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....

	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)   *
*                                        *                               *
**************************************************************************

Navigate by Author: [Previous] [Next] [Author Search Index]
Navigate by Subject: [Previous] [Next] [Subject Search Index]

[Send Reply] [Send Message with New Topic]

[Search Selection] [Mailing List Home] [Home]