Using back mount doubles as single tanks

[Lorenzoid]

You should be able to tell by checking your SPG. But you can just leave it open all the time?

One student, in the same class than I was doing, actually ran out of air during the class due to a closed manifold … when we asked him if he didn’t feel that it was weird that the SPG didn’t go down … he said he thought he had a good SAC rate … that was during a dive where we had to write down our remaining gas to infer our sac rate

My wife did that (she has a SB account but has rarely logged on, so I'm safe) many moons ago when we were first learning to dive doubles--and that was after taking the GUE Doubles Primer. We had done the GUE-EDGE pre-dive check, and she had checked her isolator, convinced herself it was open, and we proceeded to do our practice dive. We weren't exerting ourselves very much, mostly hovering above a platform at 30ft, so I suppose when she didn't see the SPG move she assumed she simply wasn't using much gas.

 

[rob.mwpropane]

With all valves fully open or fully closed, even if narked or panicked, I do not see myself having a problem at turning them properly

And this is exactly what every diver who ever died with their valve closed has probably thought. I "think" the absolute worst thing a diver can do is start to act as if "it'll never happen to me".

 

[rob.mwpropane]

You should be able to tell by checking your SPG. But you can just leave it open all the time?

I agree 100%, but we're all human. I practice valve drills on every dive, doesn't mean I'm exempt from making mistakes, it just means maybe I'm less likely then if I didn't do them.

 

[steinbil]

and every 15 minutes according to the SOPs

I do this on almost every single dive. If not every 15 minutes, at least once after descent and then another two during a 60-70min dive. Mostly for training, as the chance of a roll-off in OW is very slim. But once you're proficient at flow checks, it doesn't interrupt the dive at all. It takes a few seconds, and since you don't need the hands for swimming it's easy to do it in stride. It also gives me plenty of extra chances to catch an isolator mistake, so that added complexity doesn't worry me at all.

 

[steinbil]

I can 100% see how in a panicked diver turning the valve the wrong way can happen.

I'm wondering why the argument of the panicked diver turning a knob the wrong way keeps popping up. Can you describe a situation where this might occur?

The pre-dive check should not be done in panic. The check once you reach the bottom after descent should not be done in panic. Estimating your gas used before you look at the SPG to confirm every 5-10 minutes should not be done in panic. All of these will catch an isolator mistake before panic sets in.

And if the valves are fully open, and dung hits the air-blower, there is no "wrong way". If you want to close a valve, turn it the only way possible. If you want to isolate, same thing - turn it the only way it can be turned.

The only case I can think of right now is if you have a reg/valve failure and close the wrong valve, and then in a panicked state you forget which valve you're breathing from and what you have closed and you end up closing both valves. But that is not really an isolator issue...

 

[BlueTrin]

I agree 100%, but we're all human. I practice valve drills on every dive, doesn't mean I'm exempt from making mistakes, it just means maybe I'm less likely then if I didn't do them.

Agree, I do a random check every x mins during the dive and estimate my SAC looking at my SPG during the dive.

This is why I believe it’s unlikely to happen to me, there are a lot of potential safeguards:

• I would need to miss it during assembling, I don’t close my manifold after
• I would need to miss it when kitting up
• I would need to not check it after jumping
• I would need to not notice that my SPG does not go down or not think that it can be the manifold

If that happened, you’d probably purge your backup and switch if air comes out so you’d probably realise that it’s the manifold.

Not saying it cannot happen (forgot a couple of checks in these steps and maybe someone touched your manifold, or maybe you did a valve drill and didn’t check after they are all in the right position after aborting OR buddy didn’t look and you had a brain fart after a valve drill)

 

[rob.mwpropane]

I'm wondering why the argument of the panicked diver turning a knob the wrong way keeps popping up. Can you describe a situation where this might occur?

The pre-dive check should not be done in panic. The check once you reach the bottom after descent should not be done in panic. Estimating your gas used before you look at the SPG to confirm every 5-10 minutes should not be done in panic. All of these will catch an isolator mistake before panic sets in.

And if the valves are fully open, and dung hits the air-blower, there is no "wrong way". If you want to close a valve, turn it the only way possible. If you want to isolate, same thing - turn it the only way it can be turned.

The only case I can think of right now is if you have a reg/valve failure and close the wrong valve, and then in a panicked state you forget which valve you're breathing from and what you have closed and you end up closing both valves. But that is not really an isolator issue...

I'm only assuming based on the thread that I linked that those found dead with a gas left panicked. Maybe some were medical incidents, who knows. Maybe narked? I don't know.

I agree that you "should" be able to form a skill set that prevents or greatly minimizes the chance for there to be an issue.

I've never ran out of gas, I've never had an isolator issue, I "think" I've trained these issues out of my human brain, but never is a strong word. Humans inevitably make mistakes. That's a guarantee. We've all shown up to a dive either without fins, or forgot the hood.. maybe no lights. Why is the manifold any different? I would like to think that it can't happen to me, I've trained it out, but people with worlds more experience then me have died with air left. Enough people to pause and think based on that other thread (that no one seems to have actually read).

Agree, I do a random check every x mins during the dive and estimate my SAC looking at my SPG during the dive.

This is why I believe it’s unlikely to happen to me, there are a lot of potential safeguards:

• I would need to miss it during assembling, I don’t close my manifold after
• I would need to miss it when kitting up
• I would need to not check it after jumping
• I would need to not notice that my SPG does not go down or not think that it can be the manifold

If that happened, you’d probably purge your backup and switch if air comes out so you’d probably realise that it’s the manifold.

Not saying it cannot happen (forgot a couple of checks in these steps and maybe someone touched your manifold, or maybe you did a valve drill and didn’t check after they are all in the right position after aborting OR buddy didn’t look and you had a brain fart after a valve drill)

I don't know why it would happen, I just have it in the back of my mind that it can happen.

Where's Gareth Lock when you need him!?! Lol. I think he does worlds better at explaining the human factor, which will make mistakes one day. We all do. I don't have answers for any of it, I still dive with an isolated manifold just like you all do.

 

[steinbil]

Enough people to pause and think based on that other thread (that no one seems to have actually read).

I actually did! I just found the arguments @PfcAJ put forth then and now to be more compelling.

 

[Angelo Farina]

Most twin tanks nowadays are equipped with a manifold with a third isolation valve.
My one is old, and it has a normal manifold without that central valve.
I evaluated the possibility to switch to a modern manifold with this third valve.
But after a proper risk assessment, and based on a number of accidents and near-misses I had or witnessed, in the end I evaluated that the benefits of the isolation manifold do not outweight the disadvantages and risks.
In none of the aforementioned accidents an isolation valve had been of any advantage.
I hade a couple of O-ring extrusions, and the cure was to close the affected valve.
Same for a couple of free flows.
In all these cases I have lost just 20 bar or less, and I kept a lot of air in both tanks. So there was no urgent need to thumb the dive.
If I had closed the isolation manifold, I had lost half of the air, forcing an immediate end of the dive.

 

[steinbil]

Where's Gareth Lock when you need him!?! Lol. I think he does worlds better at explaining the human factor, which will make mistakes one day. We all do. I don't have answers for any of it, I still dive with an isolated manifold just like you all do.

I just wanted to point out that there are ways of mitigating the risk of human mistakes with isolator valves, other than eliminating it. Training, procedures and practice all contribute to dealing with the human factors. If you think the risk of a closed isolator is too high, why not change the procedures to mitigate it? You could use checklists, always analyze both posts, use buddy/team confirmation and pre and post splash protocols. Pretty sure Garett Lock dives with an isolator when he's on OC...