Why dive a twinset?

Why the twinset? Safety!
What is the most common failure?  A reg free flow.
What does it do?It throws all your gas away.
What's the solution?Turn the cylinder valve off.
But then I die?So carry two cylinders.
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.

Back to Nigel's Inverted twins page.