Jack Ingle is the BSACs Technical Diving Adviser. He is a BSAC National Instructor, an IANTD Technical Instructor, a TDI Tri-mix instructor and co-author of NSAC nitrox courses.
I dive twin 12s with a straight manifold. I use this configuration because I was told that there are fewer failure points. In the event of first-stage malfunction I can shut down one valve and still breathe both cylinders. I am now looking to advance my training level but every course I look at seems to preach the use of isolation manifolds. I can see that if a cylinder neck seal was to fail this would be a good idea, but I have asked numerous people if anyone has ever heard of this happening and have been told that it may only happen when cylinders are being filled. I look forward to your thoughts on this.
Gerald King

This is the age-old argument of straight manifolds versus isolated manifolds, and you could also throw into the equation independent twinned cylinders.
     Lets stick to the question you ask: is a straight manifold safer than an isolated manifold
     As usual there are pros and cons for both pieces of equipment. Straight manifolds do have fewer O-rings and this may mean that there are fewer failure points, as you say. However, if a failure or free-flow does occur, it will take a little time to shut down the cylinder valve that has the problem, and you can lose a fair amount of gas from both cylinders in that time. 
The centre isolator manifold has more O-rings, so potentially more chance of one of them failing, but it is far quicker to shut down a centre isolator valve if you dive with it the way I do - which is just half a turn open.
     This means that you have isolated the two cylinders within seconds of a failure occurring and are now losing gas only from one cylinder, which can then be shut down.
     This whole process sounds very simple in the relaxed atmosphere of a theory discussion, but it is far from relaxing at depth trying to cope with a free-flow and valve shutdowns. It is for this reason that I urge you to look at both systems and try them out. See if you can easily get to all valves, centre isolators and cylinder valves.
     I believe that you should be able to shut down a free-flowing regulator within 10 seconds (though you can still lose bucketloads of gas in that time). I see many divers using each system but who are unable to get to any of the valves, and that is a potential death trap.
     On your other point of a cylinder neck seal failure, youre right to say that this is slightly more of a problem with a straight manifold, but its a very rare situation for such a failure to cause a major gas leak while diving.
     You will normally see a very fine stream of bubbles if the seal is broken and you then need to get the cylinders to your local dive technician to remove the cylinder valve, check the neck and threads, replace the O-ring and refit the valve. A sealing check will be done at the dive shop once the job has been done.

We want to chat under water
I am contemplating buying a pair of Interspiro Divator Mk2 masks including radio communication for my wife and I to use. What are your views on the plus and minus values of this mask and other such masks
Tony Ellison

There are various makes of full-face mask in the marketplace but the Interspiro has a good track record and has been the workhorse of many police and military dive teams. It comes in two forms: the positive-pressure mask, where a constant flow of gas is available at all times, or a demand version in which gas flows as the diver inhales. I like the demand system, but many divers I know prefer positive pressure.

A big advantage with a full-face mask is environmental insulation, saving the diver being exposed to polluted or cold water. With a full-face mask you can breathe via both mouth and nose, which is more natural and less stressful, though it does mean that you will probably use about 10% more gas than with a standard reg.
     As you have pointed out, you can use underwater comms systems with such a mask. The STX 100 Buddy phone system works well with the Interspiro.
     These systems allow diver-to-diver or diver-to-surface communication and I have seen them usefully employed to enhance training programmes.
     One other big advantage is for disabled divers, who may not be able to hold a standard bite-type regulator in their mouth, as the spider-head harness does the job for them.
     Finally, the Interspiro, like many other types of full-face mask, is a very high-performance system and has considerable tolerance of low water temperatures. It performs very well at depth and reduces the chance of free-flows in cold temperatures.
     The disadvantages are few but need highlighting. It is important that all full-face-mask divers carry a back-up octopus rig and spare half-mask to which they can revert should the mask malfunction during a dive.
     Also, if you are carrying out dives on which you want to switch gases, perhaps to a nitrox decompression gas, it is important to use a good switching-block that allows you to be certain of which gas is being fed into the mask.
     One area that seems to have been missed by the manufacturers is training. You can buy the equipment but few places offer courses. However, the BSAC offers a course for the Interspiro, Exo 26, Scubapro and Ocean Reef masks as well as the Buddy phone comms and Thompson switch block.
     In recent years many technical divers have used full-face masks for comfort and safety. Grip a regulator in your teeth for a long time on a demanding deco schedule, and when you get out of the water your jaw can be almost locked up, hence the comfort factor.
     The safety issue is about using rich decompression gases for long periods. If a diver suffered unconsciousness or an oxygen convulsion, he or she would still be getting gas via the full-face mask when a standard regulator would have dropped out of the mouth and led to drowning.
     I have referred to the Interspiro but other full-face masks are available, including a new two-piece mask for gas-switching from Abyss Diving Systems. My advice would be to try out as many as you can before making a decision.

Best of both worlds
I have been diving using an air computer but would like to shorten my deco stops by using nitrox. I have seen versions of PC software to calculate stops and wanted to know if this is better than using a nitrox computer
Brian Small

It is very difficult to say that one is better than the other, as they do a similar job in different ways. The dive computer, whether it is for air, nitrox or mixed gas, calculates the decompression schedule for a dive taking into account the actual depths and times to which the divers have exposed themselves. With this information and the type of gases the diver has been using, the deco-stop times will be displayed on the screen for use during the dive.
     PC-generated software relies on information entered into the software before the dive, and the maximum depth entered assumes a square profile, so the software assumes that the diver will stay at maximum depth for the total bottom time.
     As we know, diving is rarely like this. Depths can vary throughout the dive but such software does build in a safety factor if the diver is ascending gradually rather than staying at one depth.
     During the ascent phase of the dive various decompression gases (nitrox) can be entered into the software to reduce the decompression schedule. This is sometimes called Ãaccelerating the tableÓ. The diver must carry out the planned dive as he or she has entered it into the software before the dive, and make the gas switches at the pre-planned depths.
     Many technical divers use both systems: a dive computer that can be programmed for the gases he is planning to use and a decompression software programme as a back-up in case the computer happens to fail, for whatever reason, during the dive.
Above: Whether to go for independent twins, a straight manifold or an isolation manifold like the one above is debatable, but is cylinder neck seal failure more likely with an non-isolated manifold: Below: Not the long-established Interspiro full-face mask but the latest offering from Abyss, its two-piece Super Mask, coming soon in Diver Tests
Above: A Uwatec Smart computer, one of many possible choices for nitrox divers