42 replies to this topic

[Dive_Girl]

The most commonly taught practice of turning on a scuba tank is to turn on the tank all the way (i.e. until it stops), then twist the knob back one-half turn.

Why? Is this what you do?

I was taught this method and one I practiced, that is until one interesting recreational dive I went on. After setting up and conducting my gear check I turned to begin donning my suit when some "helpful" other diver came behind me and "checked" my tank for me. He thought my air wasn't on and turned it all the way off and then back on half a turn. I discovered the issue at 90' on a deep wall. My buddy could not understand why I was showing him my pressure gauge with needle swinging every time I breathed in as I was pointing to my back. I ended up having to make an adjustment and turned my own air back on while hovering at 90'. I have read of other similar circumstances where the outcome was not as inconsequential.

I understand one thought is that cylinders that are going to be on for long periods of time are turned back the half turn to avoid the valve sticking in the on position. I do not believe a recreational scuba dive fits this scenario.

One possible scenario I have seen involving the need to shut a tank down was where a diver's regulator massively free flowed because of the near freezing water temperature and as a result the first stage iced over and the diver assisting couldn't turn the tank off because it simply froze over. It actually then froze his glove to the valve as well (he was eventually able to peel it off with some sharp tugging). It didn't matter what position the valve handle was in as the entire first stage and valve was frozen.

I really like to understand why something is taught in a particular way and prefer to avoid having non-applicable or no longer applicable info passed along simply "because that's the way we've always done it." So is the half turn back applicable in scuba?

[Cajun Diver]

I was taught to turn it back that way when the DM or whoever checked the tank wouldn't think it was off and "turn it back on". It's supposed to prevent exactly what you describe.

[Hipshot]

The most commonly taught practice of turning on a scuba tank is to turn on the tank all the way (i.e. until it stops), then twist the knob back one-half turn.

Why? Is this what you do?

I was taught this method and one I practiced, that is until one interesting recreational dive I went on. After setting up and conducting my gear check I turned to begin donning my suit when some "helpful" other diver came behind me and "checked" my tank for me. He thought my air wasn't on and turned it all the way off and then back on half a turn. I discovered the issue at 90' on a deep wall. My buddy could not understand why I was showing him my pressure gauge with needle swinging every time I breathed in as I was pointing to my back. I ended up having to make an adjustment and turned my own air back on while hovering at 90'. I have read of other similar circumstances where the outcome was not as inconsequential.

I understand one thought is that cylinders that are going to be on for long periods of time are turned back the half turn to avoid the valve sticking in the on position. I do not believe a recreational scuba dive fit this scenario.

One possible scenario I have seen involving the need to shut a tank down was where a diver's regulator massively free flowed because of the near freezing water temperature and as a result the first stage iced over and the diver assisting couldn't turn the tank off because it simply froze over. It actually then froze his glove to the valve as well (he was eventually able to peel it off with some sharp tugging). It didn't matter what position the valve handle was in.

I really like to understand why something is taught in a particular way and prefer to avoid having non-applicable or no longer applicable info passed along simply "because that's the way we've always done it." So is the half turn back applicable in scuba?

As was explained to me:

As a tank's air becomes depleted, it will put greater pressure on the valve's seats unless the valve is partially closed. One half-turn doesn't functionally effect the flowrate, while it does give the valve greater longevity.

Rick



A fanatic is a person who can't change his mind and won't change the subject.--Sir Winston Churchill

[Quero]

I have seen tanks valves get stuck in the on position more than once when divers have failed to turn that half-twist back, even after dives at recreational scuba depths. So you may want to adjust that particular belief.

From a safety perspective, it doesn't much matter that it's stuck on "on" since a diver won't have any trouble breathing during the dive, but when s/he gets out of the water and needs to disassemble the reg set from the tank, it can take quite a bit of strength to get the valve unstuck in order to close it, especially for people with smaller grips, like women. As well, the stress this kind of force puts on the valve knob can make it get wobbly after having to wrestle with it repeatedly. Perhaps if a diver always rents tanks, this becomes "somebody else's" problem, but if a diver has his/her own tanks, s/he'll want to care for them with as much attention as for his/her do other equipment.

