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On Tue, 21 Sep 1999, Ed Graves wrote:
 
> I got to thinking about what I wrote & how the other diver used his pony on
> every safety stop.  In this case he uses a 19cf pony with a 40% mix.  He
> does 5 dives & 5 safety stops, of at least 3 minutes.  You can't get EAN on
> the island we were on so no chance of re-filling the pony.  We were doing
> fairly deep dives, 80 - 100fsw, if he had had to use the pony in an
> out-of-air situation, let's say at the  beginning of the 5th dive, chances
> are he wouldn't have had much to work with.  Thoughts?
 
Let's assume his surface-equivalent breathing rate (a.k.a. SAC) was 0.5
cf/min, and four stops were done at 15 feet for 3 minutes. Let's also assume
there was no other gas released from the cylinder, and that it was full to
start the day. Those four stops would consume:
 
   4 * ( ( 15 / 33 ) + 1 ) * 0.5 * 3 = 8.7 cubic feet
 
The available gas in a 19 cf pony is about:
 
   ( 3000 - 500 ) * 19 / 3000 = 15.8 cubic feet
 
(Notice I've assumed his regulator will stop working at 500 psi, which is very
conservative). So he's used about half the gas in the bottle if the
assumptions all hold.
 
An ascent from 100 feet, including a safety stop, and assuming his breathing
rate DOES NOT go up (a bad assumption, but what the heck), assuming an ascent
rate of 33 ft/min (3 minutes to the surface) and a stop at 15 feet for 3 min:
 
  3 * ( ( 50 / 33 ) + 1 ) * 0.5 = 3.7 cubic feet
  3 * ( ( 15 / 33 ) + 1 ) * 0.5 = 2.1 cubic feet
 
  Total = 5.8 cubic feet
 
So, he still has a margin. If his breathing rate tripled in response to the
emergency that caused him to switch to the pony, and stayed tripled all the
way up, he might run out of gas during the safety stop.
 
A three or four fold increase in breathing rate in response to an emergency
would not be unusual.
 
--
Art Greenberg
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