Posted by seahunt on January 29, 2001 at 10:06:34:
In Reply to: Computing without a computer? posted by Wet Dreamer on January 29, 2001 at 07:17:57:
There has been disagreement from some quarters about the wisdom,
benifit or need for using a dive computer. I swear by them myself, but
really, I was trying to think about what it would take to do accurate
manual deco calculations on a multi-level dive. I came up with a
plan....
The accuracy of any decompression calculation is dependent on
the sampling rate of both time and depth, the compilation of the
sample data and then the algorithm that describes the divers
decompression obligation based on the data.
Using a manual system with just a depth guage and a bottom
timer, the sampling rate and accuracy is dependant on the diver.
The accuracy of the compilation and interpretation of the data is
again only as accurate as the diver. Due to this limitation, the
traditional method of manual deco calculation was just to mutliply
the time of the dive times the maximum depth. Since there was only
one data sample for the dive this is of limited accuracy, but will
result, by conservatism, in a safe deco solution. It also cuts out
the step of the diver having to compile different samples just to
get the data for the single calculation. With the wheel, a diver
can use more than one sample, but that is a minor change in
limitation.
Well, the problem comes from the real world where the diver's
depth profile might be so erratic (sawtoothed) that the deco
calculation using the single sample allows very reduced bottom
times over multiple dives.
What is needed is a way to increase the manual sampling. The
obvious problem is that that then introduce the risk of
error in compiling the data, but that will be avoided as best
as possible. Still, errors in the final deco calculations from
the data, will be a potential problem.
Anyway, the idea of this method is simply to track the divers
time at a series of depth increments.
Say the profile of the dive is between 50 and 90 feet.
The diver must track their time at 4 different depth
incrememnts. between 50 and 60, between 60 and 70, between 70 and
80, between 80 and 90. So they must keep track of 6 values in
their head for this dive, or just use seahunt's handy dandy
un-patented bottom timer with 6 magic memory wheels on it.
When the diver reaches their dive depth, they look at their
depth and timer. Anytime they change depth below a 10 foot level,
they record the time in their head or on one of the wheels on my
timer.
A diver goes to 50 feet and looks at their timer. After a
bit of diving, as soon as they go below 60 feet, they get the
elapsed time and remember it for that depth increment or better
yet, they turn the first wheel on my timer to that number... It's
just like one of those bevels on a dive watch. Then when they go
below 70 fet, they record the time that elapsed between 60 and 70.
Then the same when they go past 80 feet, but they record it in the
third memory place or on the third wheel of the timer.
As the dive starts moving up again, they again check time
every time they pass a 10 foot incrememnt of depth. So if the
diver starts to ascend, say they go up a wall from 80 to 60
feet real quick (it makes for an easier example). Then they
stay at 50 to 60 feet for the rest of the dive. As they start to
surface, they take the time increment again and add it to the
previous value for the 50 to 60 foot increment in their head or
on the first wheel on the timer.
They now have 4 time periods. One for between 50 and 60,
between 60 and 70, between 70 and 80, between 80 and 90. Using
the handy dandy seahunt deco table, they then figure out a value
for each of these ranges. Those 4 values are then added and the
deco obligation for the dive can be calculated from their sum.
Deco obligation for the next dive is calculated and carried over
to the next dive just the way it is done with any standard table
for repetitive diving.
Really, it can be calculated with standard tables as 4
seperate dives with no surface interval between them.
Now, since computers are available this seems like a stupid
idea to me. The very process illustrates why computers are so
much better than manual calculations are likely to be, but it
does illustrate a potentially different method for manually
calculating deco obligations from a multi-level dive.
Really, I like diving better than screwing around with
something like this.
Enjoy, seahunt