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Lars-

>On a dual manifold with isolator valve. The tankpressure will always be =
on 
>the cross-over bar, no matter which valve is being closed. What will 
>happen in the event of a malfunction of the isolator valve that results =
in 
>a leak or fast gas loss. As I can see it, all gas will more ore less 
>rapidly, be lost and there is nothing that can be done to prevent it.

True, but not likely. What is more likely is a failure of a first or =
second stage. You shut down the valve with the problem regulator and =
you still have access to both tanks of gas. With independent doubles =
you loose whatever gas you have left in that tank. Should you have a =
catastrophic failure of a valve, you shut down your isolator, *then* =
you loose whatever air might be in that tank (well, it's gone =
anyway.) But, lets face it, the weak link is the regulators and =
*thats* what you plan for. To have the same margin with with =
independent doubles you would have to have an H valve with 2 =
regulators *per tank*, total of 4.

Of course for doubles w/isolator to work you need to be able to reach =
the valves quickly, and should practice this at some point during =
every dive. As a cheapskate, I sympathize with you as I know that the =
real reason you have independent doubles is so you don't have to =
spend $300-$400 on an manifold. In this case it's not worth it. You =
are playing russian roulette if you are decom diving using =
independent doubles.

>Personally I=B4m using independent doubles instead of the dual manifold. 
>This will of course give an extra number of regulator switches and some 
>added gas management. But on the other hand you will not have to isolate 
>any valve in the case of a freeflow, certainly you will the loose the 
>"side" that malfunctions, but it can never effect the other, hopefully 
>still working bottle. In combination with the thirds rule and =
equalization 
>(through regulator changes), I feel this is a very safe system. I know 
>that a large number of very experienced divers are relying on the 
>dual-manifol. So my question is, how is this problem being addressed and 
>secondly have anyone had a malfunction as described above.

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