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Discussion Starter #1 (Edited)
Diving the Mk 6 Discovery Rebreather

I bought my unit in November of last year but with moving business premises over the New Year period didn’t have any time to really use or dive with it. I was interested to see why Poseidon had invested so much cash into developing a unit that cost the same as most others but with half the features.
I have always believed in being objective with rebreathers and the only true way to do this is buy one and dive it for a year or so, I think it’s going to be an interesting year for the Mk6 in more ways than one!

In February of this year I finally got around to firing it up with the help of my friend Hugh and he made a couple of dives with it in our local quarry, but we both struggled to get it started and after the first few dives had no success to make the unit pass it’s self checks, more about this later!

It would have been easy to look at the Mk6 with a negative attitude as the unit was so different in concept to anything I’d ever dived. I opted to keep an open mind and refrain from the cliché of ‘I know I’m going to hate this’ that user reviews often produce, and late into March saw me undertake my combined user and Instructor course.

The course itself was ‘interesting’ but not really the purpose of this post, I’m trying to forget the memories of stripping and building a Mk6 unit in the shower room (supposed to be a classroom) at Stoney Cove, nice start to the 1.5 day course it was not!

What I wanted to do was to get into the design concept of the unit and really see if this was indeed a unit to remove the complexities of CCR diving. I was looking forward to seeing how reliable the unit would / could be for everyday diving and here is what I found out over the last weekend in Vobster.
The unit has been designed to ‘fail safe’ which I initially thought meant it would never pass any checks and stop you from diving, that’s about as safe as you can get, but once over my initial cynicsm I did start to understand exactly what this meant.

You have to prepare the unit as you do any other with careful assembly, lubricated O rings, and dry cells before pre dive checks. The unit is by far the easiest unit I have ever stripped down and rebuilt, it takes little over 3 minutes to build the unit up in the morning and make ready for diving, it’s just so simple and the pre packed sofnodive cartridges make it even simpler albeit very expensive.

Once the unit is together you fire the handset up by either inserting the battery or making the wet contacts fire the unit up. Then the system takes over and pre dive checks itself through a series of 55 system checks, some of which need user participation which the hand set prompts you to do.
I have learned that the unit has to be stood up and lungs deflated prior to the checks or it will fail on the pressure tests. I have also learned that if the HUD is not secured properly this will also fail the tests, as will the calibration test if you leave the head in the van overnight and it gets cold. It is a fussy start up procedure in some ways, but actually when you think about it this means that the unit will only let you in the water if everything is absolutely perfect, whether this is a good thing or not remains to be seen in the future, for now you have to accept this as it is and allow the unit to do its own thing.

Prior to each days diving this weekend my unit would fail its checks every time on the calibration. The only way I could get around this was to take the head off (simple operation) and allow the cells to expose themselves to a warmer environment. I am unsure if this was just a co-incidence or actually made a difference, but every day I dived I had to do this then the unit worked. My biggest concern was the single cell system it employs with another cell looking at the main cell to check it’s all ok. To compensate for having no cell redundancy the unit continually dil flushes itself to check PO2 versus depth, and at the start of the dive it even does its own 02 linearity check, clever stuff indeed but the user can’t see what the results of these checks are, so we’re left trusting the unit to do its own thing. Worrying or refreshing?

Every other check was passed by the system and the only prompts you get that need acting upon are to turn the cylinders on and check the bail out valve is working by taking a few breaths. When the checks work by themselves it really could not be any simpler to fire this unit up and start the pre breathe system.
When the unit fails its checks you feel so frustrated as all you get is an error mode telling you which check has failed and the unit switches itself off so you have to start again.

When pre dive checks work first time the whole process takes less than 4 minutes, so you can build the unit up and pre dive check it in less than 10 minutes, which I think makes it the simplest unit to prepare that I’ve ever dived.

I was running a CCR Module 1 course this weekend and felt that this would be a good test to see how the unit fared on longish dives in cold water. We jumped in the water on Friday and made a dive to 12m for just over an hour and the unit was faultless, never missing a beat at any point during the dive.
I’d filled the 3 litre 02 cylinder up to 200 bar prior to the weekend and had managed to get 3 dives out of the unit and still had 60bar left in the cylinder. This is mainly due to the way in which the set point runs itself which is totally different to any other unit. On the surface the unit maintains 0.4 bar PO2 and slowly increases to a maximum of 1.2 bar PO2 as you descend. The unit I have is a no deco unit so it counts down your no stop time and when you get within 3 minutes of running into deco it tells you to go up. If you ignore it then it will run into decompression and give you ceiling information, but it won’t be happy and you’ll have all manner of alarms and warning buzzers going off, but it won’t lock you out, just sulks to itself and makes a load of noise!

When you start to ascend the set point starts to reduce but only enough to keep you out of decompression, so once you get to a depth where on gassing won’t occur then the set point starts to drop thus using a lot less 02 and making buoyancy so much easier. This is where the unit really scores as having watched loads of divers over the years struggling to hold buoyancy whilst ascending on the high 1.3bar set point I can understand why the Mk6 works in this way. But let’s not forget that you can manually run any other rebreather in exactly the same way as the Mk6, but the Mk6 doesn’t give you the option to run the higher set point all the way to the surface. By the time you are back at 6m you are not in deco and are running 0.7bar PO2 set point with loads of 02 left in the cylinder.
Obviously the decompression model due out later this year will have to run in a different way, but hopefully the low overall loop volume and excellent dump valve will help divers with their buoyancy control during ascent.

