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

[vel525]

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

 

[pipedope]

This is one of those things that is argued endlessly.

When I learned to dive we were taught the open fully turn back 1/4 turn. The idea was that someone checking your valve would turn it in the open direction and feel it move easly and stop showing that it was open. They would then turn it back the 1/4 turn so the next person would get the same result.

There really is no mechanical reason on scuba tank valves to have it fully open or slightly closed it is just for operation.

Now there are valves, mainly in chemical plants and nuclear power systems that have a back seat. These valves are opened and set to the open or back seat with a torque wrench. This keeps primary coolant pressure off of the valve stem packing. Not a consideration in scuba air.

 

[Christopher520]

Hi vel525.

Thats an excellent question, i can't find the answer for it either. I have looked through the Tnak section of the Open Water Diver manual but it doesn't mention the 1/4 turn.

I also checked in the encyclopedia of recreational diving, nothing mentioned in it either, so i would have to say that the info pipedown has given makes sense.

Good question, questions like those keep you on your toes.

christopher520.

 

[zboss]

I've heard it was because old values would sometimes stick open if they were left open all of the way.

 

[Rick Inman]

Please, Mister DM, don't touch my valve...

 

[diverduane78]

I asked this question to a PADI course director and he told me that you do it so the valve won't stick open from the built up pressure. It commonly happens to older tanks, but its a good practice to get into with any tank.

 

[nessum]

Yes, I believe it was because older valves had a tendency to stick.

Now we teach our students to always open the valves all the way to avoid confusion. If it's all the way on, it won't turn back any further when the buddy checks it. This should be follwed by breathing off both second stages. If, for any reason the tank has been closed & hadn't easily opened when the buddy checked the valve it would become obvious as the needle in the SPG drops & you receive no more air.

 

[hecker-the-wrecker]

another reason is; if the valve is full open & brutus the body building deck hand thinks the tank is shut tight, and tries to crank it open, valve damage can happen. Although it sounds far fetched, i've seen a tank cranked past the point of open, we had to use a vise grips to close it.

 

[Albion]

another reason is; if the valve is full open & brutus the body building deck hand thinks the tank is shut tight, and tries to crank it open, valve damage can happen. Although it sounds far fetched, i've seen a tank cranked past the point of open, we had to use a vise grips to close it.

Its basic valve practice both for scuba tanks and industrial valves, except as pipedope mentioned for some specialised valves. If you fully open a valve until it hits the back stop it could feel to a casual inspection that it is in fact closed

 

[StSomewhere]

RE: valves in general. Open a valve all the way and back it off just a tad. But depending on how much "throw" that particular valve has, a 1/2 turn might wind up being substantial!