Allot of people dive with a twinset or double 12 as their preferred way of diving. Especially in technical diving. But also when doing recreational shallow dives and without much knowledge of twinsets. But mainly because everyone dives a twinset. In this blog I will try to explain why you might not need a twinset to start with. But how you save your self some money when the time comes, and you do need more gas.
First let's start with some introductions. The twinset in the way it's set up now has been this way since the early 1990. Together with the birth of traditional DIR "Doing It Right" diving.
It started with the need to take more gas for deeper and longer dives. So at first tanks got bigger, then divers decided to mount two tanks beside each other on their backs and switched between regulators to deplete both tanks evenly.
This practise soon became obsolete because now the tanks were connected using a manifold.
But the first manifolds were just a bar connecting the tanks. This offered the advantage of not having to switch between regulators but it still meant that if the manifold or the tank valve o-ring broke, you would loos all your gas.
So two separate tanks offered the possibility to have two independent sources but still posed too much hassle and task loading. On the other hand simple non-isolatable manifold still did not solve the risk of loosing all of your gas because of the inability to isolate between the two tanks.
This is when the isolatable manifold came to be the preferred way of using a twinset. But... yes there is always a but. Eventough many divers think this is the holy grail of redundancy and all potential failure boxes are ticked, there is still one critical failure that can deplete your gas supply. Albeit a very small chance, but if your were to hit your manifold, and broke the knob/valve it selv you would loose the ability of isolating and still loose your gas.
This however falls under the category risk vs. benefit. We risk this potential problem for the convenience of having all the above mentioned benefits. This is also why we have s-drills :-). DIR trained divers all know that S-drills stands for safety drill or air sharing drills.
Now to the point of this blog. Do you need a twinset, or can you suffice with a single tank. And why do so many divers tell young divers to start with diving on a twinset for the sole reason that " you'll end up using this anyway" or " they all use it".
The reason for advising divers to dive this way might come from the fact that you dive a wing and backplate combination and use a longhose regulator setup when using a twinset. Like in DIR diving. With that in mind it makes sense to advise the young diver to use this configuration. But they are better of starting with a wing and backplate and a longhose configuration using a single tank DIR setup. Let me explain why.
It all has to do with two things.
The weight of the gas can make it harder for newer divers to learn to control their buoyancy.
The benefit of learning motorskills and muscle memory by the use of a consistent configuration.
Point 1: As you balance yourself as a diver you need to offset the positive buoyancy of your suit with the weight of your equipment and weights. see this video on how you do this.
Now that you have found the correct weight you start the dive with the weight of your gas in your tanks making you in affect to heavy. This is why we have a BCD buoyancy compensating device. Underwater, at the beginning of the dive you offset the weight of the gas with your BCD. This is one of the 4 roles for your BCD.
So far so good. A single tank, 10-15 liter holds about 2-3kg of gas. This means that you'r BCD offsets this. We all know that when your inhale, you go up and when you exhale, you go down. We have about 3-4kg we can move using our lungs. This is where the issue lies. Because of the fact that the gas in a single tank can be managed by your lungs, you have more control/power over your buoyancy.
Now when you dive with a twinset you carry double the amount of gas and therefore also double the amount of gas-weight. But now your lungs have much less influence on the weight of the gas or the volume in your wing.
Remember your lungs capacity stays the same, regardless of your equipment. So when you make depth changes, your wing volume wil be affected. And you will find your self moving out of the range where your can just use your lungs for control much faster. A small car towing a small trailer, has little trouble accelerating or decelerating. But the same small car has more trouble when that trailer gets heavier.
Young divers might not yet have enough experience to feel the subtile changes in buoyancy and find themselves have much grater "swings" in their buoyancy. This together with the fact that they usually perform shallower dives where buoyancy changes are greater, they end up struggling and this results in poor diving skills like bad trim etc.
So what to do?
point 2: Start using a wing and backplate? YES. Start using a longhose configured regulator? YES. Buy a twinset? YES, but... Buy a twinset and have the shop disassemble it, and plugging the manifold holes in the valves. Now you have 2 single tanks and you can start building experience using a backplate with combo and diving a DIR style hoghartian setup. But you save yourself from dealing with the extra weight of a twinset both on land and under water.
Then later when you are ready to take your diving deeper than 30m you can reassemble your twinset again, buy a larger volume wing and an ekstra 1st stage and you are good to go. All the while you have used a long hose and gotten used to your harness. Now you have your single tank wing for traveling use or the occasional single tank dive. Many dives I have advised in this practise end up buying a lightweight backplate for their single wing.
When should you use a twinset:
Depths to 30m and deeper
Overhead environments
Technical or decompression diving
I hope you found some answers in this blog but do not hesitate to leave a comment or question below.
Now go out and plan a dive, maybe try the exercise like in the video above.