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