I think we agree on the major points. The difference is that you are promoting the concepts that:I am NOT suggesting that you need to carry an extra tank with you through the entire dive. What I am saying is that diving a stage (breathing it to roughly half, then dropping it) is better than dropping a single safety bottle at x point in the dive.
Let's build a dive plan. We want to go see some cool room 2300' into the cave. Its a little tight leading up to it, but not "titties and tanks" small, or anything approaching that. We have a few options:
a) Do the whole thing on backgas, but it puts us real close to "thirds".
b) We could do it on backgas, but drag a safety to the halfway point, and still dive "thirds' out of our backgas
c) Dive a stage, drop it at half plus a bit, and dive to the cool room on backgas with a modified "thirds" turn pressure.
If I'm interpreting your posts correctly, option B is how you would approach this dive, while C is how I would do it. We both recognize that option A doesn't give enough wriggle room.
Diving a stage gives you WAY more than enough gas to complete your dive (with 104s, you get 365cuft to play with vs 288). That excess translates into extra reserve PAST your "two thirds" for exit . If you were going to carry the bottle anyways (in the form of a safety), you might as well breath it, carry it a shorter distance, and end up with extra reserves WITH YOU farther in the cave (in the cool 2300' room). You were already going to be there with your 104s (or whatever youre using), but by diving a stage, your tanks have more gas in them, anyways. Gas in the front doesn't do you any good if you need it in the back.
As far as swimming an extra (full) bottle, you not only content with the added bulk of the tank, but the added gas in your wing needed to compensate for the gas in the bottle. Its more than just a streamlining issue, too, as you're physically having to push more mass through the water.
You also can't share a single safety bottle. A single safety bottle greatly reduces your options compared to a proper stage, or even a pair of safeties.
If your dive plan puts you close to not having enough gas to complete the dive, use your resources in a manner than gives you the MOST options in the water. Filling and carrying one...more...tank... isn't really an issue. If it is, save your nickels and hit the gym
1. WAY more than enough gas is better than just "way" more gas than is needed for the dive, and
2. it's better to have the maximum possible reserve during the entire dive.
I don't disagree with either point, and all things being equal I am more than ok with that kind of over kill. There is nothing more useless than gas where you can't use it and no one will argue with that.
But everything is not neccesarily equal. In the tourist caves where this applies, "thirds" of a set of doubles is often not enough reserve, but "thirds" plus 72 or 77 cu ft is going to more than cover it. There is nothing wrong with staging the dive, except (assuming I do not want to hit the shop at lunch) I've now got 3 more stages in an already full vehicle and I am moving more equipment to and from the sink than I need to move.
Don't get me wrong, if the dive is bigger the cave is not as well known, is silty, tight, has old line, etc, I am going to lay up, stage the dive and ensure I have as much gas as possible available for exactly the reasons you suggest. I'm just not inclined to do that on every dive.
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The one point where we do disagree is in the concept that carrying the full safety bottle is in some way less efficient than carrying a stage due to the assumption the wing will have to be fuller to lift the heavier safety bottle and/or that the diver is carrying/pushing more mass.
Consider A and B in your example. In both cases a diver is going to need X cubic feet of gas to get to the 2300' room and back.
Since it is close to thirds with double 104's pumped to 3600 psi I'll assume we'd plan on using 94 cu ft to get there, and 94 cu ft to get back - basically diving to "thirds". That means if all goes as planned we will breath 188 cu ft of gas on the dive and that gas will weigh 14.1 pounds, all of it coming out of the backgas in the case of the safety bottle.
If we instead do it with a stage and breathe the stage to 1/2 plus 200, we'll breath 33 cu ft from the stage going in and 33 cu ft from the stage going out, with the balance of the 188 cu ft of gas needed on the dive coming from back gas (122 cu ft). In this case the 188 cu ft of gas still weighs the same 14.1 pounds.
In both cases we started with the same volume and weight of gas (carried in back gas and stage or in back gas and safety bottle) 360 cu ft, weighing 27 pounds and we are using the same 14.1 pounds of it during the dive.
The only difference is where the weight of that gas is distributed, In the case of the safety bottle dive, at the end of the dive 5.7 pounds of the gas is in the safety bottle and 7.2 pounds is in the back gas tanks. With the staged dive, 11 cu ft and .8 pounds is in the stage and 12.1 pounds is in the back gas tanks. In both case the ending gasis the same 12.9 pounds as the total weight of gas taken and used on the dive is the same.
It is true the stage is lighter than the saftey bottle, but the back gas tanks in that case are heavier by an equal amount, so as far as the weight the wing has to carry or the total mass is concerned, it is a wash. Combine identicial mass and wing drag with the arguably lower drag from a non butt light stage and the greater drag efficiency of the stage is highly suspect.
Now, consider that one diver can haul the safety bottle in and the other diver can haul it out, meaning each diver only carries the bottle for about 1/4 of the dive (assuming the safety is left half way to the 2300' room.) That's a bit more efficient than both divers carrying a stage a third of the way to the 2300' room and a third of the way back (a third of the dive in total) and it leave the safety bottle at about 1150' rather than at about 800 feet where you'd drop the stage in this scenario.
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Still, it just comes down to how much reserve overkill you are comfortable with and much much equipment you want to lug to increase that overkill. What matters is that you have sufficent reserve for the particular dive.
