Why use double tanks?

When asked this question, most people would answer that we use double tanks to get more gas for our dive. This is a valid answer, of course, but I think the more important reason is redundancy.

Redundancy: The inclusion of extra components which are not strictly necessary to functioning, in case of failure in other components (OED)

Running low on gas during a dive can be frustrating – we all know what it feels like to want to stay down longer and enjoy ourselves! The next logical step to overcome this limitation is to carry more gas, especially on deeper dives where a diver’s gas consumption will be much higher.

Without going into a discussion on decompression here, increasing gas capacity can be done in different ways, by carrying a larger tank, using a steel tank so you can fill it to a higher pressure, or carrying multiple tanks.

If one chooses the last option of carrying multiple tanks, one could consider simply strapping two single tanks together (called a sidemount or independent doubles configuration) to effectively double the available gas. So why not just do that? Consider a situation when a failure happens (for example, a faulty o-ring on a regulator or tank valve). The result is you lose the ability to access an entire tank of gas.

Another problem with the sidemount or independent doubles configuration is that you’ll have to double up on regulator hoses. You’ll need a primary and secondary second stage, an inflator hose and an SPG on each of the first stages. This in turn doubles the number of possible failure points. Not ideal.

That aside, we also have the issue of deciding which tank to breathe on. If we breathe on one tank fully, then what happens if failure occurs on the full tank that you were planning to switch to? (Sidemount divers will argue that there are ways to prevent this from happening, such as switching tanks at defined intervals to balance the gas consumption between both tanks, but these measures could lead to additional complications, IMHO.)

We can mitigate against some failures by using DIN instead of Yoke valves on our tanks, and by minimizing the number of hoses that we need, but the best method I’ve found is to dive with a double tank configuration with an isolator manifold.

So how is redundancy built into this configuration? 

One important feature is the isolator manifold. In this configuration, we have two first stages, each connected to a tank valve, which are in turn connected to each other via the isolator manifold.

Two 300 bar scuba cylinders connected by an isolating manifold

Two 300 bar scuba cylinders connected by an isolating manifold

With this set-up, a single failure on a regulator will not result in an immediate loss of half your gas. We could shut off the valve that the failed regulator is connected to, and effectively shut down the leak. Because of the manifold, we can still access all of the gas in that tank. In this situation, one failure will result only in the loss of some functions (one second stage and either the inflator hose or the SPG), but not in any loss of gas.

But what if the manifold itself fails, you ask? Well, this is where the isolator comes in. If any side of the manifold, or tank valve fails, then we could isolate the two tanks by closing the isolator valve. This cuts off the flow of gas from one side to the other, resulting in only one of the tanks draining, as opposed to both, if the valve was left open, or if there was no isolator.

When doing more extreme dives, whether you’re going deeper or diving in an overhead environment, you’ll want the confidence of knowing that failures won’t result in you losing so much gas that you won’t be able to get to surface safely. Personally, I’d say that’s a pretty good reason to have the heavy weight of a set of double tanks on your back!

Leon Boey is a GUE, HSA and PADI instructor based in Singapore and Bali. He runs the dive education centre Livingseas in Singapore, Bali and Jakarta. Diving since 2005, he first fell in love with wrecks. He enjoys all sorts of diving, and loves being surrounded by fish.

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