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Dave,

You don't like to repet on deep dives because of what happened 10-15 years
ago in 160 fsw diving on air, deco on air, in a wetsuit. But now, you like
4500psi O2 in your stage bottle.  We both know you are an accomplished wreck
diver.
Despite the fact that we have dove deep together, I have a different
perspective on repets and O2.  No offense.

JC

PS - I don't like your BMW either.

>
>> In 1994 I dove the Lusitania with the Tapson mob and they
>> were vehemently opposed to repet diving on trimix.
>
>The 'opposition' is to repeat (in the same day) deep diving, rather 
>than anything to do with the gases being used.
>
>For me, it all started from two factors some 10-15 years ago, when 
>'deep' dives were 50-55m. It was customary to do a main (deep) dive out 
>in the English channel, then do a 20m or shallower dive close inshore 
>on the way in. The surface interval was 2-3 hours, and using the 
>Bhulman (sp?) tables or the day you could do a 20 minute second dive 
>with little or no required decompression. Bear in mind that 
>decompression times were quite short in those days (remember 
>wet-suits?), typically 15-20 minutes on the deep dive.
>
>Anyway, a few people got 'niggles', generally a few hours after the 
>second dive, and there was one or two cases of arm joint/ muscle bends 
>that required recompression. The Royal Navy 'pot' at Portland Bill 
>forcibly told these unfortunates that they thought two dives in one day 
>was a very bad thing to do when one or more was 'deep'. (They also 
>thought that letting divers control their own decompression stops was 
>crazy). The second dives were phased out after deep first dives, and 
>the niggles stopped occurring.
>
>There was also a case with another group of divers, diving out of 
>Salcombe. Here the practice was to do a deep (50-70m) dive, then moor 
>the dive boat up in a cove for the afternoon, in 20-30 ft of water. 
>Divers would go in for a second dive as and when they chose. the theory 
>was that you couldn't get a bend in 30ft of water, and so all was safe. 
>One day, one diver took in a novice (who hadn't done the main deep 
>dive!) for a training dive in the afternoon. During the dive, the 
>novice had problems with a leaking mask, and bolted for the surface. 
>The 'instructor' when after her. The novice was ok after a bit of 
>coughing and spluttering on the surface, but the 'instructor' had a 
>major type 2 bend that took several months to fully recover from. The 
>bend was presumed to have been triggered by the fast ascent, from 20 
>ft. This rather but people off 'safe' second dives.
>
>
>Some further thoughts:
>
>1. Were repeat dives included in the base data used to calibrate any of 
>the tables?
>
>2. If you subscribe to the micro-bubble theory, you need to have a 
>long-enough surface interval such that no micro-bubbles remain in the 
>blood at the start of the second dive. Anyone know how long this is?
>
>3. Most (air) dive computers/ tables don't look at very long half-life 
>compartments. Such compartments can suffer a progressive build-up of 
>inert gas over several days' diving. Two dives a day make things 
>noticeably worse: after the second (shallow) dive of a day the 
>desaturation time given by a dive computer is often the same as the 
>time given immediately after the first.
>
>
>
>
>Dave.
>
>--
>Send mail for the `techdiver' mailing list to `techdiver@terra.net'.
>Send subscription/archive requests to `techdiver-request@terra.net'.
>
>
John Chatterton

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