Why Dive on Twin Cylinders May 24, 2019 by lukesheppard

I dive twins as much as possible and if I’m diving at home in the UK I’ll probably be on my twin 12s, I just find them better than the alternatives for the type of diving I do. Like everybody, I started on singles and then switched to twins when I started teaching and I dabbled in sidemount when it became popular a few years back, rebreathers are great too but they’re in another league so we can discuss them later. Sidemount is great for flexibility and confined spaces but for most dives, I grab my twins. Twin sets tick all the boxes for me when diving at home on a pretty standard dive but they can be an intimidating concept at first.

The first thing most divers think is “wow they must be heavy” and yes they are heavy but not in the water. I still need lead weights to get down with my twin steel 12s but nowhere near as much as on a single 12 and as my wise friend Dangerous Dave says: “I’d rather take extra gas with me then lead…” you can breathe extra gas but that lead will only make you sink.

So let’s look at Why You Should Dive on a Twinset

Redundancy

Twin sets, manifold or not, offer you two completely independent air sources. So, if something goes wrong with one air supply you can literally switch that tank off and breathe from the other tank to get you to safety. On twin cylinders, you have two valves that are connected by a manifold with an isolation valve. Both tank valves are fitted with independent 1st stages with one 2nd stage and low-pressure hose off each.

That way if, god forbid, one of your 1st stages or 2nd stages malfunctions or a hose is ruptured then you can turn that valve off and isolate your remaining gas so it isn’t wasted but you can still breathe and still control your buoyancy. On a normal dive air flows between both cylinders and you breathe from both of them so you never have to switch 2nd stages during a dive, just if something goes wrong you have to screw in a valve behind your head. Or fit a slob knob if you’re lazy…

Extended Range

Diving on twin 12s means that you’re literally diving with twice as much gas as a single cylinder diver. That way even when you get down to 50bar you still have twice as much gas as a diver on a single 12. Diving twins means that you can usually rule out air supply as a limiting factor on a dive. Diving on a single, the usual factor that ends a dive is one of you running low on gas, on twins it’ll be your NDL or body temperature or something else that turns a dive.

Multiple dives are easier too because you don’t have to do anything to your kit between dives, just close your valves so you don’t lose any gas. Single cylinder divers have to depressurise and break down their gear before rebuilding it back up again for the 2nd dive. And you don’t waste that remaining gas from your first tank just sitting there on the deck, you’re bringing that with you on the next dive.

Security

Twin sets are literally bolted onto your back so they move with you. You don’t really notice it that much until you switch to backplates and wings but normal BCDs do roll around on your back and your cylinder doesn’t always feel a part of you. With a pair of cylinders on your back, your position in the water is better and easier to control and your backplate means a lot of your weight is close to your centre of buoyancy.

Your centre of buoyancy is shifted with more weight up around your shoulders instead of around your waist and you can mount V and P weights in the space between your cylinders. You don’t have to worry about nylon straps loosening or did I tighten that cam band tight enough? Because the nuts and bolts don’t loosen when they get wet.

Flexibility

I know I’ve just sung the praises of how rigid backplate and wing setups are but at the same time, they are far more flexible than other BCDs. I know this is a bit of a tangent but bear with me; it’s worth it. Sidemount BCDs do just that sidemount, they can’t do any other types of diving. Normal recreational BCDs are ok with single cylinders and yes some can support twins but they’re not that great at it and it’s not really what they’re made for. A backplate and harness setup can be used on twins and then quickly adjusted to work on singles and you can sidemount stages because you have control of where the D-Rings are fitted.

With a proper backplate and harness setup for your twins you are ready for any occasion and have full control of how your BCD fits and feels and where D-rings and buckles and pouches are fitted because you’ve assembled it yourself.

Why not sidemount I hear you cry? They’re pretty much twins but easier to control and fix problems. Well, you can sidemount if you like I just find there’s a lot of boltsnaps and straps compared to twins. I just find twins much more straightforward and squared away especially with clunky gloves and drysuits.