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The Megalodon CCR (and other thoughts by Harry)

posted Jun 24, 2013, 11:27 PM by Richard Harris   [ updated Apr 20, 2015, 4:54 AM ]
With depths in the Pearse Resurgence increasing with every visit, there is the ever present danger of respiratory insufficiency due to work of breathing and gas density at depth. Craig Challen and I spend a lot of time discussing the factors which contribute to this problem. We cannot do anything about the laws of physics, but there are several areas we can focus our attention. The way the dives are conducted is perhaps most important with a strong focus on descent rates, respiratory pattern and remaining relaxed and diving "in slow motion" to avoid excessive effort. Scooters are used exclusively at depth to avoid swimming and over exerting.

Craig and I have presented at numerous meetings on the issue of "effort independent respiratory failure" in deep diving, a phenomenon that was described in deep technical diving by Mitchell and Cronje et al after the death of Dave Shaw in South Africa. One factor that is of interest, is the possible benefit of over the shoulder counterlungs (OTSCLs) vs backmounted counterlungs (BMCLs) in splinting the airways open during exhalation. 

All of this is a long winded explanation of why I decided to try an OTSCL rebreather after many years of diving BMCL units. Craig was an early adopter of the Megalodon in 2004 and has never seen a reason to change. The unit is extremely robust and its modular design makes it very adaptable for using different cylinders, and in Craig's case, has lent itself to developing the very successful Twin Meg for extreme dives.

Craig Challen gears up in the Twin Meg, for a 200m dive in China.

Previous experience with the KISS Classic, the Mk 15.5 and the rEVO (all BMCL) had convinced me of the benefits of reducing chest clutter for complex dives. There is no question that OTSCLs immediately fill the chest area with "stuff" making access to chest D rings, dry suit valves and the other odds and ends that tech divers hang off themselves harder. I also had concerns about the effect on breathing when you push through a tight restriction...would the CLs be squashed and limit respiration?

The Megalodon
After some discussions with Leon Scamahorn (ISC) at OzTek one year I decided to give the Meg a try. I bought the Meg with Apecs 2.7 electronics (HUD, Primary and Secondary) and the large 8lb radial scrubber. It came with the Shadowmount cylinder attachment system, and I have put an aluminium backplate and a Dive Rite Recwing on.

Like most blokes I put it all together without reference to any manual or instructions and then went back to check my work afterwards! I actually find this a good test of how intuitive equipment is and the Meg passed this foolish test :-). Some new holes in an old back plate and slight lengthening of the harness (hey it fitted perfectly 10 years ago) and that was sorted. The Shadow mounts attach to the cylinders and sit well on the can with a single large pin to secure them...a bit tight and fiddly but partly because I still have in line valves on the cylinders. Things will fit better when I change them to ?T valves (the other kind). Build quality is what the Meg is famous for and it really does have the feel of a Sherman tank. Hoses, T-pieces and all the hose fittings have a very positive feel amd are threaded to avoid incorrect assembly. The counterlungs are large and well made and offer lots of options for on and off board gas connections. Full marks for all this. The large radial scrubber (is a monster!) is well made and easy to fill. I will need to get the smaller scrubber and spacer for "normal" dives! Mick Green my instructor dived the smaller can with the 5lb scrubber with 2 x 2L ally cylinders and it looked like a very nice smaller travel unit good for some hefty dives to around 100m I reckon.

I have to say, the first time I put the unit on I immediately felt a degree of concern about all the "stuff" around my neck and chest. It really is a different feel to a BMCL unit which feels like diving an open circuit twin set in this regard. Access to the shoulder D rings was non-existent, the lungs seemed to be around my ears and my downward and side vision was diminished. I would have to wait until I was in the water to see if these were genuine concerns.

