Choosing the right twins for you
Twin tanks are increasingly sought after nowadays and many purchased with little or no thought. Twin-sets normally consist of two tanks, which are joined together using a manifold, which gives the user access to both tanks through a single regulator, although this isn’t always the case. There are three general ways of diving with twin tanks:
1. Diving Independents
This is where two tanks are mounted on your back but are run as separate systems. If you have a failure with one tank then you have lost that entire gas supply and cannot gain access to it but still have access to the remaining tank. Diving with independents involves additional task loading and superior gas management skills to ensure that you always have a gas supply available in the alternative tank to the one which you are breathing.
You must also be able to differentiate between which gauge and which regulator belongs to which tank. This is a less than optimal choice when compared to the third alternative.
2. Diving Twin Tanks with a non-isolating manifold
This time the two tanks are joined via a manifold which takes the form of a permanentely open connection running between the two tanks. This enables you to gain access to the contents of either tank via either pillar valve. This has the benefit of avoiding the duplication of gauges and eliminates the need to keep swapping regulators. However, your twin cylinders are now effectively just a single larger tank. This deprives you of the redundancy which can be achieved by running two separate systems, since one failure can affect your entire gas supply. Consequently, in many failure modes this arrangement is less safe than independent cylinders and cannot be advocated except when diving in less than demanding environments such as when a single cylinder would equally suffice.
3. Diving Twin Tanks 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 tank then you should normally still be able to gain access to the contents of both tanks through one pillar valve.
If there is a worst case scenario, such as the failure of a main tank O-ring (highly unlikely) then you can “isolate” one tank 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 redundancy which is lost by using a non-isolating type manifold. See the section on “manifolds” below for additional considerations.
Twin tanks should always be mounted with the manifold uppermost, such 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’s full functionality at all times.
Sometimes you see tanks inverted, this is a distinct no-no, since you will then need a cage guard to protect the manifold. This is something which is subject to entanglement and an added unnecessary 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 are subject to corrosion and, should they fail or seize, make it impossible to operate the valves either directly or remotely.
Simply owning an isolation manifold is not enough, you also need to know how to use it properly. See the “Valve Drill” link for training with an Isolation manifold. The chance of a failure is highly unlikely but the options available to resolve or minimize the impact are greater than the alternative equipment types.
Cylinders should be of 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 mandatory decompression stops. One twin set is normally only sufficient for one dive.
Twin 10’s have a greater volume and are suitable for moderate club diving, which involves some degree of decompression for a single dive. They may be unsuitable if you wish to use them without a gas fill/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. You may have to support their weight on your back whilst sitting down and waiting to enter the water due to their reduced height.
Twin 12’s are normally suitable for a full day’s moderate club diving (two dives), which involve limited 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 the diver has to suffer, unless he has some form of motorized propulsion available. These are only normally used for over 70 metre-ish trimix dives with multiple stages.
The choice of tank depends on the 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 – obviously this is only the norm if you are in foreign climes. If you dive in a dry suit then steel tanks will remove some of the weight from around your waist, this is the reality for British divers.
As has already been discussed, using an isolating manifold is the only real choice, but there are a number of further considerations:
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(s) which seal around the exterior of the isolation manifold insert and the surrounding interior of the pillar valve. In contrast, Facing O-ring systems use an O-ring which is trapped much like a DIN fitting. That is to say, it is trapped between two parallel vertical surfaces.
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. The editor was present on two occasions when a twin set fell from a great height due to a lapse of attention by the owner, landing on the manifold. On both occasions, the barrel style manifold successfully kept the gas supply intact, in spite of there being considerable mis-alignment. This testifies to the resilience of this design.
One of these examples is shown on the left (Scubapro). The arrows run straight through the middle of each pillar at a 90 degree angle to emphasize the extent of the misalignment present in the middle section. Please see the recommendation on Scubapro Manifolds below before buying this product
Ideally barrel O’ring designs should have two O’rings on each side of the isolator since this provides some degree of redundancy. The first O’ring will also prevent a build up of dirt and salt next to the second O’ring, helping to keep it in pristine condition.
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 consequently be avoided.
Valves tend to operate by screwing a widget in and out by turning the knob’s spindle. A balanced valve either has a groove cut into the the widget’s threads to allow gas to equalise quickly from one side to the other (MDE) or a small hole (Scubapro). Unbalanced valves rely an the gas equalising by migrating along the threads which is normally somewhat slower…effectively you will find that the greater pressure inside the tank will aid opening a valve (easier) and inhibit closing it (stiffer).
It may be possible to “balance” an “unbalanced” valve by using a hack-saw to cut a groove within the valve internals, this is not recommended unless you know what you are doing!
These are more often called “tactile handles” in dive shop speak. These are far easier to grip than normal hard plastic knobs and far more resilient. The option of using solid brass handles may at first glance seem like a good option, but in fact they transfer shocks from the handle to the shaft of the valve and can cause the valve to seize preventing opening or closure; this is an additional problem of hard plastic knobs. In comparison rubber knobs will absorb a lot of the impact, resisting damage to the valve.
The rubber handle must have a metal insert to stop the thread being stripped through use.
These are used to hold the two cylinders together.
Cam band (webbing) systems are suitable for independent cylinders but completely inappropriate if the tanks are manifolded in any way, since the tanks aren’t held rigid. The subsequent 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 a figure eight with a bolt through the middle. This bolt is tightened to hold the tanks in place.
There are certain things to avoid:
Sharp edges on the bands, which will cut into the tanks’ paint work and encourage corrosion.
Spot welds, which are not as strong as continuous (seam) welding.
D Rings on the lower (and upper) bands which can easily become entangled during a dive, this entanglement is then difficult to clear. Band mounted D Rings offer nothing but problems to the diver since they are so difficult to reach during a dive.
Low marine-grade stainless steel.
Bands, which warp under stress.
Bands, which are too narrow and do not hold the tanks rigidly aligned
Cylinder boots, whilst maybe offering the benefit of greater stability to the single tank diver, offer little if nothing to the twin set diver. Twin sets are inherently far more stable when stood upright, they can only move forwards or backwards. Sets can be easily rigged whilst leant against a boat side/wall etc and one hand is usually enough to stabilize it whilst putting it on.
All in all, when you consider the scope for corrosion with cylinder boots, you are really better off without them. For further information including a picture of tank corrosion, see the relevant section under Which Tanks? Single Cylinders
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, even if you carry 2-3 decompression cylinders, provided they are of the “right” type and you are properly weighted.
A 35lb wing represents the minimum lift you should consider but future flexibility and the ability to carry stages with this sized wing would be limited.