Really deep rebreathers

[Freedive WI]

I was reading that a diver recently died cave diving with a rebreather while trying to recover the body of another, open circut, diver who died in the same cave 10 years earlier.

When I went to the site they had some interesting stuff on there about his different rebreathers that he used for different types of diving. Seems that he liked the Inspiration for the shallower stuff, the Cis-Lunar for the extened cave stuff, and his Mk 15.5 for the really deep stuff.

So here's my question, What makes the MK 15.5 so much better for the really deep stuff than the Cis-Lunar or the Inspiration?

BTW: Thisis not a rebreather slamming question, as anything done at that depth is risky. I just was wondering about the specifics of the units.

Here's a link to the guy's site:

Jon

[Mark Y]

From what I know, and I'm no expert , the Inspiration is rated to 330 ft., mostly because the PO2 alarm can implode at deeper depths (which, I've read, is depth tested to 160 meters). This allows water into the electronics... not a good thing. There is an interesting account of this happening at 180 meters on website.

Another limitation of the Inspiration is the scrubber duration... 3 hours. From what I understand, the MK 15.5 has a much longer duration.

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Go back to click the 'dive equipment' link. The author lists the advantages (in his opinion) of the MK 15.5 over the other rebreathers.

[Chuck_Northrup]

I've read a preliminary report which suggests that the cause of death was hypercapnia. The Mark 15.5 has back mounted lungs which when over-filled would make exhalation (and therefore good gas exchange) difficult. This is thought to be what happend to Dave Shaw.

Complications resulting from CO2 retention and increased narcosis at this extreme depth/workload combination brought a tragic end to this difficult operation.

[Freedive WI]

That sounds like what I heard Chuck. Something about it being caught on video and he got caught in a line at 850'+ and built up so much CO2 while he was trying to cut himself free that he simply passed out.

I read through a lot of different things on his site and am still not sure why he picked the MK15 over his cis-lunar. It makes sense to pick it over the Inspiration if the scrubber lasts a lot longer, but I thought that the Cis-lunar was supossed to be for really long penetrations with plenty of back-up. Since it also has a split counter lung it would seem to be able to breath a bit easier than a back lung- even if it is counter weighted.

So, back to my original question, why use a MK15 over the Cis-Lunar for exteme depth? The first part, vs. the Inspiration, has been answered.

Thanks,

Jon

[Chuck_Northrup]

Jon,

There is only one person who truly knows the answer to that question and unfortunately he can't tell us.

This is pure speculation...but possibly he felt the MK15 was more streamlined and provided a lower profile for the cave. Certainly the MK15 with Hammerhead controllers would be a less complicated system. The additional "safety features" on the Cis come at a price.

Personally I would have chosen a Meg with an adapted Cis scrubber...but I need a few more years of practice before I attempt 850+

[Mark Y]

Well Chuck, we could practice on the Fitzgerald... ;D