Twinset manifolds – not the sexiest item of technical diving equipment by far and also not the most talked-about, you would think. Well, last week proved to be a little different – with three technical divers joining us in Bunaken for guided dives, we discovered that all of us had different ideas about manifold settings. So, who’s right?

We connect backmount doubles to give divers access to all of their backgas via one single regulator in case of emergency. Unless the manifold is fully shut, the diver will breathe from his main regulator attached to the right-hand tank throughout the dive, with no need to change regulators unless there is an emergency.

First things first, there will be no conclusive ‘once and for all’ answer to the question in this post. Think of it as more of a discussion starter. As such, we’ll take a look at different settings and their advantages and disadvantages.

Open Manifold

Personally, I have always dived with a fully open manifold. To me, the main advantage lies in the complete lack of sources of confusion: in case of an emergency involving a gas haemorrhage, the manifold will only turn one way which will close it. At 45 or 50 meters, simplicity is key. Every diver, even those of us who are very good at dealing with depth, is under a considerable narcotic load and even completing simple tasks can be tricky, let alone identifying and isolating a significant gas leak under stress.

Being able to move the manifold to the left and the right will add to an already high amount of task loading and can delay divers in effectively dealing with the leak. Having said that, a completely open manifold does require a larger number of turns to be completely closed than a partially closed manifold would.

So what do the training agencies say? I mainly teach TDI as well as PADI but let’s take a look at the DIR (‘Doing It Right’) rules for equipment configuration GUE (Global Underwater Explorers) have: “a closed isolator can create problems. The isolator should always be left completely open.”

Does that settle it? For some of us, it will. But let’s take a look at the other options.

Partially closed manifolds

Our three visiting divers had been taught by a mixture of TDI and SSI tech instructors. Two of them chose to have their manifolds fully closed and then opened three half turns as they had been taught and one opted to have it just cracked open.

That’s a considerable difference to the ‘fully open’ setting, and the guys’ reasoning was that in the event of a catastrophic failure it would take them less time to close down the valve. Their view is backed up by Kevin Gurr, one of IANTD’s first instructors (he’s number 6) and one of the first divers to bring technical diving to the UK. In his book Technical Diving: From the Bottom Up he states: “the centre valve should be just open during diving … In the event of a shutdown being required this is speedily achieved.” Looking at Gurr’s diving experience, which includes diving the wreck of the Britannic as well as developing rebreathers and dive computers, saying that he knows what he’s talking about would be an understatement, to say the least.

So who is correct? Is it really so easy to remember which way the manifold turns when it’s all going wrong at depth? Possibly, and in that case the diver would have an advantage by having to simply turn the valve three half turns for example. However, what if confusion sets in and the diver starts turning the manifold either way, becoming more (di)stressed in the process? Then, the above mentioned advantage might not only be lost, but the diver may even fail to close the manifold completely.

During technical diving courses, we spend a lot of time working on perfecting motor skills and trying to make them part of muscle memory, but what happens after the course is finished? How many technical divers truly keep up their motor skills and practice them again and again? Everybody should. As technical divers, we take higher risks even though we manage them by using redundant equipment, thorough planning and stricter procedures.

So, how will we conclude this? I’d like to leave it open to discussion. Personally, I remain in the ‘fully open’ camp, whilst our current (recreational) instructor candidate Fred, a technical diver himself, is on the other side. We have a plan – time permitting, we might just jump in the water and perform a timed manifold shutdown, first at 10 meters and then again at 40 meters. And, yes, we can already argue that the results will be skewed by the trial being prompted and not a real emergency. The debate continues, feel free to share your thoughts….

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