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Hi,

(Note: I posted this question in techdiver too)

I've been looking at the possibility of putting together a set of twin 80 
cu. ft. tanks and taking a deep air diving course. The diving I'd like to 
do would be more long bottom time diving than deep, in order to get more 
time on wrecks in the 100-120 foot range in warm water. This would not 
necessarily be penetration wreck diving but could include that in the future.

I have seen two types of setups for the valves in double tanks in local dive 
shops. One has twin valves, each with its own cutoff knob, connected by a pipe, 
with two standard fittings for yoke-type first stages. The two separate valves 
allow each tank to be cut off separately in case of a blown O-ring or other 
malfunction while allowing the remaining tank to be used.

This arrangement costs in the range of $350. You must dedicate two tanks to it 
as well. If you want to make a second dive with doubles you have to either get 
a second set of doubles or fill the tanks after the first dive, which is not 
always possible on a boat.

The other arrangement is essentially a pipe with yoke-type connectors on each 
end, just like on the first stage of a regulator. Each end of the pipe is 
clamped onto two standard scuba tanks. A yoke fitting in the middle of the pipe 
allows the regulator to be mounted. The standard valves on the tanks allow each 
tank to be cut off separately but would not allow the continued use of the good 
tank in the event that an O-ring blows on one.

This setup costs about $110 and would allow the tanks to be used easily as 
singles or doubles. It would also allow you to carry or rent 4 tanks to make 
two dives with doubles. You can take the connector pipe with you on trips and 
rent standard tanks when you arrive to make your dives, which wouldn't be 
practical with the dedicated setup.

The first setup is obviously better in the event of an O-ring or first stage 
failure, or even in the case of an uncontrollable second stage free flow. That 
side of the system could be shut down and you could continue to breathe off of 
a backup regulator (more $$$) on the remaining side with half of the remaining 
air supply. There are only 2 O-rings that can fail in this setup.

I normally do dives over 60 feet with a pony bottle. Would a pony bottle be 
necessary with a redundant setup like this?

The second setup has three O-rings that can fail. If any of the 3 does, the 
system will become unusable very quickly. I would imagine that the conservative 
approach to use with this setup would be to carry a 30 cu. ft. pony bottle and 
plan your dives so that you can always ascend and decompress with 30 cubic feet 
of air.

Is that practical, or would it limit your bottom time (at, say, 110 feet) to 
the extent that you might as well not use doubles to begin with?

I'm thinking that the only real advantages to the cheaper setup is that you can 
still use the tanks as singles and you can rent singles on trips to use with 
it. But, you can buy a couple of AL 80's to replace the two you use for 
doubles, for not much more than the cost of a 30 cu. ft. pony bottle and 
mounting hardware.

On the other hand, if a pony bottle is still advisable for use with the more 
expensive, isolated system, then you have the expense of the pony and a third 
set of regulators, plus the tanks to replace the doubles, unless you want to 
dive doubles all the time.

Any experience or wisdom to be imparted on this matter?

Thanks!

Rick Fincher
rnf@sp*.tb*.co*

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