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Tanks on - half turn back or not?


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#31 peterbj7

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Posted 18 June 2009 - 09:23 AM

For cave diving you want to adjust the manifold so the valve is not likely to brush against anything. Then you can safely have it barely on. In any case, you can check your gauges periodically to make sure pressures are equal. IMO rather that than lose a lot of air from both tanks if a 2nd stage lets go.

Of course, even better is to dive caves closed circuit :teeth:

#32 JimG

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Posted 18 June 2009 - 10:42 AM

For cave diving you want to adjust the manifold so the valve is not likely to brush against anything. Then you can safely have it barely on. In any case, you can check your gauges periodically to make sure pressures are equal.

I have my isolator adjusted as you describe, but I am going to respectfully disagree with your recommendation of "barely on". As I discussed in a previous posting, there is a chance that in the rush to shut it down, you could actually turn it the wrong way (i.e. open instead of closed). Again, "full on" reduces the chances that that will happen, as there is only one way to turn it. And I only dive with one SPG on my doubles, so checking for equal pressures wouldn't do me any good anyway.

Furthermore, it's my view that relying on a quick shutdown of the iso to preserve gas in an emergency, suggests to me that the overall gas plan is probably flawed to begin with (just my opinion, of course). Better to take extra reserves, or else dive in a three person team.

Of course, even better is to dive caves closed circuit :cool1:

Probably won't surprise you to learn that I am not a big fan of CCR either (and I am not talking about Creedence Clearwater Revival :thankyou:).

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#33 peterbj7

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Posted 18 June 2009 - 02:14 PM

there is a chance that in the rush to shut it down, you could actually turn it the wrong way

A bit hit-and-miss? Smacks to me of inadequate training/experience. Sounds as if there's an element of panic in dealing with the situation, and your training should most certainly have gone way beyond that.

I only dive with one SPG on my doubles, so checking for equal pressures wouldn't do me any good anyway

I think that's foolish, precisely because one reg could let go. After all, that's why you have the manifold in the first place - it isn't for the convenience of not having to switch regulators. I have never dived doubles without a pressure gauge on each post.

Furthermore, it's my view that relying on a quick shutdown of the iso to preserve gas in an emergency, suggests to me that the overall gas plan is probably flawed to begin with

Everything's relative. You know how quickly a tank can empty itself if there's a major failure of a first stage, or if a LP hose ruptures. With the manifold open you'll lose the entire contents of the twinset very quickly. Your gas planning allows for this contingency? Mine doesn't. What proportion of gas loss does your gas planning allow for?

#34 VADiver

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Posted 18 June 2009 - 02:54 PM

why do you need a pressure guage on each post? I'm with Jim on this... I have one SPG offthe left post and check it throughout the dive (every 5 minutes). If for some reason I have to shut down my left post the dive is called and our pre planned gas strategy will get the team to the 1st stop or the surface. Adding another SPG does not add anything to the dive and IMO is unneccessary.

Wwht is your valve shutdown protocol with doubles...do you isolate 1st or shutdown the right post? In an emergency my shutdown drill is
--Right Post (breath out reg), if gas is still escaping,
--Isolator

All the while I'm signaling my buddy to take a look and see whats what. A proper buddy should be there immediately, just in case. Even with a total gas loss proper gas planning will get you and your buddy to the 1st stop or the surface--just like in an OOG scenario.

#35 peterbj7

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Posted 18 June 2009 - 03:18 PM

What if you're in a restricted environment? You're breathing from a twinset with no way of knowing what's in it. Doesn't appeal to me.

My valve shutdown order depends on what I think the problem is, and how severe. In a moderate leak I'd shut down the offending post first, but in a severe one I might shut down the manifold first. If I'm not sure I shut down everything and take the rig off to inspect it.

If I have a responsible buddy close to hand that's a bonus. I generally dive as if I'm alone, even if I'm not. With more extreme diving when we are diving as a team with team reserves that clearly is different, but then of course I DO have a responsible buddy!

#36 JimG

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Posted 18 June 2009 - 07:38 PM

there is a chance that in the rush to shut it down, you could actually turn it the wrong way

Sounds as if there's an element of panic in dealing with the situation, and your training should most certainly have gone way beyond that.

Not panic per se, but sometimes the mind plays funny tricks when the excrement hits the ventilating device. Everything in this type of diving boils down to choices that are based on a comparison of risk vs benefit. I personally see no benefit to leaving the iso "barely on", but a number of potential problems with it. In my view, the risks outweigh the benefits, so I choose "full on".

I have never dived doubles without a pressure gauge on each post.

And I have never dived (dove? diven?) doubles with more than one. :cheerleader:

You know how quickly a tank can empty itself if there's a major failure of a first stage, or if a LP hose ruptures. With the manifold open you'll lose the entire contents of the twinset very quickly. Your gas planning allows for this contingency? Mine doesn't. What proportion of gas loss does your gas planning allow for?

I am well aware of how fast you can lose your gas if there is a major failure. And yes, my gas plan does account for a complete and sudden loss of the back gas at maximum penetration. In fact, I typically plan my gas such that I could lose not only the back gas, but all my stage and deco bottles as well, and still have just enough reserve (in my buddy's tanks) to get us both out.

Just out of curiousity, what would be your response to a broken isolator that was leaking gas and could not be shut down?

What if you're in a restricted environment? You're breathing from a twinset with no way of knowing what's in it.

If you have to shut down a post, then you either have enough gas to get out or you don't. Knowing how much is left won't change that. If I were to shut down a right post for example, I probably wouldn't even bother to look at the SPG, as it provides no useful information in the context of what's going on with the dive at that point. And of course once I get back to my stage bottle, it's a moot issue.

