To Manifold - Why
Cavers very often run independent side slug cylinders with their main gas either contained in both or just one. These are rarely manifolded given the configuration and the Diver breathes a set amount of gas of each on rotation.
A wreck or conventional diver uses a back mounted set of cylinders which can likewise either be independant or manifolded.
So why manifold them?
Boyles Law shows the deeper you go the more gas that would occupy a given volume (eg your lungs which typically take between 2L and 4L of volume with each breath on the surface). The Deeper you go your lung volume broadly remains the same, however the pressure at that depth makes the taken up volume equal to the Pressure at that Depth x the Lung Inspired Volume. So you end up needing a lot more gas the deeper you go. Most Technical Divers will know this basic Physics and facts well.
Technical Diver's came up with a solution, make a bigger cylinder out of two conventional cylinders - by joining them together. So two 12L cylinders became a 24L cylinder! The next challenge is that we Techies plan for problems and look to duplicate systems and components where such redundancy is not overkill, but totally necessary.
Many of the 'connecting manifiold' designs incorporate many O-rings and those could fail at depth. Early manifolds did not contain an isolator valve and the debate still rages on as to which is best - an isolating manifold and a non-isolating manifold. Even though the valve has several failure points and is a Gas leak point for sure, in our eyes its advantages far outsway its risks and disadvantages. If any of these connecting O-Rings on the manifold fail then you risk loosing your life - although this risk is rare - I have had such a failure on the RONDO at 49msw. Without the means to isolate you run the risk of dieing as you will lose all your main gas.
This isolator valve effectively provides a controlling device in this manifold between the two cylinders to effectively save one cylinders worth of gas (i.e. 50% of what you had at that failure point post isolation). Normally this is just turned on about a turn and a half so the tanks level out through a bleeding trickle of gas - some instructors teach this to be fully on - when your adreneline is flowing you can only turn it one way OFF, I take the view that just on is much better - you save a hell of a lot more gas - you decide and train so you turn it instinctively the right way.
Your shut down drills will teach you how to find and isolate the leak - once the isolator valve is shut then you will breathe from the side that survived and ending the bottom time or dive at that point.
I've used this isolator twice in 3000 dives and both times it has saved my life, both times I was a very long way from the shallower depth at which I could then Gas Switch to a stage.
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