Scott, the burst disk issue is irrelevant.  Most of us either shim the
disks or replace them with plugs anyway.  The disks are subject to
cyclic fatigue every time you fill and empty the tanks, and you dont
want one of these to fail and cause a catastrophic loss of gas when you
are x meters back in an overhead.  In the event of a significant
overpressure, the neck o-ring will extrude anyway.

The reason you need the isolator is that in an overhead, the valve
knobs can roll when contacting the ceiling, and in fact can break off. 
Take a look at the stem that the knob is connected to - it doesn't take
much.  In this situation, the right post will break off in the fully
open position (left will roll closed).  If you then have a regulator
failure on the right post, you can't shut this side off, and so need to
isolate to save half your gas.

Preventative measures you can take include changing the valve knobs - I
use the Sherwood rubber ones because they are resistant to impact and
very tactile.  On the isolator, if you don't torque down the retaining
nuts then the bar has some ability to rotate and absorb an impact. 
Also, check your valves every time you contact the ceiling.

-Sean


On Sat, 19 Feb 2000 20:55:25 EST, ScottBonis@ao*.co* wrote:

>Hi guys,
>
>I'm asking for some information.  I received a dual tank manifold that's a
little different from any I have previously seen.  So before I go ahead and
start using it in the caves here in Akumal, I appreciate any experience any of
you have had with it or similar products.
>
>The brand name and "S" symbol are from ScubaPro.  Both tank valves and the
center connecting piece say they were made in Italy.  The valves are of a 200
bar DIN type with the number "232 BAR" cast into their sides.  The right hand
valve cover is black and the left hand valve cover is light gray.  There is no
center isolation valve.
>
>An interesting feature of the valves is that they have no burst disks.  The
valve castings look as though they were never drilled and threaded for the
burst disks.  One concern I have is the strength of this portion of the casting
since I have never seen this configuration previously.  Any comments?
>
>I believe that one of the (if not "the") major reasons for an isolation valve
is the possibility of popping a burst disk, so I now need to evaluate whether
to consider the possibilitgy of using the manifold as is without an isolation
valve or to add one.  I'd really like input on whether the possibility of a
catastrophic problem with a tank O-ring, with both of the O-rings on one side
of the center connecting tube or with some other unusual manifold problem are
likely enough to warrant the addition of an isolation valve with the attendant
additional possible failure points.  With burst disks in place, the answer for
me is clearly yes, but without burst disks I'd like to get some advice from
this list.
>
>In advance, I'd like to thank you for any information and opinions that you
would care to send my way.
>
>Take care and safe diving,          Scott


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