I wonder how bad gas (thinking CO here) would affect a ccr diver on a flat profile dive. Theoretically, there would be minimal dil added to the loop, therefore a limited amount of CO, probably all absorbed eventually and bound to hemoglobin. Would that minimal (in terms of volume, not dose) amount be enough to kill a diver?
CO would kill both of them on the bottom. As they descend they have to add dil (which is presumably where the CO comes from) and the ambient pressure would increase their pCO just like it increases ppO2 and the highest concentration and largest effects would be on the bottom, also once they're on the bottom they wouldn't be touching the dil much assuming a roughly squarish profile. Same logic for pretty much any other contaminant in the gas. I think it works the same with CO in the O2 bottle although most of the volume of O2 is likely to be used on the ascent, still you have to use O2 on the bottom so the pCO would increase there as well. Also if they did a prebreathe of any sort and checked their O2 increased then the loop would have become contaminated on the surface and then when they descended it would have increased the pCO of the loop.
I believe this was also the third dive of the day according to the reports? Which makes it even more unlikely its gas contamination.
So to get this due to gas contamination it has to look something like both of them swapped tanks for the third dive. They both had been diving tanks that were fine, and both swapped to contaminated tanks. They both failed to do any prebreathing off of the contaminated tanks on the surface. The contaminant was in the O2 bottle not the dil so it didn't hammer on them on the descent as they were adding dil, but gradually they felt worse on the bottom, then thumbed the dive, and as they flushed with O2 on the way up they spiked the contaminant levels and both of them became even more symptomatic. Which just doesn't seem plausible.
More likely they were doing three deep dives and both of them did seriously insufficient deco on the third dive and both had type 2 neuro-DCS symptoms.
Its possible that they both went hypoxic, but I'd expect that to look more like they ran out of all their O2 and both wound up deco'ing on 10/70 bailout, and then both of them attempted to blow-and-go to the surface from 20 feet.
EDIT: another possibility might be that they both pushed their scrubbers out too long and with similar enough physiology and identical units they both may have had CO2 hits at the same time (3rd dive the dave without swapping canisters?). Of course that requires both of them to fail to bailout. Seems a bit unlikely, although if they bailed out then it might have turned into the hypoxic scenario above with both CCRs failed and inadequate bailout gas. But I think omitted deco is still much more plausible than anything else (see: Chris and Chrissy Rouse).