And in truth, the SPG is really just a backup to the bottom timer anyway, which is right on my wrist in front of me. At this level, a diver should have a good enough handle on their own consumption rate that they can estimate their remaining gas based on depth and run time. I typically only check my SPG about 3-4 times during the penetration, and almost never on the exit.

My valve shutdown order depends on what I think the problem is, and how severe. In a moderate leak I'd shut down the offending post first, but in a severe one I might shut down the manifold first. If I'm not sure I shut down everything and take the rig off to inspect it.

I tend to prefer a more systematic approach. Sometimes sound can be tricky underwater, and it may not be clear which post is the problem. I would never consider taking my rig off in a cave - that seems like a recipe for disaster. Better to have your buddy to look at it, since he or she is usually in a much better position to diagnose and/or correct the problem. In fact, that would be my preferred action even on an open water dive - I just don't see any benefit to removing the rig when your buddy is right there.

I generally dive as if I'm alone, even if I'm not. With more extreme diving when we are diving as a team with team reserves that clearly is different, but then of course I DO have a responsible buddy!

This is where you and I differ. I would never contemplate doing such a dive without a competent buddy. In my view, the choice of buddy is just as important (if not more so) as the choice of kit. IMO, a buddy always make for a much safer and more relaxing dive experience.

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#37 diverdeb

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Posted 19 June 2009 - 08:54 AM

This is a great discussion with a lot of good info. That's to everyone for sharing their knowledge! :thankyou:
As for me, I'm feeling pretty scubalicious. 

#38 peterbj7

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Posted 19 June 2009 - 09:32 AM

Just out of curiousity, what would be your response to a broken isolator that was leaking gas and could not be shut down?

Quite clearly I'd bug out, and I would depend on the group gas plan to get me/us out of trouble. I'd obviously have one just as much as you do. But still things would be tight.

Cave diving obviously generally produces more extreme and immediate problems than open ocean, but some of what I've written above relates more to the one environment than the other.

I can assure you that my valve shutdown procedures are highly systematic, I just don't do the same thing every time regardless of the problems presented. Like you I did very thorough training which was highly systematic, and such training is essential to develop the reflex actions that may save your life. But not every situation is immediately life threatening and intelligent choices can be made.

In my experience the most immediately critical situations do not arise in an overhead environment per se, but when diving deep regardless of environment. Especially when using a CCR, with small amounts of gas on board. I've had my fair share of those and so far I'm still here!

Without labouring the point, I don't adhere to the DIR philosophy. Though I might employ it for refresher training after a layoff.

#39 JimG

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Posted 19 June 2009 - 11:26 AM

Like you I did very thorough training which was highly systematic, and such training is essential to develop the reflex actions that may save your life. But not every situation is immediately life threatening and intelligent choices can be made.

I agree - in fact, I would say that most situations are not "immediately life threatening". It's how we respond to them that makes them so. My view on this is that the right post (which is the one I breathe and control most of my buoyancy from) is the one most likely to experience a problem during a dive. So I would typically go there first to try and sort out the problem. From there I would move to the isolator, and then lastly to the left post. I would generally use that same procedure any time I suspected a gas leak, which was what I meant by "more systematic".

Without labouring the point, I don't adhere to the DIR philosophy. Though I might employ it for refresher training after a layoff.

Well I was trying to keep the "D word" out of this, but I guess my philosophy and technical roots are pretty evident from my signature. :thankyou: I know that DIR is not for everyone, but it certainly has worked well for me and my regular dive buddies. It gives me a great deal of comfort to know that we all approach our diving from the same mindset.

BTW, I want to thank you for keeping this dialogue so civil (and I hope I have done the same). It never hurts anything to compare differences in philosophy, as long as the discussion does not become personal or revert to name-calling.

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#40 drbill

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Posted 20 June 2009 - 10:20 AM

Had a DM in Tahiti turn my tank off after I had turned it on and ended up with no air on my giant stride. Grrr... don't touch my tank.

I have heard that the rationale given about turning it back half a turn was true with the early tank valves, but no longer applies. I can't verify that as I don't remember the source. Nonetheless, I turn my valve fully open and back half a turn, and test breathe my reg while watching the SPG needle before I submerge.

Edited by drbill, 20 June 2009 - 10:20 AM.


#41 drbill

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Posted 20 June 2009 - 10:25 AM

As for reaching my tank valve, I injured my shoulder years ago which left me with limited backwards movement. It is very difficult for me to reach my tank valve. However, I always dive with a pony and it is mounted upside down so I can easily reach its valve should I need air quickly.

#42 peterbj7

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Posted 20 June 2009 - 12:53 PM

I want to thank you for keeping this dialogue so civil (and I hope I have done the same). It never hurts anything to compare differences in philosophy, as long as the discussion does not become personal or revert to name-calling

I couldn't agree more. In any case, there are lots of ways of doing it right (sorry!) and one should try always to keep an open mind. I've never got annoyed with people suggesting doing things differently to the way I do them (in any case, I change from time to time as I learn more about diving), only with people who assert that there's only ONE way to do it, that of course being their way. The real value of this forum is precisely this sort of exchange. On another well-known diving forum which I won't name I have seen hackles rise far too often, and utterly pointlessly.

#43 JimG

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Posted 20 June 2009 - 01:21 PM

On another well-known diving forum which I won't name I have seen hackles rise far too often, and utterly pointlessly.

There are other forums besides SD? :cool2:

-JimG
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