Poseidon Cis Lunar Mk6 Discovery CCR review

Let’s make this clear, I am not a rebreather diver. I have dabbled in the past, spending an afternoon in a pool on the Draeger Dolphin semi-closed unit and a couple of days with the Buddy Inspiration CCR with Martin Parker down in Cornwall, but I have never been fully convinced to take the plunge. The semi-closed circuit rebreather didn’t seem to offer much over my normal open-circuit rig, and the Inspiration – and other electronic fully closed-circuit rebreathers I have seen but not dived – were undoubtedly fantastic, very capable units which have been to extreme depths, etc, but for me personally seemed to require far too much time and discipline on the set-up and break-down stages. I am just used to sticking a BCD or wing on to a cylinder, bunging a first stage on the pillar valve, turning it on and jumping in. All that faffing about with O-rings and scrubber systems just didn’t set my world alight. However, my view on CCRs has been mellowed slightly by a couple of days spent diving the Poseidon Cis Lunar Mk6 Discovery, which has been launched on to the market aimed squarely at recreational users.

I was a bit dubious when Simon Morris, MD of Poseidon Diving Systems, invited me up to give the Discovery a trial, but I had seen prototypes of the unit at various shows, and the genuine article at Dive 2009, and I knew it was hitting the diving world designed to be used by divers in recreational situations. More importantly, it was supposed to be extremely user-friendly and great for non-rebreather divers to get their first taste of CCR diving – that’d be me, then!

So it came to pass that I drove up the A14 and M1 to Stoney Cove at the beginning of December – why when I agreed to do this did I not think of what the temperatures would be like at that time of year? – to hop on a week-long course for a couple of days to see how the unit performed firsthand.

The course was being run by well-known technical diving instructor Jack Ingle, and it was primarily for other CCR instructors to swap over to the Discovery, and non-rebreather instructors to get qualified to teach with the unit. Jack teaches CCR courses on various rebreathers, but it was interesting to hear him saying that he thought the Discovery was one of the best on the market, and that certain functions it has will soon be seen being adopted by other manufacturers. High praise indeed from someone of his calibre.

With outside temperatures hovering in the low single-figures, the first day was spent in one of the nice, warm classrooms in The Underwater Centre at Stoney Cove getting to grips with the unit. Jack basically started from scratch, opening a brand-new box in front of us and taking out and discussing each part until a disassembled but fully complete CCR was lying on the desk. There didn’t seem to be a lot there! With this CCR being aimed at recreational divers – it is designed to be used to a maximum depth of 40m with no decompression – this simplicity was a welcome sight.

Once the unit was out of the box, Jack took us through the assembly process, again step by step, and we all basically shadowed him, following what he did on our own rebreathers. The main body of the unit is solidly constructed from aluminium and durable plastics, and it has been well thought out – for example, the bottom of the Discovery will only go on one way, so you can’t get it wrong. Similarly, the head unit, which contains all the fancy electronics and sensors, can only slot into place one way. All the hoses to link the cylinder first stages to the rebreather are coloured coded so you don’t end up getting an oxygen one on the diluent, and vice versa. The BOV mouthpiece is, according to Jack, the best-designed and most-comfortable available. It does sit extremely well in your mouth, and switching from open circuit to closed circuit, and back again, is simply a case of moving a lever 90 degrees. In the event of an emergency or the unit not being happy with something and wanting you to bail out on to open circuit and go to the surface (basically the unit’s solution to any major issue while diving), you will hear a buzzer, feel the mouthpiece vibrate and see a red light flash on the HUD in front of your mask. With my ancient traditional-style twin-lens black-skirted mask, I couldn’t see the HUD, but there was no mistaking the mouthpiece vibration or the sound of the buzzer. With a teardrop-style mask, it was possible to see the HUD easily, even with a black skirt, but it is worth bearing in mind that you might need to get another mask if yours doesn’t allow you to see the HUD.

