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Thank you Mark, for telling me what I already know. But in the
interest of sharing, I am cc'ing techdiver. Haven't blown a primary
yet, and agree that it would be a major problem if it happened. As
for doubles, EVERY time some techno-weenie has brought doubles to
my boat, something got broken (either the boat took a ding, or someone's
back broke from lifting the damn things). We sometimes hang a bottle
over the boat at 20 feet too, depending on the dive. MJB
--- Mark Bush <diving@mo*.co*> wrote:
> Dear Michael (Black):
>
> >As for better systems, mine works well and is quite simple: no dives
> >below 220 feet, using a large primary, a 30cf (or 40cf) pony, and
> >40cf sling bottle(s) if needed. No doubles, and no dives that require
> >more than 45 minutes of deco (too boring with nothing to look at but
> >your buddy).
>
> If you dive long enough you ARE GOING TO BLOW YOUR PRIMARY from one of many
> possible reasons. The majority of divers assume that you are concerned
> enough to assure that you have the gas to get yourself and a buddy up to
> the surface from the deepest point of a dive.
>
> Breathing rates:
> 8 liters/min - exceptional
> 12 liters/min - good diver
> 16 liters/min - average diver
> 20 liters/min - some students and most divers in stress situations
> 28 liters(~1 cu.ft.)/min - You (exceptional diver) and your buddy under
> stress.
> 40 liters(~1.4 cu.ft.)/min - two stressed divers
>
> Maximum ascent rates and minimum acsent times:
> 220 feet to 60 feet @ 60 ft/min = 2.7 minutes
> 60 feet to surface @ 30 ft/min = 2 minutes
>
> Ideal ascent rate and time:
> 220 feet to surface @ 30 ft/min = 7.4 minutes
>
> Minimum gas consumption:
> {{[(220'/(33'/atm)) + (60'/(33'/atm))/2] x 2.7 min} + [(60'/(33'/atm))/2] x
> 2 min} x breathing rate = how much gas you consume getting up to the
> surface from 220'
> = 13.3 atm x breathing rate (cu.ft/atm) = cu.ft
>
> Minimum gas consumption at ideal ascent rate:
> Ideally you would use 30 ft/min all the way up and 220 feet to surface @ 30
> ft/min = 7.4 minutes
> [(220'/(33'/atm))/2] x 7.4 minutes = 24.7 atm x breathing rate (cu.ft/atm)
> = cu.ft
>
> And even if you aren't into deco you should make a stop which adds:
> (15'/(33'/atm)) x 3 min x breathing rate = gas consumed on safety stop
> = 1.4 atm x breathing rate (cu.ft/atm) = cubic feet
>
> So now if you ascend 220' to 15' at 30'/min and make a 3 minute stop and
> then go to the surface (let's call this ascending properly) you need 26 atm
> x your breathing rate in cubic feet per minute of gas
>
> CONCLUSIONS:
>
> Your 30 won't get two stressed divers properly up to the surface from 220'.
> Your 40 will barely get two stressed divers properly up to the surface from
> 220'.
> Your 40 won't handle any deco requirement from 220'.
>
> I'm sure that you use the 30 down to about 160 feet and the 40 down to 220'
> and carry a 40 for your deco mix on any dive requiring deco. A 40 should
> certainly handle 45 minutes of deco for one diver. If this is how you base
> your equipment choice then a mathematical evaluation shows that it is
> certainly valid. It looks okay to me and is certainly your right to make a
> personal choice about your equipment.
>
> You might want to consider that if you lost your primary tank and then a
> regulator off of one of your bottles you would be screwed.
>
> I'm sure that you will never stop your detractors from jumping on you for
> not wearing doubles.
>
> Yours truly,
> Mark Bush
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