Not everyone wearing doubles is cave diving or penetrating a wreck. Thirds is not always the answer.
I dive a longhose/bungied B/U with ID's. The hose configuration is exactly the same as MD's except I have one more SPG on the right post. As to clutter, to even things out the MD diver has an extra valve to contend with.
The free flow example is interesting. If I have one, I close the valve and if it won't resolve I move to end the dive (or at least move upwards). I still have enough gas in the second tank to surface if I plan correctly. If someone with an isolation valve has a free flow (requiring a post shut down) do they continue the dive as if nothing has happened or also move to abort? In either configuration, the diver should have enough gas to surface.
There is no argument that MD's allow the most usable gas. It's true; but there are situations where maximum usable gas is not the greatest motivator. I often hear MD divers here say that, worst case scenario, their teammate is carrying their redundancy. For the solo diver that is not an option. As I said before, I would rather swap true redundancy for max usable gas. That's why I choose ID's. I weight one more than the other. The fact that MD divers must consistently practice valve drills points to the fact that they do not have true redundancy. That is completely based on the actions of the diver, post incident, to bring into effect. If the diver cannot isolate fast enough they will lose all gas. The most I can lose is half.
People keep saying BM ID users can't reach their valves. Why? Can't MD divers reach their posts? The only difference is that my reserve gas is not dependent on reaching the valves. It is an option, not a necessity.
---------- Post added September 19th, 2013 at 07:23 PM ----------
Gabe, I can't help you there (but I'd like to see that boat) as that diving is beyond my training. But I would assume (you can tell me if I'm wrong) that on the bottom you can either switch to one of your stages if you had to OR you have a teammates BG you could switch to. In that case, a state of true redundancy does exist and MD's would seem to be the best option. Would you do that dive solo though with all your breathable bottom gas in MD's without recourse to any other bailout gas?
Here's a more likely scenario that I usually face which illustrates why I choose what I choose (and why I now am leaning more towards SM). As you can see, max usable gas is not the issue as much as dependable bailout is:
I am solo diving a remote setting at a site where there is no information regarding conditions. However, if history dictates, I anticipate the water is cold, low vis and there is an unknown entanglement potential. I plan a max depth cap/RB volume and want redundancy with more gas than a single/pony combo will afford. I am also probably shooting video. My choices come down to two small steel cylinders because they won't kill me getting in and out of the water (I'm alone if I fall and can't get up) and I need to wear less lead yet can achieve neutral buoyancy at the beginning of the dive. I can either use a manifold or dive them independently. The manifold offers me nothing, because I don't need to use all available gas, and the manifold requires a step, post incident, to achieve true redundancy. Independent tanks offer me all the gas I need and two completely separate gas systems.
My potential to survive human error is also better with ID's. I can miss my turn pressure and the worst that will happen is I will breath down one tank. I will still have a backup to switch to, surface and slap myself silly with. I can also fail (for whatever reason) to shut down the offending valve and still have enough gas to exit (only losing half gas). With MD's I could, potentially, breath right through my turn pressure to the point I do not have enough gas for exit or fail to isolate quickly or properly. I have thought about this a lot and can say that, for myself, human error is a far more likely failure I will have to contend with than equipment failure. My choice highly anticipates and attempts to address human error while solo diving.
very stream lined and out of the way, but kind of a pain to read.
