For most recreational divers, when they dive to their computer, the limiting factor is the amount of air they have in their tanks.
A twelve litre tank filled to 220 bar holds 2,640 litres of air and a fifteen litre cylinder holds 3,300 litres. A sidemount system using Ali 80 cylinders hold 4,840 litres of air, twin twelve cylinders 5,280 litres of air.
Increase the duration of your dives
Your eRDP will, for the first dive of the day give you an NDL time of 56 minutes at 17 meters. With descent, assent and a three-minute safety stop your dive time will be over an hour. To complete that dive most recreational divers will run out of air before they run out of time. So to get the most out of that dive plan you need additional air. There are a few ways to do this, you could dive with a stage, you could learn to dive side mounts or you could learn to dive a twin set.
Clearly this means that diving a twin set does not mean diving deep or technically. Diving with a stage, side mount or twin sets as a recreational diver not only means you can dive for longer it adds several additional layers of safety. These include redundancy, self-reliance, and increased gas. Also, diving with twinsets means a wing so your position in the water is improved making you more streamlined
Why is a twinset safer?
Twinset is very similar to how we teach students in a wing and long hose set up. The tanks are on your back and joined together with a tank band and a manifold between the valves. You still have two regulators to breath from. Your Octo is on a necklace around your neck and you have a long hose primary. There is a low pressure that connects to your wing and a second one connecting to your dry suit. You also have a pressure gauge.
Redundancy can be a divers best friend…
The difference is that you have two first stages, one connected to each cylinder. This means that if one fails you have a second one. Also, if any of your low-pressure hoses fail or your pressure gauge blows an O-ring, you can isolate that side and continue to use the other cylinder to end the dive.
You can do all of this without needing to rely on your buddy and their equipment making you self-reliant. The fact that you have two cylinders usually means that you have more gas to use. Twin 12 litre cylinders obviously means you have twice the air as a single 12 litre cylinder. You could have twin seven litre cylinders, give more air than a single 12 but less than a 15. However they will give you the added safety margin of redundancy, so the cylinders do not have to be big and heavy to give you some really positive safety features.
How does this redundancy work?
You have two first stages, one on each cylinder. One regulator and one low pressure inflator is connected to each side. This means that if you have to turn one off, you have a functioning second one. You can also buy wings with double bladders. An inflator on either side allows you to dive on one bladder and if this fails you can use the second one. Meaning again, that you do not have to rely on your buddy as you have a spare system, redundancy. In a free flow on a single cylinder you would have to buddy breath and to turn your air off. This would result in you also having no working low-pressure inflator.
Shut down drills
Redundancy only works with a manifold. The manifold is the third knob, which is found between the valves. Without the manifold you have effectively one big tank. So if you encounter a problem, the first thing that you do is to close the manifold. With the manifold closed you have two separate systems to dive from. This is why training is important when you first start to use a twinset. The shut down drills teach you to do this over and over again. This is a skill you practice every time you get in the water. This training amongst other things allows your hands to develop the muscle memory to turn your cylinder valves on and off effectively. I can assure you for some of us this took a lot of practice, particularly in a dry suit in the lake at Capernwray.
If I am planning a two tank diving day, diving on my twin 12s and my buddy is on a single 12 litre cylinder, I do not have to change cylinders between dives. In addition to this, if we use the same air on the dive and my buddy on the single tank surfaces with 70 bars, when he changes his cylinder he looses the remaining 70 bars. This means an additional 70 bars in my tanks when we start the second dive. Win, win!! In volume this is an additional 840 liters of air!
Twinset vs Sidemount
I am certified to dive both Sidemount and Twinset and people often ask which do I prefer. This is not an easy question to answer as they both have pros and cons. The extra air in both set up add value when supporting students who are completing continuous education programs and in particular the deep courses.
Sidemount is usually undertaken with aluminum cylinders these become quite floaty as you empty them, this is not a problem with steel twins. When you are diving sidemount, cylinders can be unclipped and held in front of you (they can even be removed completely) so you can pass through an opening as small as you are. This is not possible with any back mounted system.
With students, the twin set system looks very similar to the back mount system they are using, so there are no additional areas to brief them on. When diving a side mount system you switch breathing from each cylinder in turn so that they are both floating at the same level. This means that in an out of air situation you may not be donating the regulator in your mouth as you do on a back mount system. This may cause a panicking diver additional confusion if not properly briefed. Even if it is properly briefed in the heat of the moment this may not be recalled.
Both setups have their place!
I find that twins are better when diving from a RHIB as they are easier to balance and handle when getting in and out of the water as you are removing one system rather than two. Sidemount cylinders can be removed and left at the waters edge. This means that if you walk in and walk out you can do this without any cylinders. Twinset systems are heavier and you will need to wear these into the water. You either need a ledge, van back or a strong buddy to help you on with the system.
In conclusion I like them both. If diving on a tight wreck or in a cave I would use sidemount. When diving with students or from a RHIB, I prefer the twinset. Also, when diving with an additional stage I prefer the twinset –which is how I completed my PADI TEC40 course.
You do not need to be a TEC diver to use a twinset and there are many, many benefits of using twins for recreational diving. It is also a great gateway into TEC diving if this is the route that you are hoping to dive down.