IT HAS OFTEN BEEN ASSUMED that Bill Stone, the rebreather pioneer, was pursuing a contract with space agency NASA for an astronauts life-support system when he embarked on some extensive Florida cave-diving expeditions more than two decades ago.
For this purpose, Stone developed the CIS-Lunar closed-circuit rebreather in its various manifestations.
It was over-engineered with multiple redundancy systems, it was the size of a small wardrobe, and the clue to its origins was in the name.
Cut to the international diving trade show DEMA three years ago. There, to everyones surprise, was Bill Stone again, but this time with a prototype of an electronically operated rebreather, aimed not at the most extreme of technical divers, but at ordinary leisure divers, those who simply wants to swim about, not that deep, looking at fish.
This time, Stone had focused on designing a rebreather that would be as simple to set up and use as possible.
The Poseidon CIS-Lunar Mk6 used a pre-packed cartridge canister for its scrubber, and was computer-driven.
If you have read anything about closed-circuit rebreathers, the advantages are clear. In the main, what matters is the ability to maintain the ideal gas mix at any given depth, keeping the partial pressure of oxygen (PO2) at around the same value.
Unlike with conventional open-circuit scuba, doing this keeps decompression obligations to a minimum.
Most CCRs have three oxygen-sensing cells and some kind of voting logic (or redundancy system) to account for cell failures. What really separated this prototype CCR from other rebreathers was that it required only one oxygen-sensing cell.
This was constantly and automatically recalibrated, using two micro-valves that alternately blew air from the diluent cylinder as well as pure oxygen over the cell, to give two known fixed points for the PO2.
That night, after seeing the Poseidon CCR, I had dinner with Bret Gilliam, the founder of technical training agency TDI, but now making the majority of his income from diving expertise pertaining to law-suits.
With us were a couple of lawyers, and a diving insurance expert. They were anticipating plenty of business if and when this single-cell rebreather finally came to market. I was fascinated.

New Discovery
There has been plenty of water under the bridge since then. After a lot of development work, the Swedish manufacturer Poseidon has finally reached the point at which it feels
it can offer its rebreather for sale.
The single-cell idea fell by the wayside, although the calibration of the two oxygen-sensing cells using both air and oxygen has not. One cell acts as big brother to the other, constantly checking how it is performing. The CCR has a slightly different name, too. Its called the Poseidon Discovery MkVI.
Most people progress to closed-circuit diving after getting into deep diving using helium in the breathing mix. Helium is an expensive gas to exhale wastefully into the water, as you do on open circuit, and the prime motivating force for such a move seems to be the enormous saving in gas costs.
Rebreathers therefore usually appeal mainly to technical divers.
Less vociferous as a group are those divers who use rebreathers with air as the diluent (with the machine making nitrox on the fly) to extend their bottom times and get closer to marine life. Nowadays, I put myself firmly into this category.
For example, I have dived with scalloped hammerhead sharks on five trips to the Cocos Islands. I used closed-circuit on four of these visits and conventional scuba on the other, and noted the palpable difference that lack of bubbles made to my ability to get near these skittish animals.
You may not be invisible, but without exhaled bubbles to give away your position, and a noisy regulator, you can stalk animals under water much as a hunter/photographer might do on land. You also have time on your side.
Many leisure divers would like to use a rebreather in this way, but are not prepared to embark on the long training regime that prepares them for the technicalities of deco-stop diving.
They dont want the bother of preparing and maintaining a unit, nor are they prepared to accept the perceived additional risks of using closed-circuit equipment. They want safe, simple, easily prepared equipment that allows them to do straightforward, no-deco-stop diving in the less-than-40m range.
The Poseidon Discovery aims to appeal to such divers, and even to those just about to take up diving.