One thing I was impressed with was the scrubber canister. Unlike other units where you need to pour loose absorbent into a canister, here they are pre-packed and just slot into place. No need to spend time patting and shaking canisters to ensure there are no air spaces or channels in the ‘sorb, just slide the pre-packed canister into the unit. Dive time on a single canister is advised as three hours, and if you have done a couple of hours one day and are going to be diving again the next, you can put the used canister in a ziplock bag or slot the lids back into place and then reuse it for another hour the following day. It has been tested beyond three hours, but Poseidon are playing safe by advising users stick to replacing the canisters after three hours of use. There is a cost implication in using pre-packed canisters – they come in at around £20 each – but the piece of mind you get from knowing these were professionally packed in the factory that makes the absorbent, along with the ease of use, helps offset this to an extent.

Once the Discovery is assembled, the really clever part kicks in, which is the automated pre-dive system PST (power up self-test). You turn on the handset, or ‘paddle’ as it is referred to, by wetting two contacts on the back, and then the unit starts running rapidly through a series of checks. Once it has reached a certain stage, you are finally asked to do something – turn on the oxygen. Once it has monitored both the oxygen content and the fill of the three-litre cylinder, it asks you to turn on the diluent – in this case air, other gases will be online in 2010 (see later) – and again checks the pressure, etc. It then asks you to take a couple of breaths, and then is it off again on another series of checks. Watching your unit buzz, light up, vibrate and inflate and deflate by itself is slightly unnerving, but it shows how advanced the electronics are in this rebreather. Finally it will ask you to turn the mouthpiece to closed circuit, then back to open circuit, and last but not least, breathe the loop on closed circuit for a few minutes to ensure it is maintaining the right PPo2 level. After that it is a case of turning the mouthpiece to open circuit and that’s that, you are ready to go diving. Written here, this PST seems to take forever, but in reality it is all over and done with in a matter of minutes.

For our first foray into the water, we went down to The Underwater Centre’s indoor pool. Though the water is lovely and warm, we went in with our drysuits on, so that we could get our weight sorted out ready to dive in the Cove.

One by one we got into the water and Jack worked with us individually to get us submerged and weighted correctly. Just getting underwater in a rebreather takes a lot more weight than I use on open circuit, and while Jack did say that as your experience on CCRs grows, you can drop some of it, you will always use more weight as, in his words, ‘a CCR is just a big bag of wind attached to you’.

A long-time open-circuit diver, I am used to breathing out and sinking, but with the Discovery, I had to get used to changing my buoyancy by venting air through my nose, so it bubbled out of the skirt of my mask. I think that was the hardest thing to get used to – on open circuit you are able to control your buoyancy by breathing in or breathing out, here it makes no difference as the air in your lungs just goes into the counterlungs, or back again, so there is no change in your buoyancy whatsoever. Once you’ve achieved neutral buoyancy, and you are just hanging there with no sound or bubbles, it is an awesome feeling, but it does feel very alien.

We all spent an hour or so in the pool, getting used to the feel of the unit, moving up and down in the water column, switching back and forth from open circuit to closed circuit, and monitoring the read-out on the ‘paddle’. Once Jack was happy that we’d all got to grips with the Discovery, it was time to move on.

A coffee and some essential diver-fuel later – bacon-and-egg cobs are the way forward – and we were ready to get into the Cove. We all ‘booted up’ our units again and ran through the PST before we walked down to the water’s edge and made our way into the chilly waters. Once in the water, Jack checked that we were all comfortable and that we were monitoring our ‘paddles’ and then we slowly descended and made our way down to the 6-7m ledge. We spent the next 35 minutes swimming up and down the ledge, from the Viscount cockpit to the Nemo submarine, fine-tuning our buoyancy now that we could achieve a bit more depth, and running through a few drills, including switching back and forth between open and closed circuit, going up and dropping down a few metres, and emptying the loop of any gloop that had gathered during our breathing cycles.

We ended the session with a quick swim down the ‘road’ to 15m, which gave us another chance to work on our buoyancy both descending and ascending, before heading back to the side to get out for a debrief.
As I was just joining for a few days to get a taster on the unit, this was where I left off, but the remaining course members would be working progressively deeper over the next few days, running through more drills, including working with their back-up side-slung stage cylinders. However, the Poseidon Cis Lunar Mk6 Discovery has captured my imagination, and I can see myself signing up for a full course in 2010…