I couldn't agree more. It's interesting that BSAC is taking gas density seriously on the basis of Simin Mitchell's work, and so we all should, but don't seem to pay attention to his work on ICD. I hate to paraphrase but his work (and others) pretty much concluded that it isn't an issue at normoxic depths, which is backed up by successful use of standard gasses for decades.
Hello,
What Elmo says is pretty much on the money. One correction though. The data that defined a gas density threshold beyond which there is a substantially greater chance of CO2 retention were collected by Gavin Anthony, recently retired from QinetiQ. He deserves the credit for that. I was the catalyst for getting the data published. I wrote the paper and Gavin and I published it together [1].
I agree that the risk of inner ear DCS following helium to nitrogen switches is being over-called on this thread. Our previous work [2] suggested that the increase in supersaturation in the inner ear provoked by gas counter diffusion after a helium to nitrogen switch was relatively small. It would only be likely to cause harm if the inner ear was already substantially supersaturated as a result of decompression in the period leading up to the gas switch. This in turn would be most likely during the early decompression phase of a very deep dive.
It may interest you to know that David Doolette published an NEDU study [3] in which divers performed switches from
pure heliox to
air (massive violation of related technical diving ‘rules’) in decompression from dives to ‘intermediate’ depths between 45 and 67m (150 – 220ft). In 104 dives they never saw any inner ear DCS, and just to be clear, the inner ear is the only organ that could plausibly be adversely affected by gas counter diffusion.
I think that in an intermediate depth dive (eg, the range of the NEDU study) on open circuit where gas switches are obviously necessary, I would be much more concerned about gas density and the potential for CO2 retention than I would be about a risk of gas counter diffusion and DCS implied by having to use a higher helium trimix that kept me within the density recommendations. I doubt that many dives deeper than 90m / 300ft get done on open circuit these days and when using rebreathers there is much less reason to do gas switches (dil flushes). We used to do that quite a lot in the 2000s but most people are just staying on the bottom dil all the way these days.
References
1. ANTHONY TG, MITCHELL SJ. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM (Editors). Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA, 66-79, 2016. Available from:
Rebreathers and Scientific Diving Workshop Proceedings
2. DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003. Available from:
https://journals.physiology.org/doi...l_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org
3. DOOLETTE DJ, GERTH WA. Safe inner ear gas tensions for switch from helium to air breathing during decompression. NEDU TR 12-04. Panama City (FL): Naval Experimental Diving Unit; 2013.
Simon M