Nothing makes me cringe more than watching a diver enter a wreck utilizing a single tank with a K-valve and a single regulator. With this configuration, in the event of a regulator failure, it may be possible to go to an octopus, but you do not have a redundant first stage regulator. You cannot shut down a regulator with a freeflow. Penetrating a wreck or entering into an overhead environment with a single regulator requires that you rely heavily on luck, and I don't like to dive that way; in spite of the fact that I am very lucky.
The vast majority of divers are single tank divers, and single tank diving totally works for them. They have their setup, they know their equipment, and they know their limitations. However, some of you may want to extend those single tank limitations, but are just not interested in a big, heavy, twin set of killer LP130's!!! Or even a twin set of aluminum 80's for that matter??
On my back, I love steel tanks. I always have. My first set of doubles was a mismatched pair of steel 72's, of course with a single outlet manifold. I replaced them with a very pretty set of galvanized steel Healthways 80's with an original hydro stamp of 1969!! They were just like the steel 72's, but longer. They fit me, they held lots of air (relatively speaking), and they were 2400 psi cylinders, so on occasion I accidentally overfilled them a little on purpose. They were great tanks for my early years of deep air diving, however with the single outlet manifolds, and thereby a single regulator, we carried 20, 30, or even 40 cf pony bottles for bailout. In a primary regulator failure, you could not access your primary back gas, so your pony bottle could not be big enough. Regardless of how much air we had, penetrating the Doria relying on a single regulator was obviously foolish.
For me, aluminum cylinders are okay for side slung, or stage bottles, but I really don't like them on my back because they do not have that much gas, I don't like the way they feel, and I have to wear more lead on my waist. The only advantage to aluminum cylinders is that they are cheap. Aside from that, I once had a set of twin aluminum 50's with an original hydro date of 1972. Because of industry warnings, and for good reason, no one would fill them because they were too old. 🙁 I sold them to the scrap guy for $23. I now have one set of aluminum 80 Doubles, but I really don't prefer to use them.
For the sake of argument, let's forget about rebreathers right now. If I am diving deep on open circuit, I will match my doubles to the dive, which usually means a big set of steel, HP 120's (3,500psi). They are big and heavy, but I like having all the gas, and I am 6'1" so the tanks fit me well both on the bench and in the water. I could dive steel HP 100's, but the tanks are shorter and for the difference in weight, I personally prefer the 120's.
However, I am not always diving deep, and I am not a big fan of the H or Y valves. For regular old recreational dives, my favorite tank(s) of all time are…..(drum roll please)…..my little twin LP steel 50's (2400psi)!!!!! They rock!
The center of gravity for these 5.5" diameter tanks is closer to my back, so they don't move around as much, which gives them a more natural feel. The buoyancy characteristics of the steel tanks mean I need little or no lead. I prefer this, but some divers might need the additional security of big lead weights they can ditch? The dual outlet manifold means I can utilize a primary and secondary regulator, and even an isolation valve if I want? Regardless, in the event of a regulator failure, I have some excellent options. In addition, the dry weight is pretty user friendly, and they provide lots of gas for most all recreational dives. Aside from all that, I use the same backplate and harness for pretty much all of my diving.
But wait, I also have a couple of 45cf double sets. These are even smaller and lighter than the 50's, but still 5.5" diameter. Because I am tall, I prefer the 50's when I am suiting up on the bench, but for divers shorter than me, the 45's are a very comfortable set of cylinders.
The dual outlet manifold, where two independent regulators can be utilized, was a huge development in diving equipment. It meant that the primary bottom gas supply can be accessed, even after a regulator failure and shutdown. So, I am a big fan of the dual outlet manifold for safety reasons, regardless of cylinder size, or severity of the dive. Divers using a Side Mount system cannot do this, and I see this as a weakness in their setup. Personally, I think Side Mount is great for certain mission specific applications, like in cave diving, but in general it has more disadvantages than advantages to me. So, I avoid Side Mount, but diving is about having fun and that does not mean someone else should not enjoy that setup if it makes them somehow happy.
I always choose utility over fashion, but IMHO the twin set 5.5" cylinders provide both for recreational divers. If you come down to Pompano, and see a guy a guy on the boat with Tiny Doubles, it is most likely me (or Megan). But maybe..... some day in the future (when we are disembodied heads in jars?)..... everyone will be diving them??? Then again, probably not as they are not for everyone, and it would be boring if we all dived the same.
Notice that I did not even mention how they make me more... self reliant. 🙂
everyone sing along now...