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On Wed, 21 Sep 1994, Rick Fincher wrote: > > > 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. > <Snip> > > 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. > <snip> Rick, I think you need to consider a couple of very important things here. I regularly dive to deep wrecks off the North Atlantic coast, and have been faced with making the same decisions. Others will undoubtedly have other opinions. I have both types of setups (sort of). A yoke is a bad investment for deep diving. Indeed, a yoke offers NO redundancy at all, can be knocked loose somewhat easily when wreck diving, offers three O-rings as failure points (not including tank valve o-rings), and does not offer much stability to a doubles rig. In addition, it is often mistaken for a handle by well intentioned dive shop personnel, and can be ruined quite easily. Also on the subject of yoke fittings: I think you are underestimating the ease of conversion for this fitting. Double tanks need bands, stainless steel binders that hold the two tanks together and allow them to be easily fastened to a backpack. In order to tear down a set of yoked doubles and reassemble them (or a separate pair) takes a ratchet, a wrench, and more time then you probably would have between dives. I have never tried it on a cramped, rolling boat, but I would imagine it to be a miserable experience. I would contend that this method of diving is not practical for most people. If I havent't dissuaded you yet :-) , In order to dive safely with yoked doubles, you will need at least a pony bottle (requiring extra expense) for redundancy. Most books that I have read on deep diving (and deep is a relative term) do not recommend yokes at all. This may be a little conservative, but as you have acknowledged safety is very important for any diving, especially deep. The decision that I made when I was young and foolish (and poor) was to buy a yoke, dedicate two tanks to it, to buy bands and a backplate/harness, and to buy a pony bottle/reg setup for redundancy. Total price of the setup was approx. $450 not including the tanks, which I already owned. When I got older and wiser (still poor), I bought a manifold from Dive-Rite which is modular. It consists of two K valves and an isolator manifold. If there is failure at the regulator seat/DIN level, the offending tank valve can be closed. With the isolator manifold open, air from both tanks is still available for ascent/decompression/general use. If the valve fails at the tank o-ring level, the isolator can be closed and air from the good valve can be conserved and breathed. The cost of this set up (including bands, backplate/harness, manifold, reg and gauges was $525, not including tanks. If you are looking to start serious deep diving, safety is key. As a very subjective observation, I feel safer with the manifold system. Many would contend that the manifold system IS safer and they are probably right. If the difference between a fundamentally safe system (manifold) and a system where safety is kind of Kludged (yoke/pony bottle) is $75, an objective decision is relatively simple. Choose the manifold. But whatever decision you make remember that redundancy is a key to safety. Oh, and a third option that you didn't consider is two cylinders banded together, each with a separate regulator and gauges. This setup is redundant, and relatively cheap at around $300 (N/I tanks). Use proper gas planning rules though. Regards, Eric
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