Why close the valve 1/4 or 1/2 turn???

[Dive-aholic]

I've head the 1/4 turn back thing comes from the old valves that could lock open under pressure. Today's valves typically won't do that. If they do, just drain the tank through your regulator. I keep my valve fully open.

I am not sure with valves these days if there is any reason not to open them all the way. I understand when diving in an overhead enviornment that opening all the way creates a "lock" and makes it harder for an accidental roll off, so I can see the "it may be dangerous" part...

Not so. The reason for keeping valves open all the way when diving doubles is to facilitate valve shut down drills. There's only one way to turn the valve. If it's open all the way, then you don't waste time turning the valve 1/4 or 1/2 way the wrong way. Rolls offs will happen regardless of how open the valve is. Prevention of roll offs is done by staying off the ceiling.

 

[soudan]

This is exactly why this is done. When you open the valve all the way, you are squashing an internal oring. By backing it off 1/4 you relieve the pressure you are applying on that o-ring, making it last longer and less likely to fail on the dive.

It is the same reason they tell you never to over-tighten an oil filter or oil-pan screw, because you will damage the oring and have leaks.

OK, i am not an expert but if there is an o-ring to be squashed, wouldn't that be when the valve is closed not when it opened??

if i am not mistaken the thing with the valve getting stuck or damaged was in the old valves, the current designs do not allow that to happen.
so the 1/4 turn thing was made in a time where the valves could get damaged, but now its just a good way to know that your valve is open and not closed.
i am only speaking from my experience and from the knowledge i have gained. so please correct me if i am wrong.

Happy Holidays.

 

[Crazy Fingers]

For what it's worth, I don't leave any hand valves open all the way. Many types of valves whose actuation depends on a screw (such as bellows-sealed, globe, gate, etc.) will sometimes stick after awhile when opened fully. Closing them a 1/4 turn prevents it from sticking and has a negligible effect on the flow. Now just because it sticks doesn't mean it can't still be closed fairly easily...

Go try it with your garden spigot.

Whether or not it happens to scuba tank valves, I haven't really seen it but haven't really been looking.

 

[The Horn]

Turning the valve back a quarter turn is common practice in the Marine engineering world and it comes from years of experience.

1. Steam valves would heat up and thermally expand. If opened fully and steam/heat was stopped the valve would cool and jam in the open position. You may get this experience if you had your tanks in the trunk on a hot summer day and then plunged into 47 degree bottom temp and had to close the valve.

2. When checking a valve it is easy to turn the 1/4 turn and know (confirm) the valve is open. This was mentioned earlier by Submariner. In situations where you need to be positive the valve is open it is good practice.

3. Valves with large diameter handles can be spun open and can get bound up if spun fully open (treads damaged, packing crushed etc) by turning back 1/4 a person can quickly close without having to fight the valve. This happened on a Naptha carrier and in the rush to close a tank filling valve they thought it was closed (it was jammed open) the tanker exploded when the tank overflowed into the pump room killing alot of engineers.

4. For the smaller scuba tanks the knob is small and would take greater effort to jam. Most have a spring that retains the knob and will not allow you to crank the tank valve.

5. You will get minimal if any reduction in flow by turning the knob back 1/4 turn. The valve will be fully open in about two turns, the rest of the turns are to ensure it is fully open.

6. Some valves are designed to be fully open. Oxygen tank valves back seat, or seal when fully open to prevent leakage at the packing gland. Refrigeration valves are similar.

I'll find out more if you wish

 

[Iruka]

You don't have to. The 'open fully and then back off a half-turn' is an old technique without any real purpose. It also can be dangerous. Every diver is responsible for their own valve setting, and nobody else has any business touching or checking them.

I agree that every diver should check his/her own air....but a lot of them don't. The ocean bottom here on Guam would be COVERED with dead divers if the guides didn't check their customers' air. Other guides who work in areas where many of the divers are infrequent, once or twice per year divers know what I'm talking about. I know some people don't want anyone to touch their gear...if that was the case with my customer, fine...except I'd need to personally watch them check their air. Normally a lot simpler just to check it myself. I have 6,270 dives as of yesterday, and most of the time when I'm about to do my entry, the deckhand will check my air. I already KNOW it's on, but it's not hurting anything and lets him be another "link" in the chain to increase diver safety, so I never ask them not to do that.

As guides, the customer's safety is the most important consideration, and some people need closer attention (sometimes LITERALLY...and I know the difference between literally & figuratively...having their hands held throughout the entire dive, because they are incapable of effectively moving through the water themselves.) For people who are really opposed to things like having their air checked, it is, of course, possible to dive unguided or solo, in which case your safety is your own concern, and there's no DM/guide who has to explain "I didn't check that person's air because they didn't want me to." Again, the guide doesn't have to "touch" the customers' gear, but one way or another must be SURE that the air is on.

 

[DandyDon]

I always teach my students to have the valve either all the way open, or all the way closed. I'd rather have a stuck open valve than a valve 1/4 open as can be the case.

I like that. And I don't like anyone farting with my valves without my expressed approval. Actually, I may ask that it be turned on because I forgot :silly: but I have never seen that go bad. What I have seen is divers running out of air pressure at depth after some crew member turned one off, then half a turn on. :light:

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[vondo]

I've also seen a divemaster turn someones tank nearly off, open only a half turn. This individual probably had trouble with reverse image perception.

I've had this happen to me. Fortunately I noticed on the surface and turned it back on. It happened to someone else on the same liveaboard trip. That one was caught by someone else on the boat before the diver splashed.

 

[Al Mialkovsky]

We turn them on and leave them on. I do not like the partially turned on posistion as it can lead to being nearly turned on by some divers. Make it all the way on or all the way off.

 

[tonka97]

SCUBA gear is Life Support Gear

No one should touch or adjust your gear without your permission. At the least, divers should be asked before the dive if they want to opt out of last minute valve twists.:no

I turn my full valve full open, then just a smidgen toward closed. Let's me check it easily, if necessary, and be confident that it is not tightly closed. I always slightly inflate my bc and take breaths watching my gauge to confirm A-OK.

 

[Diver Dennis]

When I travel, 90% of people have their gear set up for them. I don't, I just tell them nicely that I want to set up my own stuff.