My TDI Nitrox book References the NOAA Diving Manual for the 90-minute SI, but it also says if there are one or more dives in a 24-hour period that have reached the time limit for a normal single exposure, the minimum SI is two hours.
So, the source should be the the NOAA Diving Manual. However, the TDI manual says nothing about this 50% 'reduction'.
If it's in the Nitrox or Mixed gas chapters (15 and 16) I can't find it, nor did I see it in the Physiology chapter (3, IIRR) or the index. I'll have another look through.
First, the US came late to the party on this one:
Perhaps the best-kept secret of WWII was the use of oxygen-enriched air re-breathers by the British commandos defending Gibraltar. Those attacking the British stronghold were using 100% O2 re-breathers. The deeper maximum operating depth of the British mixes (45-60% O2) was a distinctive underwater combat advantage since opposing divers (using 100 % O2) would be at “convulsive depths” while the British divers was still within their operating parameters. A major component of the British strategy was to simply take the opponent down until convulsions overwhelmed the enemy diver. This secret was so well kept, that much of this was not even revealed to the US Navy until the 1950’s. (Larry "Harris" Taylor, Ph.D)
Interesting, I wonder when they introduced this, because they had several tox incidents and one fatality while training their Charioteers (human torpedo operators using 100% O2 rebreathers.
Really interesting question. No luck yet, but it has to be somewhere between Oxygen Poisoning in Man Donald, K.W British Medical Journal May 24, 1947 pgs 712-717 and A Model for Predicting Central Nervous System Oxygen Toxicity from Hyperbaric Oxygen Exposures in Humans Toxicology and Applied Pharmacology 132, 19-26 (1995) The first reference is a surprisingly interesting and readable article that details the experiences of divers undergoing CNS ‘hits’. (I highly recommend this one to any tech diver) The second reference details how rapidly the risk accumulates. A quick check shows that my IANTD CNS O2 tables are all linear and show a 75% increase over any 90 minute period. My guess, somebody linearized the data (from the source the OP is looking for) and added a safety factor. What I think that I see is that the oxygen clock runs backward at 50% and runs forward at a 75% increase every hour and a half. Pretty simple, but I’m always suspicious of a linear model to any biological process.
OK, we did our homework. Now I’m curious too. Any US NAVY or NOAA input? -who put the pin at 50% every hour and a half?
I've been reading the 2001 Dan Nitrox Conference and 2008 Dan Tech Diver Conference reports, as well as the paper by Baker. The presentation on CNS toxicity in the 2008 DAN conference (by Vann and Hamilton) is quite interesting, as it presents Donald's information and results among other research. I've never seen the original, so this was great to learn.
It also discusses Italian and British combat and human torpedo divers and research related to them (Donald). I was already somewhat familiar with the British aspect from an operational perspective, as I've long owned a copy of "The Midget Raiders" by Warren and Benson; this describes British Chariot and midget submarine development, training and use (attacks on Tirpitz, Takao etc.) during the war. I also have a copy of "Subsunk" by W.O. Shelford, which describes accidents, escapes and rescues from sunken subs, and training and equipment development (DSEA, Drager, Mommsen, free ascents, etc.).
IIRR, both of these date from the 1950s. They're popular works rather than scientific ones, but do bring the people and problems into the foreground, as opposed to "Case #6".
Guy
