Crossbar and isolator valve: do they help?

[PfcAJ]

So, my dives are such that I could not complete my deco on back gas if I lost my deco gas. I am reliant on my buddy to give me the extra half of his deco gas if I lose mine.

I would recommend making some adjustments so that's not the case.

 

[Basking Ridge Diver]

So, my dives are such that I could not complete my deco on back gas if I lost my deco gas.

TDI/SDI suggests 2 times the amount of backgas on top of the bottom gas you would use - this is considered as reserve for deco...
Certainly that should be more than ample to complete deco?

 

[Beau640]

My scenario is currently only being trained to use 1 deco gas. I'm starting Trimix and will be trained for 2 gases pretty soon. But, at the moment, I dive according to my training and only use 1 deco gas. So, my dives are such that I could not complete my deco on back gas if I lost my deco gas. I am reliant on my buddy to give me the extra half of his deco gas if I lose mine.

Run it through subsurface or equivalent calculator. If you are using no more/planning for no more than 2/3 of gas for your dive and saving an entire third for contingency, with almost any deco dive sub 150ft that 1/3rd should be enough to get you out of the water if you lost your entire O2 bottle.

 

[Centrals]

My scenario is currently only being trained to use 1 deco gas. I'm starting Trimix and will be trained for 2 gases pretty soon. But, at the moment, I dive according to my training and only use 1 deco gas. So, my dives are such that I could not complete my deco on back gas if I lost my deco gas. I am reliant on my buddy to give me the extra half of his deco gas if I lose mine.

Can you bring an extra bottle of the same deco mix?

 

[leadduck]

Resurrecting the thread, going back to the initial topic:
(Note that this is about isolation vs. non-isolation manifolds. It is not about independent doubles / sidemount).

Seems to me that in-water burst disk failure is the main reason for having the isolation valve, instead of using a plain straight bar without a third valve:

Right post roll on/ valve break followed by a free flow would cook your goose without the isolator, plus the inability to isolate in event of an o-ring or burst disk issue (and everyone in America has burst disks).

I have two friends who have had in-water burst disk issues. It happens, even if it's rare. I'm not too keen on drowning so I'll keep my isolator and make sure it's open

However, the isolator valve enables a new human error: accidentally diving with the isolator valve closed, or filling tanks with the isolator valve closed:

Another thing to consider is how many incidents have occurred that are attributable in some way to the presence of the isolator that would not have occurred in the absence of the isolator. For example, how many people have discovered an isolator that was closed at some point when it shouldn't have been?

I know of one serious accident and one fatality that both had isolator closures as contributing factors.

Gotta check it. It'll cook your goose.

fun fact for anyone curious. The founder of the WKPP and namesake of the Hogarthian system, who is still an active cave explorer fwiw, doesn't use an isolator on his doubles... Straight bar

There have been at least two fatalities that I am aware of that may possibly have been prevented if the isolator was opened. Both of those should have been caught in a pre-dive check, and at least one of those should have been caught during the dive when the pressure gauge wasn't dropping.

I also found this text: “Ideal” Manifolds… Not So Ideal? by Jeffrey Bozanic about the topic, saying
"One might argue that these incidents did not need to occur, and that it was the divers’ fault for not checking the isolation valve prior to their dives. I do not disagree with this. However, when a piece of equipment opens itself up to a multitude of cases of “pilot error,” while not providing any concrete improvement in other areas of safety, then the net result is one of additional risk with a commensurate reduction in safety. For this reason, and the history of misuse of the manifolds in the field, my belief is that we should go back to using the standard dual valve manifold of the 1980’s or adopt another type of technology."

Furthermore, burst disks on scuba tanks are required by law only in the US, but are forbidden in the EU for safety reasons. See Pressure Release Devices aka Burst Disk | Dive Gear Express®

So diving European style no-burst-disk tanks tips the scale even more in favour of a non-isolation manifold.
The only "near miss" incident I witnessed so far was also caused by a closed isolation manifold, on doubles without a burst disk.

Nevertheless, the isolation manifold is not only recommended but required by many tec classes in Europe. Why?

 

[helodriver87]

Nevertheless, the isolation manifold is not only recommended but required by many tec classes in Europe. Why?

It's an odds game. Think of all the o rings, moving parts, hoses, diaphragms, etc involved from the DIN o ring downstream. Compare that to the number of possible failure modes on the tank neck/valve. Plus, consider the wear on regs and hoses compared to something like a tank neck o ring. Based on that alone, if you're gonna see a gas source failure underwater, it's probably going to be reg related. I suspect that if we took a non scientific poll here of people who have had failures, they'd be predominantly reg or hose failures. The isolator manifold dramatically simplifies reg failures by providing access to all the gas after a simple shutdown and gives you an option for the less common failure types.

 

[leadduck]

It's an odds game. Think of all the o rings, moving parts, hoses, diaphragms, etc involved from the DIN o ring downstream. Compare that to the number of possible failure modes on the tank neck/valve. Plus, consider the wear on regs and hoses compared to something like a tank neck o ring. Based on that alone, if you're gonna see a gas source failure underwater, it's probably going to be reg related. I suspect that if we took a non scientific poll here of people who have had failures, they'd be predominantly reg or hose failures. The isolator manifold dramatically simplifies reg failures by providing access to all the gas after a simple shutdown and gives you an option for the less common failure types.

I'm sorry if my last question was not clear; I wasn't suggesting to remove the manifold, but to remove the valve in the manifold. Why do training agencies not recommend or at least allow the straight bar non-isolation manifold?

 

[ls23]

I'm sorry if my last question was not clear; I wasn't suggesting to remove the manifold, but to remove the valve in the manifold. Why do training agencies not recommend or at least allow the straight bar non-isolation manifold?

Because a burst disk failure isn't the only scenario you need the isolator valve for. There are also possibly problems with tank o-rings, barrel o-rings, or a complete valve failure + free flow like PfcAJ said earlier.

 

[leadduck]

Because a burst disk failure isn't the only scenario you need the isolator valve for. There are also possibly problems with tank o-rings, barrel o-rings, or a complete valve failure + free flow like PfcAJ said earlier.

True for the European non-burst-disk valves, the o-rings are still there. But in general still all of these problems are very unlikely and it's doubtful if there was ever a case where closing the isolator valve saved a diver's life. Might be because such failures happen with high tank pressures, i.e. while filling or in hot cars, but rarely at lower tank pressures of a cold tank mid dive.
Whereas there are plenty of cases where human error with the isolator valve (accidentally closed while filling or accidentally closed while diving) caused accidents and fatalities.

Of course everybody can say "I'm careful, it can't happen to me", but truth is that human error by distraction has happened to the best divers too, and it's wise to make one's gear more foolproof not only for beginners but for everybody if you use it often.

Hence on average over the complete population of divers, if a training agency would recommend the non-isolation manifold instead of the isolation manifold, their students' safety record after class should improve. I think the problem here is it's hard to convince people that taking away an option (not being able anymore to shut off the manifold) improves their safety. People tend to underestimate their personal risk of human error and overestimate risk of gear failure.

 

[ls23]

The combination of overhead environment and a scooter make these failures a possibility mid dive, however unlikely. A closed isolator valve will be caught with proper training and procedures. Taking away options to combat training shortcomings should not be the approach in technical diving.