Charlie99:
Since you actually answer questions, here's a follow-up question.
When going first to 100'/30m for a while, then afterwards to 60'/18m it makes sense to average.
How about the case where you are at 60'/18m for say, something like 15 minutes, and then head on down to 100'? (Just assume that there IS a valid reason for such profile, such a 60' shelf and a 100' shelf on a shore dive). Let's further assume that you are using a minimum deco table with limits of 30 minutes at 80'/24m and 20 minutes at 100'/30m.
Do you do the very conservative "treat it all as 100'/30m'" and figure you have 5 minutes left at 100'/30m / 20 minutes total bottom time to stay within minimum deco limits?
It seems that doing just a simple average and figuring that you have 15 minutes left at 100'/30m, for total bottom time of 30 minutes is too aggressive or liberal. Agreed?
Or do you just kind of fudge it and figure that some where around 10 minutes is about right?
Thanks in advance,
Charlie Allen
gas loading is all about exponential curves. if you do a forward profile where the you go from 110 feet to 90 feet you can use a linear approximation to how the gas loading changes in the beginning of the dive where a small percentage change around the depth produces a roughly linear difference in gas loading.
this is similar to what you do in physics when you take a pendulum described by:
F = -k sin theta
and replace it by a simple harmonic oscillator (hooke's law):
F = -k theta
because when theta is small:
sin theta ~= theta
if you graph sin theta you can see that around theta=0 it is nearly a 45 degree angle.
now, if you do a reverse profile this approximation is no longer valid, and its better to just use the table value for the max depth that you wound up at.
however, we still do depth averaging even on reverse profiles (at least I use this in the near-to-NDL regime) the slow compartments aren't going to saturate and can still be treated with depth averaging, which affects the shallow stops. the fast compartments are going to get hammered by whatever depth you wind up at. so, on a reverse profile betwen 100-120 where the average depth is 110, i'd do shallow stops timed like it was 110, but start deep stops like it was 120.
(this isn't taught in any DIR class, its just applying approximation methods to exponential functions...)
also, if you're doing large differences in depths, i'd suggest using different approaches. for example, on a dive where you do 10 mins at 100 fsw and then move up to 60 fsw, I'd double time time at 100 fsw and take that of the NDL at 60 fsw. the logic here is that the really fast compartments hammered by 100 fsw are going to actually offgas a little bit at 60 fsw, and you mostly want to look at the effect on the controlling compartment at 60 fsw (the controlling compartment won't actually be identical, but it'll be close), so i'd take the 55 min NDL value (air) and subtract 20 mins to get 35 mins.
