On the topic of forgotten questions, I asked you elsewhere "One question for you as a diver: How do you personally plan and calculate your bail out gas given your knowledge? It's kinda like asking a Dentist if he flosses..........
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Sorry Peter, I have been travelling and busy. My apologies.
Can I be clear that I don't consider my opinion / practice any more valid than anyone else's on this subject. Whilst I have a deep understanding of CO2 physiology, and while a CO2 "hit" is almost certainly the worst case scenario in terms of bailout gas consumption, I think the issue of planning the amount of gas you will actually carry with you is complex and
potentially affected by many factors (of which the predicted peak and decay in minute volume as you bring a CO2 "hit" under control is only one). These include (in no particular order): depth; deco / no deco dive; overhead environment (or not); availability of team bailout; staged gas plans - where, what and how much; probability of not being able to find / reach staged gas (visibility, currents, dive plan); the levels of surface and underwater support (incl. safety divers etc); the hazards associated with carrying multiple bailout cylinders (eg inside very tight wrecks) etc etc.
I would have to say that only rarely does it reduce to a simple matter of picking a worst case scenario RMV and performing calculations from there. I would also have to say that there have been occasions in my diving where a severe CO2 hit would have exposed the carriage of inadequate bailout gas, but such situations have been a calculated risk which was accepted based on the prevailing circumstances.
So, having said all that, to generate some indicative numbers for an "ideal goal" I have tended over the years to see what sort of gas requirement numbers I get for 5 minutes of breathing at 30L/min (SEV) followed by 5 minutes at 20 L/min and 12 L/min thereafter. Why 30L/min given that it is theoretically possible to breathe harder? Well, this is partly based on what I see in the clinical environment. The reality is that if I ventilate a hypercapnic patient at 30L/min the end tidal CO2 falls very quickly. You eliminate a lot of CO2 in a relatively short time. Now, an anaesthetised patient is producing very little CO2 and production will be higher in an exercising diver. The two situations are therefore difficult to compare. Nevertheless, if a hypercapnic diver
stops exercising and actually achieves 30L/min minute volume they should lower thier CO2 reasonably quickly. The difficulty may be in ceasing exercise, and actually achieving a minute volume of 30L when breathing dense gas through a regulator (+ other potential impediments to ventilation underwater).
For depths down to 80m with bottom times less than 30 min this sort of regimen applied to a single 12L cylinder of bottom mix should under most circumstances facilitate return to the downline and ascent to a depth where a switch to a second cylinder containing something with more oxygen in it could be made. I do not worry about ICD in bailout scenarios, and not infrequently this cylinder would be air or a nitrox mix either carried or staged depending on my confidence in being able to access it if staged. The bailout plan ends with oxygen, again, either carried or staged. It would be unusual for there to be gas volume problems using a single cylinder of each of these gases on a typical
bailout deco plan for a dive to <80min for <30 min.
Once you start going deeper than 90m or so for long bottom times bailout plans almost inevitably draw heavily on assumptions about strategies like team bailout and staged gas. Resigation to risk acceptance is frequently a feature of such dives!
Simon M
