When you get used to diving with a single cylinder (maybe with a pony as well) you will notice that other divers around you are diving on two cylinders (again sometimes with ponys and side slung cylinders) ! You may be asking yourself why these people seem intent on putting even more weight on their back, and what the advantages of using a twin set is. All is explained below ...........


Why dive with a twin set ?

Twin sets have one big advantage over single cylinders; they give you a larger volume of gas to breathe so that you can increase your dive time. This can make the difference between getting 5mins bottom time and getting 15mins bottom time. Then if also have your twin sets filled with Nitrox then you can get even longer bottom times and shorter decompression stops. As well as using a twin set, you may opt for having pony cylinders and side slung cylinders attached to the twin set and you. With all this extra gas you can do long dives and see all that you can on the dive (you could view all of a wreck in a single dive instead of multiple dives), and using EANx 50 etc you can do Oxygen-rich decompression for maximum safety and shorter deco stops.


What do you need to consider when getting a twin set ?

When buying or putting together a twin set there are several things that you must consider, configuration of the twin set, how your going to hold the cylinders together, what size cylinder you should choose...... Read on then !



More and more people are going out and buying twin sets fully made up by a dive store, however if you already own your cylinders then they will be suitable for adapting to a twin-set. Twin-sets normally consist of two cylinders, which are joined together using a manifold, which gives you access to both cylinders through a single regulator; although this isn't always the case. There are three general ways of diving with twin cylinders:

1. Diving independents

This is where two cylinders are mounted on your back but are run as separate systems. If you have a failure with one cylinder then you have lost that entire gas supply and cannot use it. Diving with independents involves more equipment and much better gas management skills. You must be able to differentiate between which gauge and which regulator belong to which cylinder. This is a poor choice when compared to the alternatives.

2. Diving twin cylinders with a non-isolating manifold

This time the two cylinders are joined via a manifold which takes the form of a permanently open connection running between the two cylinders. This enables you to gain access to the contents of either cylinder via either pillar valve. This has the benefit of avoiding the use of two gauges and eliminates the need to keep swapping regulators. However, your twin cylinders are now effectively just a single large cylinder. Since one failure can affect your entire gas supply, it leaves you without the safety margin of having independent access to both cylinders. Considering this you should take the extra time and money to get the extra option.

3. Diving twin cylinders with an Isolation manifold

This is without a doubt the best of the three options since it has all the benefits of both the previous systems and none of the disadvantages. It enables you to gain access to the contents of both tanks through one regulator and read the total gas supply off one gauge.

If you suffer a failure in one cylinder then you should normally still be able to gain access to the contents of both cylinders through one pillar valve. If there is a worst case scenario, then you can "isolate" one cylinder from the other by turning a handle, which is in the middle of the manifold, this will protect half your gas supply. This retains the safety aspect which is lost by using a non-isolating type manifold.

Twin cylinders should always be mounted with the manifold as high as possible, so that you can reach over your shoulder and turn your own valves on or off whilst in the water. This ensures that you can use the manifold to it's full potential.

Sometimes you see tanks inverted, this is a definite no-no, since you will then need a cage guard to protect the manifold. This is something else which can snag hoses etc, this is an added complication which makes access to the valves harder; completely defeating the idea of turning the set upside down in the first place. In addition, all hoses must be custom made.

You also occasionally see remote valve winders (known as "slob-knobs"), which consist of a handle on the end of a long hose-like type connector. These are attached to the pillar valves and isolation manifold and allow you to turn these pieces of equipment on or off without reaching behind you. Unfortunately they can corrode and should they fail or seize, it can make it impossible to operate the valves either directly or remotely.

Cylinder Size:

Cylinders should be the same size and dimensions if they are to be "twinned" up. Twin sets normally consist of either twin 7's, 10's, 12's or 15 litre cylinders.

Twin 7's are generally used for the types of dives which fall within the "normal" club type diving description.     i.e. under 35 metres, not involving any decompression stops.

Twin 10's have a greater volume and are suitable for sport diving, which involves some degree of decompression for a single dive. They may be unsuitable if you want to use them without a top-up for a full day's normal club diving i.e. two dives under 35 metres with no stage decompression stops. This is due to their reduced volume compared to diving twin 12's.

Twin 12's are normally suitable for a full day's sport diving (two dives), which involve minimal decompression stops for the first dive. They may only be sufficient for one dive if it is to a greater depth/for a greater time with a lengthy decompression. Twin 12's seem to represent the best choice for the average diver.

Twin 15's tend not to be used, due to their size, weight and the amount of drag that they cause.


Cylinder Alloy

The choice of cylinder depends on the type of suit you wear due to their buoyancy characteristics. If you are diving in a wetsuit then aluminium cylinders represent the best choice since they won't cause you to be over weighted. If you dive in a dry suit then steel tanks will remove some of the weight from around your waist.



As has already been discussed, using an isolating manifold is the only real choice, but there are a number of further considerations:

1. Barrel O-Rings Vs Facing O-Rings

This consideration is about the way in which the manifold is sealed against the two pillar valves. Barrel O-ring systems have an O-ring which seals around the outside of the isolation manifold insert and the interior of the pillar valve. Facing O-ring systems use an O-ring which is trapped much like a DIN fitting; it is trapped between the cylinder valve and the insert of the isolation manifold.

Barrel O-rings are more tolerant of movement within the manifold and are less likely to be forced out of position, which would cause a gas leak.

2. Balanced Valves Vs Unbalanced Valves

If a pillar valve is balanced, then it should be just as easy to close the valve as open it. Unbalanced valves are easier to open than they are to shut and should be avoided.


Rubber Knobs instead of the normal plastic knobs on the cylinder valve

Normal hard plastic knobs may break if accidentally knocked whereas rubber knobs are less likely to break off and it is easier to get a grip on them. The option of using solid brass handles is a strict no-no as they transfer shocks from the handle to the shaft of the valve and can cause the valve to seize; preventing you from closing them etc.


Cylinder Bands

These are used to hold the two cylinders together.

Material cam band systems are suitable for independent cylinders but not if the cylinders are manifolded in any way, since the cylinders aren't held rigid. Any movement stresses the connections between the manifold itself and the pillar valves.

There are a number of stainless steel bands available. They generally look like strips of stainless steel, which have been formed in the shape of the number eight with a bolt through the middle. This bolt is tightened to hold the tanks in place.

There are certain things to avoid:

1. Sharp edges on the bands, which will cut into the cylinders paint work and encourage corrosion
2. Spot welds, which are not as strong as continuous (seam) welding
3. D Rings on the lower (and upper) bands which can easily become entangled during a dive
4. Low marine-grade stainless steel
5. Bands which warp under stress
6. Bands which are too narrow and so do not hold the tanks rigidly



Obviously a larger form of buoyancy is needed for a twin set due to the additional weight. A 55lb wing should be sufficient for your needs. You can use a Buddy BCD with an additional Buddy Redwing attached to it (to increase the weight capacity sufficiently. However the best method is to use a wing-style BCD like the custom divers TDB, and bolt the cylinders to the back plate of the wing.

A back plate is essential if you are going to use a wing-style BCD, as it reduces the stress on your back and spreads the weight of the cylinders over your whole back. Whether you choose an aluminium or steel back plate is up to you, although if you choose a steel back plate you won't need as much additional weight.


Weighting with a twin set

Another advantage of using a twin set is that you don't need as much weight, as the whole set-up adds significant weight to your body.

Typically, a diver who uses 30lb of weight with a single cylinder, would only need 8lbs of weight with twin 12s in sea water, and no extra weight at all in freshwater.



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