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> 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.

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