SO WHAT ARE THE ADVANTAGES of this sort of kit over open-circuit scuba
It doesnt allow you to go any deeper, because as a nitrox rebreather it is intended for use at a maximum of 40m, like OC kit. With a CCR you use only the gas you actually metabolise, which is very little, so what it does do is remove the worry of running out of gas.
It gives you lots of bottom-time but without the deco-stops, because you breathe the minimum of nitrogen in the constantly readjusted-by-depth nitrox supply. So it should make diving simpler and safer.
With OC scuba, you need only to be sure of having enough gas to breathe. With CCRs you can always breathe, but need to know which gas you are inhaling. Poseidon has attempted to use modern computer technology to take this important decision-making away from the Discovery MkVI diver.
Incidentally, ever-conscious of excess-baggage charges, I note that the all-up weight of the unit is only 18kg, which makes it practicable to take to warmwater destinations.
Lets face it, the sort of people who have the asking price of £4700 to spend looking at the pretty fishes will not be jumping into freezing lakes, or engaging in club-talk at the end of a British jetty!
Our first report on the Discovery was last year, when a non-CCR diver tried it out, on the basis that he would be typical of the market (Bubble-Free Poseidon, June 2009). Now I wanted a go.

I HAVE ALWAYS BEEN A GREAT FAN of the closed-circuit idea. I have used rebreathers for 18 years and have had experience on the Prism, the semi-closed Dräger, the Inspiration and Evolution (including time with the prototypes of the Inspiration) and, more recently, the Sentinel. I have also turned down the use of another rebreather in which I had no confidence - subsequent sales have indicated that I wasnt alone.
Long ago, I wrote an article for DIVER entitled: I Have Seen the Future and It Works! You wont find it online because I wrote it before the Internet was in popular use.
However, since then I have been shocked by the number of fatalities associated with rebreather use, and have often wondered if I was right to promote it so enthusiastically in those early days.
Poseidon has promised to provide a rebreather that is foolproof to set up and use, and which absolves the user from too much individual responsibility in assembling it correctly and ready for use.
I went to Malta to get in the water with the Discovery under the watchful eye of chirpy Cockney Poseidon Discovery instructor-trainer Jack Ingle.
I was joined by Continental diving journalists and potential distributors. Many of them had never dived a closed-circuit rebreather before. Our host was Alan Whitehead of Divewise.

At first glance the Discovery looks as complicated as any other rebreather, even if its dimensions do seem far more manageable.
The scrubber canister with its double O-rings and four locking bolts is quite fiddly to assemble successfully, even though it uses factory pre-packed cartridges. It becomes especially fiddly if the rebreather has been part-assembled already. You really need to assemble the scrubber before putting any of the other components together.
Use of a pre-packed cartridge of CO2 scrubber absolves the user of responsibility for packing it correctly. This said, I have already noticed various alternative accessories being developed by independent manufacturers to allow the use of loose scrubber material that you pack yourself - to save a few quid on the cost of Sofnolime.
I thought the corrugated breathing-hoses were a little toy-like, as are their connections.
The on-board computer has four processors. One is in the special battery-pack, one in the electronic head, one in the handset and one in the mouthpiece.
These work together to do all the pre-dive checks. The only contribution the diver makes is to turn on the oxygen and diluent valves when prompted. There are two oxygen control solenoids, and two for the air diluent.
You have to know what a problem is and how to rectify it should the computer flag one up during this automatic pre-dive sequence.
The computer checks around 36 items automatically, and does so very quickly. If it discovers a problem, it wont let you continue until its been rectified.
Jack Ingle had conveniently provided a list abstracted from the instruction manual of fault numbers as displayed, and their 62 different remedies.
I suggested that it would be good if this list was printed on a small set of plastic slates that could be carried easily and survive handling at a dive site.
You still need to do a three-minute pre-dive breathe to check that all is well.

In the water
The mouthpiece was very easy to switch from closed- to open-circuit. It is closed in the OC bail-out position, but I did find it easy to over- or under-shoot the relevant positions of the control lever. In this case the computer handset display shows NC, meaning no circuit, the mouthpiece vibrates and the red light shows on the head-up display (HUD).
Most other rebreathers permanently show something on their HUD, if only a comforting green light (green is good). The Discovery HUD reveals itself only when the red light shows, and at first I was a bit worried that I might miss it.
My mistake on opening the mouthpiece too far to get NC on the display also gave me a chance to see this red light working.
The mouthpiece is very clever, because it tells the computer whether youre on open-circuit, closed-circuit or have failed to move the setting lever all the way and got neither.
You do need to check the handset every 30 seconds or so, and as a working underwater photographer I felt it would have been better mounted on the wrist rather than on a console, to avoid groping for it.
Others must have suggested this before me, because I saw a prototype wrist-mount while I was with the Poseidon team.
Diving in the rather chilly Maltese water in February and using the warm OThree drysuit required to wear with the Discovery meant that I carried a massive 21kg of ballast.
We were diving in shallow water to start with, and even with this inordinate load, and zero gas in my suit or the integral Poseidon BeSea wing, I found myself floating upwards whenever I was photographing other Discovery divers within a metre or so of the surface.

