Half-Turn to close the tank - why???

[fmw625]

I asked the tech at my LDS about this. His reply was that the valve will last longer if it is closed about 1/4 turn from full open.

 

[matt_unique]

hey everyone,

sorry for such a basic question, but i was having a discussion with a friend about the procedure for turning on air before a dive. i always opened the tank fully, followed by a half-turn in the direction to close the tank. i wanted to verfiy:

1. is this procedure always performed?

2. why is it performed?

my friend says that you only do the half-turn back for boat diving. his reason being, that this helps the divemaster/boat captain who is assisting divers into the water verify that the tank is open. sounds confusing, buthis reasoning is it's easy to see if the tank is open if you only have to turn it a little bit.

okay, my girlfriend/buddy and i think his reasoning is incorrect, and we have both always done the half-turn back for every dive we've ever done. however, we cannot think of a reason why we've been doing this!!! the books are packed as we're moving, so we can't look it up.

someone, please settle this for us! thanks for any insight.

victor

As pointed out, with some valves "half a turn" could restrict air flow. You would not often notice this until you were at depth. I personally turn the valve back *just a pinch*. It is also helpful for valve shutoff needs - if you (or a buddy or whatever) are trying to shut off a valve and it stops after a very short turn you (they) know they are going in the wrong direction.

--Matt

 

[simbrooks]

Hopefully folks know which direction a valve turns to open and closed, both from behind the diver and when doing it yourself with the arm over the shoulder motion. I open the valves all the way on my singles and doubles, never had a sticking valve and always seem to be able to turn them off during valve drills.

 

[Shawn95]

I had a write up regarding this that I can't find right now. I will post it later. A few years ago there was a regulator manufacturer that recomended a 1/4 back because of disimilar metals in the regulator. This ended up as common practice now. Turning it back does save a little pressure on the seat but for the sake of a $0.10 part leave you should leave your air open. It's either on or off. I have been on the recieving end of a well meaning but not well knowing divemaster who turned off my air and turned it a 1/4 turn on.


Shawn
Quite typing and get in the water.

 

[ScubaDadMiami]

Precisely due to incidents that have been mentioned here, you should open your valve all the way and leave it that way. If you are in a stressful situation underwater, needing to shut down a valve, you might not (with all the stress going on) turn the valve in the right direction, losing valuable time and gas. Also, it has happened that someone has actually shut down a valve, thinking that s/he was turning it on. Sounds crazy, but it happens.

Technology has improved greatly since the days when valves getting stuck was an issue. Trying to save the seat is also really not significant enough to warrant leaving the valve partially opened.

People just keep repeating old habits without wondering why. There was once a child that asked his mother why she would purchase a ham, bring it home, cut off half of it and throw the half away before cooking the ham. The mother's response was that this was the way her mother taught her to cook. Little did she know, the mother did this because they were too poor to afford a refrigerator, and their frying pan was too small to cook the whole ham. Never stop questioning!

 

[cerich]

It's been years since I've seen a valve stick in the open position, at least 10 years (and for 5 of those years worked in Cayman as instructor). You are more likely to damage the valve by shutting it too tightly(seat and copper washer)

I now teach to open the valve all the way and don't let anybody touch it. For buddy checks I recommend students to ask their buddy if the tank is full and valve open then have them breath on both second stages while both members of the team watch the SPG for any pressure drops. A drop will mean a valve issue(busted or only slightly open or a first stage issue). As mentioned above some tank valves have a "short throw" and 1/4 or 1/2 turn back will restrict gas flow(genesis valves(not the 3500 DIN valve) come to mind, also why I won't use that valve with Nitrox or o2)

I had my fellow instructors/buddy turn my gas off by mistake and have seen them do it to many divers.

 

[knotical]

I was once told by an old salt / special ops diver that when shore diving (or whatever) you can get grains of sand wedged under your tank valve knob that make it hard to turn. If the sand grain lodged just right with a fully open valve, you have only one way to try to turn it. With the valve closed a tad, you have a shot at “rocking” the sand out. Of course he might have just been passing on what someone else told him. I didn’t ask if he’d had personal experience with it.

Also found this thread:
http://www.scubaboard.com/t38474-why-do-you-not-turn-a-tank-full-on-.html

 

[jonnythan]

I leave mine full open. If I need to close it or open it, I want the only way it turns to be the right way.

 

[Walter]

This really isn't an issue. Full on works. Back just a crack also works. Who cares? The concept of someone turning off your valve when they think they are turning it on can happen regardless of your choice to open it all the way or to back it off a tad. It's a non-issue.

 

[pilot fish]

I've been turning my a quarter turn from full since OW cert but after reading this thread I think I'll open it full.