and instead of an answer, get back blabber.
Where is this place Aqua?
And what is it about a tank configuration that leads someone to dive in a place or at a depth they are not qualified for?
Doubles are a perfectly legitimate open water configuration in some circumstances. I am going to try them out myself, not due to intending to do planned deco diving or the like at the present time, but because I often spearfish and when spearfishing we are often "same ocean" buddies more or less by necessity.
As such having a fully reundant kit on my back means that a single equipment failure is far less likely to lead to me needing to do a CESA from 110' - while I might well make it doing that, making it unnecessary is, for me, worth the (relatively moderate) expense and hassle. It also means that I can do two dives on one set of gas provided that I consume no more than half of the supply on the first dive; obviating the need to switch tanks and regulators on my kit during the diving day. The safety factor here is even better than you might expect; here's the logic:
1. I start with doubled AL80s each with 3000 psi.
2. If I was breathing one, I would likely surface with ~500 psi, or 1/6th the capacity of the tank, remaining.
3. If I dive doubles, instead of 1500 psi remaining, I will be surfacing with 1750 psi remaining, assuming I breathe the same cuft of gas.
4. The second dive therefore is commenced with about 93 cuft of gas, which means I have an even greater margin on Dive #2 than I would have by taking a second single full tank!
5. If for some reason I surface with insufficient gas (under 1500 psi) in the doubles, I not only have prevented an OOG emergency from happening on the first dive BUT I simply can skip the second dive or, if I have a third tank and whip on board, transfill the third tank into the doubles. Provided I can get over 1500 psi from that transfill I have sufficient gas for the second open-water dive.
So I win several ways:
1. I have a greater supply of gas, ergo, more time to solve problems underwater.
2. No single failure in my gas supply chain will lead to a need to do a CESA. I can shut down a failed post and in an extreme case isolate, even if my buddy is not within reach.
3. In the event of an inadvertant trip into deco land, I have the gas supply to manage the situation and complete the obligation.
4. In the event that my buddy has an OOA emergency for some reason, I have more gas on my back to give him/her.
5. I don't have to futz around with my kit between two dives on the same day; I can just put it back on and jump back in the water.
The disadvantages are:
1. I have to buy a set of bands (~60) and a manifold (~160 from Diver's Supply).
2. I will have to be satisfied diving the same mix (Nitrox) on two successive dives in the same day. This is not a major consideration from my perspective.
3. The kit weighs more out of the water and has more mass and drag underwater (possibly greater effort to propel myself.)
Never mind that getting comfortable with that configuration means that if/when I take formal decompression training I will be changing fewer things in my dive and will already be familiar and comfortable with the equipment I am using and its configuration.
If I can deal with the added weight and bulk of the doubled tanks underwater and on the deck I see no "lose" to this configuration and plenty of "wins" in the safety column. Since I don't dive on cattle boats whether the doubles will fit in some boat's tank racks is not a concern.