As for the particular air starvation problem described in the OP, it can easily be avoided altogether by performing a complete pre-dive safety check immediately before entering the water. The way I teach the "A = air" part of the pre-dive safety check is to have the buddy pair look at the SPG while purging (or breathing from) one of the second stages. If the valve is not far enough open, that dancing needle thing will become apparent.

So yeah, I always turn the half-turn back. I don't think leaving the valve all the way open is the best solution to avoiding an air starvation episode on the dive; furthermore, doing so does cause significant and avoidable wear and tear on the tank valve.

Edited by Quero, 11 June 2009 - 07:06 PM.

[JimG]

I consider the "half turn back" to be tantamount to the SCUBA version of an urban legend.

I always turn my valves full on until they "just stop" (i.e. do not wrench them on) or else leave them full off - no in between. That removes all ambiguity as to whether they are on or off, and which direction they need to rotate to be turned off or on (this can be a real issue if you ever dive doubles). I also watch the SPG while test breathing the tank, and for the same reasons that Dive_Girl mentioned.

I've serviced plenty of my own valves over the years (I personally own four sets of doubles, three singles, and about a dozen stage/deco/argon bottles), and the notion that a half turn back reduces wear on the seat makes no sense to me. A typical valve only has one "moving" part, which is the threaded slug that encapsulates the seat. The seat itself is fully unloaded as soon as you make the first half turn on, so going halfway on or almost all the way on or full on shouldn't increase any stress on it. The thing about a reduction in tank pressure leading to an increase in pressure on the valve mechanism makes no sense to me - in point of fact, the opposite would be true.

If the valve stem threads are galled or corroded toward the knob end, then that could certainly cause a problem. Routine (and regular) inspection and service on the valve should prevent that from happening however.

FWIW, I have never had or seen a valve stick "on" in almost 20 years of diving, and over 15 years of teaching diving.

-JimG

[dustbowl diver]

I can say that I had a buddy 'turn' my air on for me in Belize. The local authorities are still trying to recover his body!!!!

[Mermaid Lady]

I turn mine on fully and then back off a quarter turn. Right before I enter the water I reach back and check it (still flexible enough to do so), just in case someone has been "helpful".

Oh yeah, and I have started looking more carefully at the gauge whilst test breathing prior to entry.

Edited by Mermaid Lady, 11 June 2009 - 08:33 PM.

[Landlocked Dive Nut]

Righty-tighty, lefty-loosey. What's the confusion on whether a tank is on or off, regardless of whether the tank knob is backed off slightly or not? Even if you're checking from the "back side" of the tank! The same saying applies to nuts/bolts, screws, lightbulbs, and all kinds of everyday stuff.

I turn my tank valve knob back slightly, too, mainly just because I was taught to, and I could not come up with any mechanical reason not to follow what I was taught and my diving was not affected by following the practice.

Jim, your experience with the inner workings of the tank valve was interesting to read. It sounds like "no harm, no foul" applies for both backing off the knob and not backing off the knob from a mechinical standpoint. So if it doesn't hurt the valve either way, then logically the only reason to back off the knob (or not) would be if personal experiences prove it's a wise thing to do.

Any other personal experiences for or against the backing off the tank knob?

[Quero]

FWIW, I have never had or seen a valve stick "on" in almost 20 years of diving, and over 15 years of teaching diving.

-JimG

Lucky you! Granted that the stuck valve doesn't happen every time (or even most times) when divers don't do the half turn back, but *every* time I have seen one get stuck open the diver has failed to do that little twist back. The fact that it does, in fact, happen (even though it is outside your own personal experience) means that it cannot be dismissed simply an "urban legend."

Your explanation of your own inspection and maintenance of your personal tanks gives you an authoritative voice which is mirrored in my own equipment maintenance experience (I own over 50 tanks and six sets of doubles, though I typically get somebody else to do the servicing). Yes, theoretically, there is no reason that a valve being turned all the way on, and only finger tight, should stick. But the fact is that in practice valve handwheels do stick. It could be that the divers who come up and discover stuck handwheels have simply cranked the valve open too far, making it hard to close, or it could be that our tanks here see a lot more wear and tear than yours do since they undergo an average of three fills a day, seven days a week, and of course all of the associated open-close-open-close-open-close .....

