I didn't go into the depths of the gas volume and at which points i switch, just that the switches took place... i made no mention of how many times. This was intentional, as the OP asked what is clearly a very basic question, so to go on and baffle him with a complicated answer (complicated to somebody who appears to be new to this) would not have been a good option (in my opinion)..../
~No offense was intended, I just wanted to clarify that keeping the tank pressures "close" has a wide range of definitions.
With 1/3, 2/3, 1/3 and 2 switches I'm on the minimalist end, while with 1/6, 1/3, 1/3, 1/6 and 4 switches you're more middle of the road.
What I caution against are divers who think they need to with every few hundred psi, and/or don't understand how much gas they really need on one side to exit if gas is lost on the other side. That's over kill on the one hand, and bad planning on the other.
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As for giving a long answer rather than a short answer the reader can absorb as much as he or she likes, and even if they skim over most of it, the message is sent that there are different ways to do it, but that it's part of an overall plan - or should be. Again, what I want to avoid is giving someone the impression they have to switch every few hundred psi to keep the tanks "balanced". I've seen divers with that misperception going back 25 years to back mounted independent doubles, where the same 1/3, 2/3, 1/3 gas switch methodology works just fine.
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We used to use a 7' hose on the right tank and a short hose on the left tank. But given that we dive in mixed teams now and then, we eventually migrated to a 5' hose on each tank. I run the left side hose up and around the back of my neck, like you normally would with a stage bottle in back mount - but it has a bungee loop on the reg, loose enough to pull free if needed by an OOG diver. This leaves a very short u shaped loop of hose on the tank. The reg for the right tank is clipped to the right shoulder D-ring, and with either a zip tie, or a rubber snorkel keeper (placed over the mouthpiece, through the bolt snap, and back over the mouth piece) it can again be pulled free quickly if needed by an OOG diver - or by yourself when the OOG diver takes the one in your mouth - with no need to fumble with a bolt snap. The hose itself, makes one loop down to the bottom of the tank and back up so it's clean and easy to re-stow. The major risk is running the tail and bolt snap through the loop, which would prevent quickly donating all 5'. However an S drill will easily detect that issue if you miss it when clipping on the tank.
In any case, it means that regardless of what reg you are using, it's on a 5' hose and can be easily donated and kept by the OOG diver. 5' is also long enough to get divers through restrictions in trail - it's snug but do-able, and in larger passage it's as easy to swim as a 7 ft hose.
---------- Post added December 3rd, 2015 at 05:39 PM ----------
Jesus when you say get back to the stage and its ''gone'' you mean as if someone else took it? That would scare the **** out of me. I like how meticulous you are with the planning though so that that wouldn't even phase you!
---------- Post added December 2nd, 2015 at 11:02 PM ----------
I really didn't mean gone so much as when you were finished with that tank. I didn't realize multiple switches were made during the dive.
If it's not kept on you for the entire dive, you never want to have to rely on it being there when you need it. Even when the gas stays with you for the entire dive, you want to plan lost gas scenarios assuming lost gas from any one tank.
That means a stage in this case, but it also applies to a deco bottle you may leave in the cavern zone. It's not unknown for an open water diver, even a well meaning open water diver to "find" your tank and rescue it for you. Now and then one gets turned in at the front desk at Ginnie Springs - where I assume the would be good Samaritan gets yelled at for removing it.
If you're planning to do your deco on 100% O2, it will take about twice as long to do on 32% nitrox, and longer on a leaner mix, so you need the reserve gas in your primary tanks to support the longer deco. On a deeper trimix dive, with a shallow section of cave before the deep point of the dive, we'll take a travel gas like 32% (or 30/30 on dives with higher percentage helium trimixes where isobaric counter diffusion issues might develop) in a stage to use as a travel mix to avoid using the leaner oxygen content (and much more expensive) high percentage trimix we need on the deep portion of the dive in shallow cave where it's not needed. It also serves as a first deco gas on the way out. If the O2 isn't in the cavern when you get back, it's helpful to have had that for initial deco, and for what's left to speed the deco before you have to switch to the bottom mix to finish the longer lost gas deco schedule.
More than you need to know maybe, but feel free to ignore it. The main point is that gas planning is one of the primary skills learned in technical wreck, cave, or decompression dive training - and one of the major reasons you need that training, and the major reason why you can't just fly a computer.
