Hello Steve
Good to hear that you are putting all your learned skills to work. It
seems that you also now understand more fully the problems of mixing. if
you need any help,then as you know I have been doing this in large
amounts for some time and might be able to iron out any probs.
I do know that americans do not seem to mix on board between dives, they
bring all gas needed for the trip with them. This of course removes
mixing probs and allows you to concentrate on the diving.
Maybe consider a premix heliox (90/10) and then you can pump air on top
of the decanted gas,a simple operation. heliox is available in any ratio
of he to o2, simple calc will give you what you want.

Cheers
>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....
>
>       What actually happened ??  
>
>       Well for the 1st trimix long weekend that we have ever planned,
everyone 
>came back safely from all their dives and the diving was enjoyable.  
>That was the good news.  However, the organisation of the overall dive plan
did 
>not go so smoothly.  The question of whether to do 1 or 2 trimix dives/day 
>became irrelevant...  
>
>       When we arrived, we found out from our skipper that the boat only had 
>one engine working & that it wouldn't be ready until mid-morning.  It was, 
>but this put paid to 2 trimix dives on the 1st day.
>
>       Coming back quite late after diving on the 1st day meant that we could 
>only decant helium & oxygen in the evening & had to leave the noisy
compressing 
>until the morning of the 2nd day to avoid upsetting every non-diver living 
>around us.  
>
>       A couple of filling mistakes were made in the morning that resulted in
4 
>bottles of trimix being dumped (!).  This was not only expensive & left us
tight 
>on both oxygen & helium for the 3rd day, but we were then struggling to make
the 
>first slack water for the 2nd day.
>
>       After a lot of effort to make this 2nd dive together with the weather 
>deteriorating into continual rainfall, everyone decided to take the gas mixing 
>more steadily and to get eveything ready for just 1 dive on day 3 in a more 
>relaxed manner.  This we did.
>
>       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 
>think the USA divers doing 2 dives in one day must have either :
>
>       -       No tidal problems to work with.  Cave diving doesn't have the 
>inflexible deadlines imposed by the moon (although I'm sure it has many 
>others..)
>       
>       -       If you are taking ready mixed twinsets and stage bottles to the 
>dive site to carry out both dives, for either sea or cave diving then the 
>logistics of getting enough gas mixed between dives becomes a non-issue.
>
>       -       If you can mix your gases at the dive site then the logistical 
>problems are much reduced.
>
>       For our trip we had a wreck dive site about 1 hours boat trip away from 
>the harbour.  There is of course a 6 hour period from one slack water period
to 
>the next.
>
>        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.
>
>       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.
>
>       Any comments anyone ??
>
>     Regards, Steve M.
>
>**************************************************************************
>*                                        *                               *
>*    Dr. S. G. Millard,                  * E-Mail : ec96@li*.ac*.uk*       *
>*    Senior Lecturer,                   *                               *
>*    Department of Civil Engineering     * Tel :    0151 794 5224 (UK)   *
>*    University of Liverpool,            *        44 151 794 5224        *
>*    PO Box 147,                         *             (International)   *
>*    Liverpool L69 3BX,                  *                               *
>*    UK.                                 * Fax :    0151 794 5218 (UK)   *
>*                                        *        44 151 794 5218        *
>*                                        *             (International)   *
>*                                        *                               *
>**************************************************************************
>
>
>
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-- 
john thornton

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