In message <ECS9605311258A@li*.ac*.uk*>, Steve Millard
<ec96@li*.ac*.uk*> writes
>       Many thanks to all the divers who responded to my query about the
safety 
>of doing 2 trimix dives in one day.  The overall response I got was that in
the 
>USA 2 dives/day using trimix was not thought to be a problem, as long as some 
>extra stops at 30m and at 6m or 3m were put in, in addition to the stops 
>indicated by the tables cut for the dives.  The surface interval recommended 
>varied from 2 hours to 6 hours, but should be kept as long as possible.  
>
>       There was a 'deafening silence' on the Net from this side of the 
>Atlantic.  The verbal feedback I have recieved is that we are crazy to be even 
>thinking of two trimix dives in one day.  We would get 'bent' for sure....

Missed this one Steve. If I had you would have bot an 'affirmative'. The
trick is to run high He content on both dives. This maximises offgass
during the surface interval. Have a chat with Billy Deans when he's over
in June.
>
>       A couple of filling mistakes were made in the morning that resulted in
4 
>bottles of trimix being dumped (!). 

Ooops........ The trick is to be a little more careful and to have an
ordered filling protocol that everyone understands.

>
>       So, what did we learn from this ??  If we want to get 2 trimix dives 
>into one day then we need to be a lot slicker out of the water than we were. 
I 

Agreed - without teamwork the whole issue becomes a circus act.

>        If we assume that a typical dive takes around 1 hour, with another 1/2 
>hour to get everyone back on board & de-kitted then that makes 1+1/2 hours.  1 
>hour trip back to harbour makes 2+1/2 hours.  1/2 hour to unload all the
bottles 
>& get them up to the mixing site makes a total of 3 hours.  Now if reloading
the 
>bottles & the return trip to the dive site takes another 1+1/2 hours, and we 
>would like to get there 1/2 hour before the next slack water, then we have
just 
>1 hour to do all our mixing !!  This just isn't going to happen.  Even if we 
>just do air top-offs for the 2nd trimix, we still need to top up 2 nitrox
stage 
>bottles for between 5 to 8 divers.

Interestingly enough it is usually the bottom and final deco gas that
require topping for a second dive. Even running the calcs as regards
quantity most folks have enough travel for the sort of diving you are
envisiging. Aside from all the 'we use 80% because arguaments....' the
main reason 100% O2 is used is phsyiologic and if you have an emergency
you have loads to go at. But secondary to this is 100% in a cylinder is
damn easy to top off - no calcs - no frigging around with a compressor.
So if you get slick enough you only need to top/refill the twinsets for
the second dive and top the deco cyls with 100%.

>
>       Our conclusion was that we either need to purchase a lot more cylinders 
>or we are stuck with just one trimix dive a day, regardless of the
decompression 
>arguments.

You still need more cylinders Steve!!!.

Regards

Chris
Chris Hellas

e-mail:- chris@de*.de*.co*.uk*
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