Brad_Horn
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This occurred per the article on a Drager LAR7000; which has two modes O2 only and SCR Nitrox at a constant flow. Typically in the order of 5L/min but it varies dependent on the exact gas 40/60, 60/40 etc etc and planned depth. You have a switch as in physical lever that you can change the mode from O2 CCR to SCR and back. As the diver did he also closed off the OPV in O2 mode to stop escaping bubbles and essentially meaning as he consumed the oxygen he was then just rebreathing the remaining nitrogen.
Typically in O2 mode you purge the loop to ensure you are just breathing pure O2 but when switching to SCR mode it is more a case of continuously flushing fresh gas at a constant rate into the loop to prevent hypoxia.
An O2 demand valve is exactly that providing O2 on demand as you breath down the loop. By turning off the O2 cylinder if he had surfaced he would have been able to prove that he wasn't diving in O2 mode and consuming oxygen.
No electronics are fitted to these units and therefore no PPO2 monitoring. This style of rebreather being designed for covert tactical operations as opposed to MCM which nowadays typically do have PPo2 controllers but not in all instances especially if SCR.
NEDU did try fitting temp stik style CO2 monitors to the O2 only Mk25 which is the same base Drager CCR design but I think that went the way of the dodo when they worked out how inaccurate they are.
Due to the extremes that these sets are pushed to you'd likely also find that the scrubber is completely exhausted at the end of these dives with very little buffer remaining. Depending how far into the dive this was Hypercapnia may have also played a part.
Functionally it looks like they were pushing physics through trying to extend the duration of their Nitrox gas endurance at 6m finning hard probably over a straight line course in the thousands of yards. The sets actual Nitrox duration being fixed based on the amount of gas supplied in the cylinder 2L 200bar typically for a Drager unit and the depth of the dive. The only variable being the diver themselves whom will be very very fit and their build giving an indication of the amount of oxygen they will consume. You'd probably find in the Drager User Manual for this set a fixed nitrox gas duration at constant depth that would provide the average max endurance of the set.
In this case it would appear that the diver was in SCR mode but then switching to the O2 CCR mode but with the O2 turned off in order to stop the flow of the Nitrox. In concept extending his duration through pauses to the flow of Nitrox albeit without any means of verifying the oxygen content of the loop. In reality he would have had a very Hypoxic loop quite rapidly.
There are some good NEDU trials that I think DAN published a few years back which identified that they found on a different variant of the same make of rebreather you could actually get the loop to go Hypoxic without doing anything wrong. Just being the wrong diver at the wrong time finning hard was sufficient. Which is why in some newer military SCR units Nitrox is supplied at a slightly higher rate of flow at the cost of gas endurance or fitment of larger capacity cylinders.
OSELs preference is that for SCR diving of this nature our Incursion CMR which can also be specced with an O2/SCR switched capability are optioned with PPO2 monitoring as standard but this is not always suitable for the specific mission that the sets are used for. For our back mounted SCR sets we mandate it due to the likelihood of their use on deeper dives down to 100m+. In both cases they enable an element of longer endurance under high workload through the much lower WOB that the sets provide through design and therein reduced gas density. Though they are still limited by the same physics.
