Isolation Manifolds

[Sat Diver]

Hi,
I have a question as to whether there is any compelling reason to dive twin tanks with an isolation manifold and not just with twin regs and spg's.
I spent a large part of my career diving umbilical supply on oil platforms in the north sea, and even when we did the occasional open circuit expedition away from surface supply and the diving bell, we had a 4 tank configuration.
Recently, in the last 3 years since I retired and started diving for fun, I started diving separate tanks with no manifold for my 2 back tanks containing my bottom gas.
Just seems simpler and wanted to know if anyone thinks it's a bad idea.

 

[wedivebc]

Hi,
I have a question as to whether there is any compelling reason to dive twin tanks with an isolation manifold and not just with twin regs and spg's.
I spent a large part of my career diving umbilical supply on oil platforms in the north sea, and even when we did the occasional open circuit expedition away from surface supply and the diving bell, we had a 4 tank configuration.
Recently, in the last 3 years since I retired and started diving for fun, I started diving separate tanks with no manifold for my 2 back tanks containing my bottom gas.
Just seems simpler and wanted to know if anyone thinks it's a bad idea.

There are lots of people on here who will tell you it is a bad idea but most anyone who has actually used this system will tell you it has quite a few advantages and as long as you are careful about keeping track of consumption (which most experienced divers are) it can be at least as safe as a manifold if not more so (at least in theory). Enjoy your independent twins.
Having said that, this should be fun.

 

[parzdiver]

I think there are arguments for and against each configuration. I currently dive manifolded doubles, but have been considering an independent doubles setup for travel. When in the Caribbean, it is difficult to get manifolded doubles, but taking two AL-80's and strapping them together is easy. Also, if you lose one tank (blown o-ring) you don't need to worry about valve drills and isolating, you will always have the other tank.

The primary drawback I see for the independent doubles is the added complexity, having to manage two gas supplies during the dive, and a second reg/spg pair. Then again, this is something the sidemount folks and plenty of other divers are doing anyway.

There is always more than one way to skin a cat and I don't think either answer is "wrong" they are just different.

 

[Blackwood]

Manifolded doubles grant access to all your gas in the case of a post failure, independent doubled don't.

The decision to use one or the other is a personal one.

 

[Trace Malinowski]

The most compelling reason for not diving independent twins is that "the mean girls of diving" will laugh at you. Indie twins have become about as stylish as a polyester leisure suit. Sidemount allows you to do the same thing and you'll look far cooler, plus have total access to your cylinders for many forms of aggressive diving. The mean girls won't laugh. They'll know you can go places they can't.

 

[rx7diver]

My first doubles were twins (primarily because I needed to save up to purchase an isolation manifold for these HP 80's). Managing gas with them was not difficult at all (for me). More recently, I tried diving twin HP 120's. Again gas management was no problem. But moving the tanks around was a huge PITA, since there was no easy way to grip them. Twin HP 120's are long enough and heavy enough that trying to move them by gripping one post really doesn't work well at all. I also recently tried diving twin Al 80's, which worked very well--except they are suboptimal when diving dry.

I'm currently hoping to find a second PST 72 soon so that I can try twin 72's. I believe I will find that this setup will be exactly what I'm looking for, for doing deeper (though not deep) walk-out solo drysuit dives--a simpler set up IMHO than a single with slung buddy bottle, and a bit more reliable IMHO since no stand-by system (no buddy bottle) is involved.

Of course, for rocky or steep or otherwise difficult walk-out entries/exits, I'll probably prefer a single with slung buddy bottle, since this configuration will allow the rig to be separated for easier portage.

Safe Diving,

Ronald

 

[Sat Diver]

So far I don't see any compelling reason to not dive independent twins. I usually side mount my travel/ deco gas anyway, and I don't find it too complicated, as far I can see, you add an spy and subtract a manifold. I have not done any deep penetration of caves, since I do not have adequate training, but I have done deep dives (350 feet+) and more than 400ft in saturation though that was more than ten years ago. Now I do mostly wreck and reef diving, usually without any overhead environment.
I have been bent twice and had mild osteonecrosis which reversed itself. Neither of which have had long term I'll effects.

 

[Akimbo]

So far I don't see any compelling reason to not dive independent twins. ...

Scuba isolation manifolds for doubles are [-]not unlike[/-] similar to crossover manifolds in the gas shack/bell control console -- regulator failures and leaks can be isolated and different gas banks brought online independently (on a mini scale). Given the reliability of Scuba regulators in the recreational setting, failure mode analysis can offset the advantage due to diver complexity and failure potential of the gear you can't see on your back.

Is there ever a clear compelling reason for any added safety devices? Every added layer usually makes accurate assessment of a failure more complex, and the probability that a quick corrective measure will be wrong.

Remember that recreational divers don't train or know their systems like sat divers who live them 24/7 for months before even getting in the barrel.

... I spent a large part of my career diving umbilical supply on oil platforms in the north sea, and even when we did the occasional open circuit expedition away from surface supply and the diving bell, we had a 4 tank configuration...

Quad cylinders must be after my time, what are they like? How were they manifolded and how much did they carry?

 

[DaleC]

I'm another independant doubles diver and in the recreational realm I think they work just fine. While it may be true that an isolation manifold allows access to all ones gas in case of a post failure it is equally true that a post failure can mean the loss of all ones gas. Depends on how one looks at it.

This question comes up quite often and there is always a lively debate surrounding it. I have my own personal opinion, based on actually diving indies, but I also understand the reasoning of those who prefer it [-]the wrong [/-] a different way

... and I don't worry about the mean girls, they're all bulimic anyways.

 

[DaleC]

Given the reliability of Scuba regulators in the recreational setting, failure mode analysis can offset the advantage due to diver complexity and failure potential of the gear you can’t see on your back.

Is there ever a clear compelling reason for any added safety devices? Every added layer usually makes accurate assessment of a failure more complex, and the probability that a quick corrective measure will be wrong.

Akimbo, I actually understand that. One reason I prefer ID's over MD's recreationally is that I believe there is more of a risk due to incorrectly shutting down hard to reach valves (wasting time and/or losing gas) rather than encountering a circumstance that will require all available gas. I reserve rock bottom volumes in both tanks so I always have enough gas to surface safely.

I also believe that the skill needed for diving ID's safely (gas monitoring) is performed under normal, non stressed conditions while the skill needed for diving MD's safely (valve shutdown procedures) is performed under emergency, stressed circumstances. To recognise what this means one has to look at the great emphasis placed on practicing valve shutdown drills repetitively. Some divers do this at the beginning of every dive. This indicates that the actions won't ordinarily be reflexive under stress conditions and must be "ingrained" in the MD users muscle memory.

I rarely hear about that potential failure point in MD use brought up in ID use discussions, presumably because it seems to be assumed that divers will do the drills and perform the shutdowns correctly when they are actually needed. For me, the simpler my bailout plan is, the better. Failure: switch regs, ascend, figure it out on the surface.

Some may advocate that another disadvantage ID divers have is needing to perform reg switches every few 100 psi but that is a cave diving mindset and not required for recreational diving. My experience has been that I can breath the first tank down to rock bottom volume, do a reg switch, and finish on the other tank (using either Al 80's or St 72's).