Spectre
Contributor
sheck33 once bubbled...
If i make a 40 min dive and i spend 35 minutes at 40 ft with one short bounce to 100 ft then these few 100 ft readings will influence the average you get significantly and you would get an average that would not be too useful.
Huh? 35 * 40 = 1400 + 5 * 100 = 1900 /40 = 47.5 ft average
In actuality it would be more like:
35 * 40 = 1400 + 1 * 70 + 2 * 100 + 2 * 70 = 45.25
Yes, you loose data with zig-zagging profiles, as the average doesn't take into account travel time [e.g. if you are at 40 ft on one data point, and 100 at the next data point, you obviously travelled, and the 'depth' from 40 to 100 would be 70].
However the subtle points missed doesn't really matter as your only looking for an idea of your SAC. When you use your SAC for gas management planning, you're going to be using your bad SAC rates, and you'll be calculating based on depth floors. So if it's actually .55 or .57 instead of .56, it don't matter, you'd use .6 or .65 for planning anyway.
Now the only difference between the calculus method and the 'easy' method is that the calculus method is calculating the SAC rate for each data point and then averaging them together througout the series. The other method is averaging the depth and then calculating the SAC rate from that point, which is a lot easier.
If you _have_ the data to have the pressure reading for every time T, then the calculus method works well as you could then calculate intermediate SAC rates. e.g. T=3 to T=10 I was working, so my working SAC rate is... 1-2 and 11-20 I was resting, and that SAC rate is...
But calculating the SAC rate for the whole dive... you'll get the same result with the calculus method as you get with the average depth method.