Crossbar and isolator valve: do they help?

[2airishuman]

Manifolded twins have long been the gold standard for safety for technical dives.

I wonder how much safety benefit the isolator valve actually provides. Have there been any accidents or incidents where the ability (or lack of it) to close the isolator valve made a difference in the outcome of the dive? Have there been any accidents or incidents where a sensibly maintained single-cylinder configuration suffered an abrupt loss of gas below the valve after the dive was started?

I also wonder about the advantage of the crossbar itself. Sidemount diving has become accepted, and sidemount configurations don't have a connection between cylinders comparable to a crossbar. Do sidemount divers plan their gas based on loss of the most critical cylinder at the most critical point? Does sidemount provide a material advantage in safety over independent backmount doubles? Enough so that backmount doubles really do require a crossbar while sidemount does not?

 

[runsongas]

the isolation valve is to prevent an extruded neck oring or blown burst disc from causing loss of gas from both tanks. in a sidemount or independent backmount doubles, the tanks are separate so this isn't an issue. a rare failure, but possible.

the benefit of the manifold is allowing access to the gas from both tanks during a reg failure without resorting to valve feathering or a first stage swap underwater. valve feathering is easier to perform in sidemount.

 

[HantsDiver]

Sidemount isn't safer than a twinset. If the reg freeflows then you have to mess around with turning the valve on and off. With a twinset you just close the offending valve and get on with it.

I think you mean mannifold when you say crossbar, but it's only there to help you preserve the total gas in the event of a freeflow or problem.

Pretty much no one who partakes in technical diving will use a burst disk. Well certainly not anyone I know. And I'm not convinced that a protruding oring will suddenly implode underwater. If anything it's less likely to my mind. With din regs the chances of a valve failing on dive even if smashed are tiny. I mean really tiny. The isolator on the maniffold is really there in case you can't shut down the valve (either because it's jammed or because you are inept). It's also there because we can't see behind us so some people will go for it first to preserve some gas. However in reality you will hear which post has gone. If it's a hose that is a different matter and can be a pain.

 

[PfcAJ]

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

 

[HantsDiver]

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

If it happens then surely common sense dictates that you remove the problem. In this case the burst disks - which seems like an apt name as they seem prone to bursting when you are diving. I'm yet to hear of people being blown up with their tanks in more developed parts of the world which don't have these dangerous additions

 

[tbone1004]

If it happens then surely common sense dictates that you remove the problem. In this case the burst disks - which seems like an apt name as they seem prone to bursting when you are diving. I'm yet to hear of people being blown up with their tanks in more developed parts of the world which don't have these dangerous additions

illegal for tanks in the US to have burst discs removed btw. Doesn't stop us from doing it, but on any rental/shop tanks, you can't do it in this country per the DOT.

I'll stick to sidemount in overhead and manifolded doubles in OW. Feathering the valve is annoying, but oh well.

I am torn on the isolator being required or not in doubles, but tend to lean to better to have and not need. Most valves are prone to failing when open, so worst case you close it off and switch if the valve knob o-rings start leaking, but I don't want all of my gas to be in one source in the event of a tank neck o-ring or burst disc going *as unlikely as those are*

 

[stuartv]

I think the OP's question highlights a funny/ironic point.

There are people who are quick to dismiss many things in technical diving because "it adds a failure point."

Sidemount diving shows us clearly that pretty much any kind of diving can be accomplished safely while using independent doubles.

Having an isolator manifold clearly introduces numerous additional failure points, as compared to independent doubles.

And yet I suspect that some of the same people who dismiss other things for adding failure points are also people who would voice strong support for using an isolator manifold.

 

[Basking Ridge Diver]

I made a list for myself when I decided to dive ID's... Somewhat rudimentary but I wanted to consider my options...

 

[Akimbo]

@stuartv

I concur. One of the most important lessons in my career is that safety systems can conspire to make you less safe because troubleshooting gets more complex and adds failure points. US Navy saturation diving systems are the poster-child. Life, and safety, is all about choosing options that suck the least.

 

[Lorenzoid]

. . . Have there been any accidents or incidents where the ability (or lack of it) to close the isolator valve made a difference in the outcome of the dive? . . .

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?