Dear Readers:
A very noteworthy report however, one would need to read the actual HSE
computer model report of Dr. Flook before considering a specific recommendation. It is
very specific and her conclusions are proportionately less applicable past 60'. In Dr. Flooks' model, most comparisons are done to ~< 50', where the NSL is ~ 80 min {PADI RDP}, with proportionately better results the shallower the dives are.
I wholely support Dr. D's position on the current recommendation that yoyo diving, which is diving ... bottom time ... and fully surfacing, SIT then diving again, is risky practice generally speaking {FYI a sawtooth profile does not break the surface but what is most important in both profiles is relative changes in pressure and depth-time spent before surfacing}.
http://www.hse.gov.uk/research/rrhtm/rr214.htm
At this time I have not had time to read the paper and dissect the extensive details.
Dr. Flooks' own conclusions provides a far different perspective than the news article. In general, what is new is that her model parameters suggests a window of safety in the 30-40' range were yoyo diving maybe safer than spending a continuous 40' dive to NSL.
5.0 CONCLUSIONS
The diving considered in this report is all to relatively shallow depths and it may be incorrect
to extrapolate these conclusions for diving at greater depths. Examination of Figure 10 shows
that not only do the quantitative results changing as depth increases so too may the qualitative
conclusions. Figure 10 shows how the temporal pattern of bubble formation in the brain
changes as depth increases. For even deeper diving, deeper than 30 msw, it may be that the
surface interval has so little effect on total gas load that yo-yo diving is no different from
diving the envelope. That being said it may be that deep yo-yo diving is never an attractive
option as the allowed bottom time become shorter and the distance to be travelled increases.
One conclusion that can be drawn for all depths is that No-D diving puts the brain at particular
risk. The rates of ascent though generally slow enough to protect the faster tissues are not
slow enough to protect the brain. In general the accepted No-D tables do not protect the brain.
A second conclusion that can be drawn for all depths is that tables drawn up for repet diving
to not take into account the way in which the presence of bubbles slows down the removal of
gas from the body. Though not a major part of this report this aspect of decompression is
touched on in discussion of the profiles shown in Figure 7.
These two conclusions lead the author to believe that dive computers currently in use may not
be suitable as a means of determining safe yo-yo diving procedures.
The main conclusions from this work are that yo-yo diving of the type traditionally practised
in fish farm diving can be very safe and that dividing the total bottom time into several shorter
dives alternating with a surface interval is less of a risk than diving the envelope.
Though, as far as we have the requisite knowledge, it seems that for the depths covered in this
report bubble formation in the brain does not reach levels which can be said to be dangerous,
once bubbles form there is a random element to the risk. For some individuals, on some
occasions, one bubble in the wrong place can lead to severe consequences. This is what
makes bubble formation in the brain a particular concern. This reasoning would strengthen the
conclusion that several shorter dives, with less time for gas uptake in the brain, could be a
much better approach than a single long dive. If the pattern of diving can be such that the brain
clears its gas before the end of the subsequent dive then the risk of a single bubble growing
large enough to cause trouble is greatly reduced.