First dives
My first few dives were in a beautiful clear sinkhole with Mick running me through my crossover course. Easy diving off a pontoon in 60m water makes for an awesome training site. From the moment I submerged I was struck by a couple of things. The work of breathing on this unit really does feel good! I have always been cynical of people who claim they can feel a big differenece between unit A and unit B, but after 10years of diving BMCL units and sidemount rebreathers I was (pleasantly) shocked to feel how nice this unit was to breathe. Real or imagined I cannot be sure but it felt genuinely good. And the best part is, that feeling stayed with me in any position...prone, supine, lateral or erect on the shotline. Very impressive! The second thing was that my trim was very easy to maintain in a nice horizontal attitude instead of the classic 30 degree head up position you often see CCR divers in. The large couterlungs suited my lung volumes well and unlike some units, I felt I could really take a full breath unimpeded. A degree of bouyancy control using these lungs seemed easier than with BMCLs also. The ADV worked well, the unit is easy to perform dil flush and O2 flush (at decompression) and everything felt very comfortable after just a couple of hours in the water. The HUD is the best I have used and sits in the perfect position. It is dimmable and aside from not having a vibrate function, is near perfect. It uses Smithers code and operates from the secondary display.

The displays
The two handsets are quite large, the menus are intuitive and easy to work through. For a man with poor close up vision the displays are not easy to read. LCD displays with a backlighting option are not a patch on the new OLED displays so I really struggle to see the PO2 display clearly on my arm. I tried the primary clipped off on a bungee and was able to read it OK when I flipped up to read square on. But really, this is the only significant criticism I can find with the unit and if my eyes were better maybe it would not be a problem. Anyway I hope ISC will produce an upgrade at some stage in the future.

Set point control is very tight and the HUD rarely wavered from setpoint during the dive. I really liked ISC's "ASP" setpoint control which gradually ramps up the setpoint during descent.

The last dive of the weekend followed the completion of the course when I did a long dive out to the back of Tank Cave with Craig Challen. I really enjoyed diving the unit in a cave that is not generally well suited to rebreather diving. Tank is a very shallow, gently undulating site that is unforgiving of the poor bouyancy control that often goes with diving rebreathers in the shallows. Anyway for some reason the bouyancy control on the Meg seemed easier than the backmounted units I have dived in there, perhaps because the lungs seem to lend themselves to use as bouyancy compensators more than BMCLs. Not in keeping with the concept of "minimum lung volume" I know, but it did seem a nice way to dive. I got to test one of my other concerns about OTSCLs; that is what happens when you get squashed in a restriction. Well I unintentionally put that to the test in a dramatic way when trying to ascend the "Goat Track"; a narrow flattener that heads up a rock collapse. I got comprehensively wedged halfway up (I'd normally be on sidemounts through there but I knew a lot of OC backmount divers pass OK), unable to reverse or advance. I was a little concerned because I wasn't sure which part of the unit was catching...the cylinders, the top of the can or the loop hoses. Anyway I had to force myself through in the end with OC bailout at the ready in case the loop flooded. I learn't several things on that dive (aside from the obvious about being an idiot). Firstly the can and cylinders take the impact of the rock and the loop is well protected. Secondly, the lungs can be squashed flat beneath you and breathing is unaffected, as the usable volume is residing in the top of the lungs at your shoulders. Thirdly, the lungs must be tough because I really gave them a bad time! It was a really good dive for learning a lot about the unit, but I think I'll stick with sidemounting for such resrictions. At least you have some more options for taking off gear and escaping!

Conclusion
There is no such thing as the perfect rebreather. Not yet anyway. For every feature that has a benefit there is usually a drawback. The rEVO was close to the perfect cave diving rebreather for me with its uncluttered chest, bright Shearwater display and excellent work of breathing. However it is not a modular design in so far as the possibility of sidemounting it or twinning it up. I dived it to over 200m and it worked beautifully. I think the Meg will suit my kind of diving even better with its tough as nails construction and excellent "breathability" in different diving attitudes. At this early stage, I'm impressed! I just need new eyeballs ;-)
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