Regardless of whether the stuck valve is due to over-opening or slight wear and a need for a cleaning/repair, the simple precaution of a quarter or a half turn back will ensure that the valve doesn't get stuck in the open position. A properly performed pre-dive safety check will also ensure that the diver doesn't "get confused" about whether the valve is sufficiently open.

Edited by Quero, 11 June 2009 - 10:53 PM.

[Wakemaker]

...From a safety perspective, it doesn't much matter that it's stuck on "on" since a diver won't have any trouble breathing during the dive...

Yes! If something is going to malfunction, it should be in the diver's favor! I concur with Hipshot, PlatypusMan and Quero. Valves can be impacted by the high pressure in the tank when you start breathing off the system.

ALL of my friends on the job back-off a quarter turn. Firefighters to marine engineers. If they don't, their probably not my friend. Firefighters do it when they don their breathing aparatus. It's a sound habit! If you work with gate valves and don't back it off, you're going to get a whipp'n from the boss. We especially do this on gate valves. Some kinds of valves are prone to getting stuck open due to pressure in the seal when all the way open (or all the way closed too) if there is high pressure involved. Some types are simply not vulernable to seizing in this manner.

I've never researched what kinds of valves we divers use on our equipment; check valves, relief valves, gate valves, butterfly valve, ball valves... I wonder. There might be all these, and more, on a dive boat.

Does anyone else have their tanks serviced if ambient air is suspected to have entered their tank? I always end my dive with something left. I want to shut my tank valve, and clean my gear. The valve needs to function so I can get that properly accomplished. An idiom for divers; Take care of your dive gear and your dive gear will take care of you.

Edited by Wakemaker, 12 June 2009 - 07:19 AM.

[JimG]

Cylinder valves are of the type that is commonly referred to as a "globe" valve. They are very similar in construction to the common faucet. Scubapro sold a SCUBA ball valve a few years ago, but as far as I know they are no longer manufactured. I have a couple of them that I use on my argon bottles. The issue with gate valves is that the fluid flowing through the valve puts lateral pressure on the gate mechanism, which can jam it. With most other types of valves, the pressure is applied axially along the stem, which is less of a problem.

With respect to the confusion over whether a valve is on or off, it's not the pre-dive checks that I am worried about. It's what happens when you are shutting the valve down in an emergency. If the valve is fully on, then there is only one possible way to turn it to shut it off. This helps eliminate confusion, particularly when dealing with an isolator or left post valve on a set of doubles. In an overhead environment, it's possible to brush the left post partially closed, and turning the knob back to start with can exacerbate that effect. Obviously that situation would not apply to recreational diving, but I tend to be a simple-minded guy, and prefer simple-minded procedures. "All the way" on or "all the way off" is about as simple as it gets, and works for any type of SCUBA valve, and in any situation.

Note that I don't claim that SCUBA valves "cannot possibly" or "will never" stick on. What I am saying is that if they are properly maintained, then it is virtually impossible for them to stick on when fully opened. What happens is that dirt and corrosion can enter in through the stem area, causing the threads to fill with rust or grit over time. If the valves are taken apart and cleaned periodically, then that will not typically cause a problem. Since the valve has to be removed anyway for annual inspection of the cylinder, that's a pretty good time to have a look at it and see if it needs service. It should turn smoothly and freely throughout the entire range of motion - if it doesn't, or if there is any sign of corrosion around the stem, then take it apart and fix it.

In every case where I have had a problem with a valve, it was due to infrequent maintenance. A simple cleaning and replacement of the worn parts will almost always bring it right back to "like new" condition.

-JimG

Edited by JimG, 12 June 2009 - 09:05 AM.

[Dive_Girl]

I consider the "half turn back" to be tantamount to the SCUBA version of an urban legend.

I always turn my valves full on until they "just stop" (i.e. do not wrench them on) or else leave them full off - no in between. That removes all ambiguity as to whether they are on or off, and which direction they need to rotate to be turned off or on (this can be a real issue if you ever dive doubles). I also watch the SPG while test breathing the tank, and for the same reasons that Dive_Girl mentioned.

