Why do you not turn a tank full on ?

[SOUTH DEVON]

JHAM once bubbled...
I have to admit, I've been told to do this and do. I however never knew the reason. I really never thought to ask.
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well I'm glad I'm not the only one lol !
I think it's because when you're learning you're been told so many new things that you learn in a parrot fashion to try and absorb all the information particularly when learning in a crash course!
Anyway thanks guys for the information.

 

[Genesis]

is that you're not sure if its (mostly) on or (mostly) off, or which way to turn it if its not!

Consider this - you hit the water, suck, and get nothing.

You reach back for your valve. The first way you turn it moves. You continue and hit a stop. You just CLOSED IT, and still have no gas! Ditto if you have it on and need it off NOW.

If its against the "on" stop, but just lightly, then you KNOW which way to crank it for the opposite state.

Now on a SHIP, yes, FredT is right. Same for process control applications. But Scuba is neither, as valves in the former two are often not moved for days, months or even years until suddenly they NEED TO BE moved RIGHT NOW.

Scuba valves today are a bit different. When you need to move them, you need to move them RIGHT NOW, but you also need to know with some reliability WHICH WAY to move them, and you're usually, when in that situation, not looking at them at the time.

If, for example, you lose a burst disc underwater on a manifold, you must isolate quickly, or you risk losing ALL the gas in both tanks. Turning the isolator the wrong way could easily cost you enough gas to screw you. Since the "usual protocol" is to close the post you believe is leaking first (since the most common failure is in the first stage, not the burst valve) if THAT is also not up against a stop then you potentially have to make TWO mistakes before you stop the leak. That's quite a bit of extra time in a situation where you don't have any.

I want my tank valves LIGHTLY against the open stop when in actual use. Not hard against, just lightly against. If I try to turn it the wrong way in the heat of the moment, I thus know INSTANTLY that I've done so.

If the valves are stiff or otherwise hard to turn then you need to service them and fix that. Unlike gate valves and such which may not be torn down for years at a time scuba valves usually either are or should be.

 

[BILLB]

What an interesting set of explanations. I personally do not like gimmicks that are used to replace knowledge and mastery of skills.

My students are instructed to turn the tank valve completely on. Which way is completely ON is a skill that every OW student MUST master. As a diver, you must first be able to do all the required skills without assistance from others. Depending on others to check your tank valve is a buddy function but without the gimmick of a 1/4 turn, etc.

As stated in this thread, there are no longer any mechanical reason to continue the practice of backing off 1/4 turn. Cave Divers always turn the tank completely on.

 

[MechDiver]

SOUTH DEVON once bubbled...

JHAM once bubbled...

I think it's because when you're learning you're been told so many new things that you learn in a parrot fashion to try and absorb all the information particularly when learning in a crash course!

This is probably one of the scariest things I've read on this board yet.

 

[drbill]

Why RICHinNC, you're not an "old guy." You were probably just a freshman when I was a senior. By the way... some of us "old guys" can oitdive the twentysomethings (well, at least some of them).

A J-valve of course.

This us an interesting thread because I'd always wondered this too. My instructor may have said why, but that was back in the 60's so I can't be expected to remember it.

Dr. Bill

 

[fritzer]

I think the original reason a lot of instructors used to (and some still do) teach their students to turn the valve back a quarter-turn was to keep the valve from becoming stuck in the open position after being at depth. Supposedly, and it seems feasible, the pressure at depth can actually tighten the valve to the point that it's impossible to turn once you're back on the surface. There's been a lot of flack about this in recent years because of potential accidents due to a buddy turning a valve off because they didn't think it was all the way open.

 

[Don Burke]

fritzer once bubbled... I think the original reason a lot of instructors used to (and some still do) teach their students to turn the valve back a quarter-turn was to keep the valve from becoming stuck in the open position after being at depth. Supposedly, and it seems feasible, the pressure at depth can actually tighten the valve to the point that it's impossible to turn once you're back on the surface. There's been a lot of flack about this in recent years because of potential accidents due to a buddy turning a valve off because they didn't think it was all the way open.

I think the idea of water pressure tightening the valve is one of those "submerged legends" we have to deal with from time to time.

On some valve designs, you can really screw up the valve if you open it hard into the backseat. Some valves are expected to be backseated (I saw some of those in nuclear power plants.).

Your garden variety SCUBA tank valve deals with backseating pretty well these days. I've never seen one stuck on the backseat and it may indeed be impossible to get a modern one stuck.

I still turn the knob back just a bit since I'm looking for the loose feel of an open valve when I check it open. It's an old submarine habit that has served me well.

 

[MikeFerrara]

Wow!

Here's the thing, if a valve gets turned off instead of on and then 1/4 or 1/2 turn back you'll be able to breath on it just fine. That is until you get to some depth when the demand is more than the flow. You might be at 50 ft.

Now if some one turns your air off on you you can only breath untill the hose pressure drops. In other words you'll know be fore you get very far.

I teach that we only EVER use two positions of the valve and those are ALL the way on and ALL the way off.

 

[sasdasdaf]

Hence the importance of being able to manipulate your valves underwater quickly and instinctively.

 

[MCL]

I learned to turn the tank all the way on or all the way off, but my instructor was a cave diver and related all of the OW stuff to that.