Recreational doubles

[Storker]

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A ScubaBoard Staff Message...

This thread has been split off from Scalloping, Scandinavian style and refers to the gear setup seen in the video there

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if you look at my buddy's gear, that's a very popular configuration around here: Drysuit and recreational doubles (in this case a double 6L 300 bar without an iso valve on the manifold). Long hose optional, however both my buddy on that dive and I prefer a LH/BO config with a 150cm (5') long hose. Even if I only dive open water with a single tank rig.

 

[Z Gear]

I did noticed you and or your dive buddy with the dual tank set up, makes total sense when using those lift bags.

 

[Storker]

I did noticed you and or your dive buddy with the dual tank set up, makes total sense when using those lift bags.

Why would the lift bags make a difference?

The point with small recreational doubles is to have the weight of your tank(s) closer to your back, giving you a better center of gravity (COG) and less tendency to turn turtle. Plus of course the coolness factor Otherwise, there's no difference from using a single tank setup. As I said, my buddy that day used a twin 6L 300 bar set. I have clubmates who like the single 12L 300 bar, a not uncommon choice if your gas consumption is a little on the high side. The 12L single and the twin 6L have exactly the same gas capacity, about 110-120 cu.ft. (127 nominal, but you lose some capacity due to compressibility between 250 and 300 bar). Other popular recreational twinsets are D7x300 or D8.5x232, both with a capacity of about 4000 surface liters, or 140 cu.ft.

 

[BoltSnap]

Why would the lift bags make a difference?

The point with small recreational doubles is to have the weight of your tank(s) closer to your back, giving you a better center of gravity (COG) and less tendency to turn turtle. Plus of course the coolness factor Otherwise, there's no difference from using a single tank setup. As I said, my buddy that day used a twin 6L 300 bar set. I have clubmates who like the single 12L 300 bar, a not uncommon choice if your gas consumption is a little on the high side. The 12L single and the twin 6L have exactly the same gas capacity, about 110-120 cu.ft. (127 nominal, but you lose some capacity due to compressibility between 250 and 300 bar). Other popular recreational twinsets are D7x300 or D8.5x232, both with a capacity of about 4000 surface liters, or 140 cu.ft.

What about the 10LX300B Tanks, do you use them? I was thinking they make great twinsets when diving "deep" but within recreational limits for their gas capacity and as a way to provide redundancy to eliminate the pony.

 

[Storker]

What about the 10LX300B Tanks, do you use them?

I prefer the 10x300s since they're rather negative, but only as singles. A D10x300 would be about the same capacity as a D12x232 (roughly twin 100CF), but interestingly, it's the latter that seems to be the most popular with tech divers here. Don't ask me why.

 

[BoltSnap]

Anything wrong with using 10LX300B in double setup?

 

[Storker]

Like I said, I don't know why the 232s are preferred by the tech guys. But if you PP blend, compressibility makes the calculations a bit more complicated when you go over 230-250bar. Up to 232 bar, non-ideality really isn't an issue. That may be one reason.

 

[Z Gear]

Yeah this is something I need to to learn about.
I was assuming for some reason that it was better in some way when using this set up when utilizing a lift bag. I can see what you are referring to now and its good to know that this set really helps in this area ( center of gravity). Helping to improve on the center of gravity and keeping it close to you I think it is a really good topic to bring up. I knew that some liked this set up but really didn't know why. An eye opener for me.
Thanks for clarifying!

 

[Z Gear]

Why aren't more divers using this set up? I would think improving on the center of gravity and having a lower profile is great for every aspect in diving.

What are the pros and cons when using recreational doubles?

Thanks in advance
Frank G

 

[Storker]

What are the pros and cons when using recreational doubles?

Those of my clubmates who like them claim that a small twinset balances much better on their back than a single tank rig does. A small twinset without an isolation valve is effectively no different from a large, flat single tank, albeit with two 1st stages.

I prefer singles because it makes it possible to swap tanks between dives. Those twins hold only 50% more gas than one of my singles, so if you don't have a compressor available, you're basically limited to one and a half decent dive during a day outing. Besides, a broken down singles rig with two tanks is easier to stow under the benches in my boat.

Tech doubles are, as I've said, usually D12x232 around here, and although they provide enough gas for two good no-stop dives, they're no fun to schlep from the car to the boat, or to carry down to the water on shore dives. And since larger tanks means more buoyancy, the total weight of your rig and your weight belt increases with increasing tank volume