Hence the manifolded twinset as popularised by US cave divers (deep cave
diving being so ludicrously suicidal that anybody who lives long enough to
bang on about safe kit is clearly onto something).
Two cylinders with two first stages and two second stages. So it isn't too
complicated (keep it simple when your brain is full of nitrogen) a joining
manifold. When something goes wrong and there isn't an obvious fix FIRST shut
the manifold. Now whatever has happened half your gas is safe. If the fault was
a pillar valve half your gas will go but if it is a reg (most common fault) you
can shut the pillar valve and reopen the manifold and you have all your gas to
signal your buddy and get out of there.
Why breathe the long hose? Safety!
We all like wreck diving but having your buddy suffer a serious failure might
mean getting him out of somewhere tight and if he has just done a serious OOA
swim it will be silted up big time. Rule 1: take the reg in my mouth. Don't ask. Don't signal. You are in
trouble and haven't got time to be polite and I am a big boy so I can cope. I
promise that this reg works, that this reg is the right mix for the depth and
it is right where you can see it. I have another hung round my neck. Get
some of my gas into your lungs and live. I can find my secondary in a total
blackout in a moment. When you have got yourself back together we can make our
way out/up. You go first if it is narrow, because the hose is long you can swim
through the passage/bulkhead and because I'm not pulling you you don't get the
reg pulled out of your mouth. We are not pinned face to face so we can both
work together and do a normal ascent and because we have lots of gas we can do
any deco owed.
Why the wing? Safety!
In the twinset there is 6 kilos of gas. In the stage bottle or accelerated deco
bottle is another 2 kilos. If I am weighted to complete a stop on empty (I may
have a buddy breathing all my spare) I need to start 8 Kilos negative. If I hit
the bottom of the shot and my neck seal goes suddenly I can lose 10(?)kilos.
Now I have to ascend 16 kilos negative and then pin myself on the surface while
my buddy dekits me (I am not going to climb a ladder with full kit with many
kilos of water in the suit while I am ice cold). So I need, say 18-20 kilos of
lift. That's not a stab that's a wing. If something serious fails the object is
to end up back in the boat saying "that was a crap dive" not being
recovered two weeks later in a body bag.
Why the backplate? Safety!
A twinset is a lot of weight and naturally it needs a harness to support all
that weight out of the water. Since it hangs off your back the physics of the
situation is that it swings and pulls at the top and digs on at the bottom.
Digging in is bad news but we get round that with a big plate that rests flat
against the big bones of the pelvis. Now add unbroken harness loops over the
shoulders a loose waist band and a crotch strap and you are surrounded so the
rig can't move about but nothing is tight. Tight is bad news decompression
diving as anything that impedes blood-flow makes off gassing unpredictable.