it gets stuck if fully opened.
I've never heard to turn it back half a turn....I've only ever heard to turn it back a quarter of the way. I certainly wouldn't be turning it half way back as it may not deliver air at depth (deeper than X feet). Personally, I don't turn it back a quarter of the way either and have never had freezing issues....though I also don't dive in super cold water.
Not half the way back, half a turn. It takes several full turns to go from full off to full on.
I was told to do it, so that the person checking the tank could easily tell it's state. All the way on, and they might presume all the way off.
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