ON DESCENT, THE SET-POINT for the PO2 in the mix automatically increases in increments of one-10th of a bar.
At the surface you get a PO2 of 0.4bar, and this will reach a maximum of 1.2bar.
The handset shows this along with depth, maximum depth, dive time, how much gas remains in each of the two tanks and, most importantly, remaining no-stop time. Its all very simple.
The handset display tells you what percentage of gas you have left in each of the 3-litre cylinders. For an average leisure dive, you might use around 35% of the total available, unless you need to bail out to open circuit.
The Discoverys inhalation and exhalation counterlungs are mounted at the upper chest. This could result in that Mae West feeling, but it didnt.
The over-the-shoulder counterlung design keeps breathing effort to a minimum for most body attitudes, and the Discovery proved overall to offer a very low work of breathing.
The lightweight inhalation and exhalation hoses are angled downwards from the mouthpiece so that they dont become intrusive, as on some other units. The exhalation lung traps any water that might make its way past the mouthpiece, and after a days diving enough had collected to make it worth emptying out.
The divers who had never used a rebreather before seemed to get on with it as well as I did. An over-pressure valve on the exhalation lung takes care of expanding gas for those unaware of the technique of blowing it out through their noses, and I never suffered from hamster cheeks on an ascent.

THINGS DIDNT ALWAYS GO WELL for me. For my third dive, I made the mistake of swapping to a privately owned rig that belonged to a skipper living in Malta. He was shorter than me, and dived in a wetsuit.
He had customised his rig with a Hollis wing and steel backplate, to which he had permanently attached 8kg of lead. This meant that my ballast was positioned much higher up my body than it had been with the BeSea wing, with its integrated-weights. Over a metre of my long legs was left unweighted.
Once in the water, it was clear that my trim was going to turn me head-down.
I had a very uncomfortable dive fighting the inversion being forced by a badly trimmed rig and the legs of my drysuit. You need a lot of lead with the Discovery MkVI, but you must be circumspect about where you position it.

Keep it simple
I also felt that the Discovery MkVI had too many straps. This makes donning it something of a chore, and indeed I think many divers would be too daunted by this to wear it. Its a pity you cant simply slip it on and go diving.
The user isnt expected to learn how to cope with all the possible problems one might encounter with a rebreather. If anything goes wrong, the Discovery simply asks the diver to do one thing, which is to switch to open-circuit and bail out to the surface.
Despite the theory that one can always bail out to the on-board diluent supply, the training agencies, because of their own product-liability insurance, insist that all CCR divers carry a separate and independent sling-tank of unspecified size with an open-circuit regulator.
This is slightly onerous for shallow dives, and immediately makes the whole set-up look like technical diving.
Considering that one is never deeper than 40m, and always within a no-decompression stop dive profile, I would have thought a 5-litre tank of air or nitrox 28 (an upsized diluent tank) would be more than adequate to get me back to the surface safely if need be.
Other than that, the Discovery feels like a conventional rebreather. Its just that the pre-preparation has been taken over by a computer, so you dont have to act on information displayed.
As such, this is not a technical diving rebreather, but one firmly aimed at leisure divers. Whether the demand among such divers will make Poseidons investment in this complex technology worthwhile, considering its present purchase price, remains to be seen.
Like all mass-produced goods, the price will come down as the volume of sales goes up.
Poseidon needs to identify, and sell to, the first few thousand customers before this can happen. I have seen a possible future and it might work!

The Poseidon Discovery MkVI costs £4700 with tanks but without BC.