I've serviced plenty of my own valves over the years (I personally own four sets of doubles, three singles, and about a dozen stage/deco/argon bottles), and the notion that a half turn back reduces wear on the seat makes no sense to me. A typical valve only has one "moving" part, which is the threaded slug that encapsulates the seat. The seat itself is fully unloaded as soon as you make the first half turn on, so going halfway on or almost all the way on or full on shouldn't increase any stress on it. The thing about a reduction in tank pressure leading to an increase in pressure on the valve mechanism makes no sense to me - in point of fact, the opposite would be true.

If the valve stem threads are galled or corroded toward the knob end, then that could certainly cause a problem. Routine (and regular) inspection and service on the valve should prevent that from happening however.

FWIW, I have never had or seen a valve stick "on" in almost 20 years of diving, and over 15 years of teaching diving.

-JimG

I am in line with these thoughts as well. However, Quero, I respect your experience too. Interestingly I too have the similar amount of diving and dive professional experience as both of you and I work with a fleet of 30-50 tanks including aluminum and steel tanks of all sizes and doubles. We also teach hundreds of students each year in harsh conditions. Although other of my staff instructors teach the roll back method, I no longer do as I have never experienced or seen a stuck valve. I have, however, seen first stages that have remained stuck even after they have been depressurized. The first stage issues typically occcur because of one or two reasons or a combination of both: 1) corroded yoke screw or DIN screw ; and/or 2) first stage over tightened during assembly. I could see the same reasons being applied to tank valves, but I am not so certain I can say we know exactly when the valve became stuck - upon assembly or after the dive, because you typically only discover it is stuck after the dive when you are dissasembling.

Why would the duration of a 30-60 minute dive where you are continually decreasing the tank pressure cause the valve to stick open? Wouldn't it really already be stuck the moment it was cranked open? Perhaps the diver "didn't roll it back" becuase they couldn't, they dove with it, and then left the problem to the boat crew.

This is a great conversation as I really enjoy understanding the detailed why behind some very routine things we do in scuba that are explained in detail less and less. I especially appreciate those of you, such as Jim and Quero, sharing your real experiences.

[JimG]

Interestingly I too have the similar amount of diving and dive professional experience as both of you and I work with a fleet of 30-50 tanks including aluminum and steel tanks of all sizes and doubles. We also teach hundreds of students each year in harsh conditions. Although other of my staff instructors teach the roll back method, I no longer so as I have never experienced or seen a stuck valve, however other of my staff instructors do.

I teach at a University that owns its own tanks (about 30 of them). On average we have about a class a week in the water (8-10 students and staff at a time) and we have never experience any problems with sticking. The tanks almost never go in salt water though, so that might be the reason. Fortunately I am in a position to set policy about what is taught, so we teach "full on" in our courses.

Why would the duration of a 30-60 minute dive where you are continually decreasing the tank pressure cause the valve to stick open? Wouldn't it really already be stuck the moment it was cranked open?

I'll take a crack at explaining this. It's actually true that decreasing cylinder pressure can affect the mechanism slightly. As the pressure decreases, the threaded seat slug can relax slightly in the bore, and if there is any crufty stuff in there, then it might cause the mechanism to bind. This will be more of a problem when the valve is open and the slug is out towards the knob end, which is a more likely location to collect dust, rust, and contamination from salt water. Regular cleaning and lubrication of the valve mechanism can eliminate this problem however, which is the basis for my comment that "yes it's possible, but it can be 100% avoided".

One other thing to keep in mind is that high O2 pressure can cause the stem O-rings and teflon bushings to flake or shred, so if you are PP blending your mixed gas, then that will cause accelerated wear as well. If the flaked off pieces get ground up in the threads, then that will cause problems (in fact, this is the most common issue that I have with my personal cylinders). My experience however, has been that the wear becomes evident long before the sticking begins, so if you pay attention to how the knob turns (with most valves it should be smooth enough to do with just your thumb and forefinger), then you will catch it before it catches you.

Again, the key here is regular inspection and maintenance. As Wakemaker said - "Take care of your gear and it will take care of you".

-JimG

[VADiver]

I am in line with these thoughts as well. However, Quero, I respect your experience too.

Nicolle, ever the peacemaker! lol

How are you doing?

BTW, I keep my valves open all the way during a dive. and look at the SPG while I pre-breath the reg.