You don’t need to exhale through your mouth / nose during the ascent either, you just open the valve on the exhale counter lung and let the expanding gas escape this way. It also means that any water that ingresses into the lung through your mouthpiece also gets cleared automatically as you ascend, just like the Meg can do but without the huge oversized counterlungs! If you do get water in the loop then you can purge the water out through the purge valve.

On the Friday I had the lungs set up improperly but on Saturday I got it right and I will post some photos to show the differences in lung position which just made the unit even more comfortable. I was using a standard back plate harness and wing set up and the lungs work very well with this rig, I believe you can also use a standard BCD but I wonder how neat the lungs would be.

On the second day the unit only gave me problems only on the Calibration but once it was in dive mode I kept the unit switched on all day (battery life is 30 hours) and the second dive only momentarily flashed up a cell warning after being stood unused for 2 hours during the surface interval. This means you simply keep breathing the unit until the cell warning disappears, I had been warned this was quite common to see in between dives when the unit isn’t being used.
We made 2 dives of 70 minutes each in 6 degrees to a maximum depth of 25m. The unit was still using the same Sofnodive cartridge from the day before and when I exited the water after the second dive the heat was just over half way on the canister. I have no doubt I could have dived another 2 hours on the same canister but with no way of knowing and 30m dives planned for the next day I opted for another expensive lime fill.

During the dives I found the unit so simple and easy to use I forgot for a moment what I was diving. The mouthpiece is by far the most lightweight and comfortable of any unit and with the hoses poking downwards you have 100% clear vision, for film and photography divers this will be a huge advantage as you have no idea that there is a loop in front of you, brilliant design. I also found that the OC to CC switch was the simplest and easiest to be found on a CCR to date, very straightforward and with true one handed operation made for easier bail out switches.

Weighting was also impressive as I only needed 7kg in full winter underwear and with ally 3 litre cylinders. If you put steel 3 litres on instead I can only imagine you’d need much less lead. The trim was also just as good as the JJ which is by far the best trimmed unit I own right now, again I can only put this down to the position of the valves (head up) which just seem to give excellent natural trim and positioning.

Work of breathing is ok but not as good as other units, but definitely good enough. When you look at the hose diameter and overall size of the system and counter lungs it’s amazing it breathes so well being so small.
The only part of being underwater with the Mk6 I didn’t like was the handset, it just didn’t seem to sit particularly well anywhere on my rig and looking at it was a pain in the arse, I am sure in time this will be redesigned and they will go for a wireless through water version, which would make the unit even easier to use but possibly less reliable.

Unfortunately that’s where the good stuff ended! On Sunday morning my unit wouldn’t start and the error message was ‘insufficient battery power’. The unit only tells you an error number so you have to consult a check sheet to see what the number refers too. My handset was showing over 50% battery power on the handset yet it failed 6 times in a row to start each time referring to test 48 (battery power level). I recharged the battery and even then it wouldn’t start so early bath for us all, very disappointing and frustrating and left me wondering if I’d trust the unit ‘at work’ again.

Overall it was a real shame as up until that point I was genuinely enjoying the unit and even more so the simplicity of setting it up and diving it. I regularly use a Sentinel for my ‘working’ dives and find I spend too much time prepping and making it ready for diving. It was nice to just turn the Mk6 on and let it run through its checks, but not when you can’t get it in the water, it was a huge blow for my confidence in the unit.

I also know that loads other Instructors all have similar problems, and I have no doubt that a Company such as Poseidon have the ability to make this unit really, really good. Until that point arrives my own unit may have to take a back seat for a while, which for me personally is a real shame as I was just starting feeling good about it.

I hope that this report is of interest to some of you, if anyone has any questions please feel free to post here or email me to [email protected].

I will be posting some photos of the unit stripped down and rebuilt at some point over the coming week.


Rich Stevenson
Deep Blue Diving

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9,612 Posts
What an excellent, very detailed warts and all report.
I don't dive CCR, but like to keep an eye on what is out there, and that was very informative.

Notice my avatar. I am hard astern.
9,347 Posts
I had a similar problem in Malta with 'Low Battery Power' but it took only a few minutes on the high-tech charger to get it up to speed. (It isn't a conventional battery and charger.)

This CCR does not compete with Inspiration/Sentinel/JJ/rEvo and the like. It is for leisure diving shallower than 40m.
I think that the CC/OC set-up is terrific but it was obviously designed for no-stop diving with 'go OC' at the click of a switch as the only underwater option in case of a diagnosed problem.

I went in with a camera on each dive and photographed others (inc. Jack Ingle) using the Discovery while using one myself. It was that easy to use one-handed.

I found it was spoiled by the training agency's need for divers to carry separate OC bail-out because this was obviously not Bill Stone's concept and makes it look like technical diving as opposed to an alternative to no-stop leisure diving with OC. (Full details of my own experience with the Discovery is in Diver Mag and DYK Mag's May issue.)
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