i've seen twin set 7l tanks, i wonder, why would you not just use a 15l single tank?

sometimes i browse ebay just to see if there are any deals, and a couple of times i've seen 7l twin sets. i just think it's weird that you wouldn't just use a single 15l tank. is there any advantages?

Redundancy, and they might balance or trim out better for certain individuals.

If you know how to use a manifold then the tanks can be redundant and you can still have access to the air in both tanks if you should have a regulator failure.

Your preferences may evolve. I started with a single 72 when buddy breathing was taught. I later added an octopus. Then I added a pony bottle. Next came twin 50's. Then a manifold to use 2 regulators. And finally I changed to valves-down. My 72 is too long to comfortably wear valve-down.

Glenn08:

Your preferences may evolve. I started with a single 72 when buddy breathing was taught. I later added an octopus. Then I added a pony bottle. Next came twin 50's. Then a manifold to use 2 regulators. And finally I changed to valves-down. My 72 is too long to comfortably wear valve-down.

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do you mean you dive with inverted doubles?

It really sucks when your only first stage starts to free flow and you have to abort a nice dive.
It sucks even more if you happen to be under ice.
Two small tanks are balanced and lovely to carry.

Inverted doubles makes no sense to me. How do you get your gear on and off?

I dive inverted doubles myself, but it's called a rebreather.

Invertet doubles are quite common in commercial/public safety SCUBA-apparatuses., IE Interspiro.

- Mikko Laakkonen -

I love diving and teaching others to dive.

Rusty Liam Booth:

sometimes i browse ebay just to see if there are any deals, and a couple of times i've seen 7l twin sets. i just think it's weird that you wouldn't just use a single 15l tank. is there any advantages?

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The primary advantage is redundancy. You have two regulators, fed by two cylinders. If one regulator failed, you still have a working regulator that will function. You can shut-down a free-flowing regulator and abort the dive. If the rig has an isolation valve manifold, you can access all the gas (in both cylinders) through either regulator. Essentially, with the manifold open, it is one big tank. Should a tank valve, or the manifold itself, fail, you can shut down the isolation valve (separating the tanks). This flexibility give the diver a myriad of solutions to a gas emergency and/or equipment failure.

Proper training is advised on how to conduct shut-down drills and manifold drills before you use doubles. Many technical-level instructors run informal 'Intro-To-Doubles' courses (normally 1-2 days duration). There are also many useful skills/techniques videos available to view on Youtube, which show the protocols and procedures necessary.

See also: Technical Diving Skills - Valve Shutdown Drill Explained

The secondary advantage, cited by many, is that twin tanks (especially small 7L cylinders) tend to be more stable in the water. They are closer to your body and the weight is distributed. Many find them more comfortable than a single, easily off-balanced, large cylinder.

I have twin manifolded 7s for shore diving, twin manifolded 12s for boat diving down to 50m and twin manifolded 15s for boat diving down to 70m. The twin 12s and 15s are a bugger to carry for shore diving, the sevens are much easier.

The only thing that changes between dives is the size of the twinset and whether or not I take decompression stage(s).

It makes adapting for different types of diving very easy - I don't have to change regulator set-ups or put STAs on or any other faff; I just bolt my backplate and wing onto the appropriate set of cylinders then assemble everything else exactly as normal plus I get all the advantages DD describes above.

In case you're wondering, the centres on the twin 7s are only marginally narrower than those of the 12s and 15s so I don't get a huge bat-wing effect.