Calculating average depth

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tchil01

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OK, this is a pretty simple question, but I am having a brain cramp today :confused:

I have an Oceanic Datmax Pro Plus. Oceanic’s download program Oceanlog is pretty anemic, to say the least. One of the things it does not do is calculate SAC for a dive. It does give me a readout of my depth every 30 seconds as well as my beginning and ending PSI. Is calculating average depth of a dive as simple as taking the average of those logged readings?

Also, does anyone have a better program that will interface with the Oceanic computers?

Thanks

Ty
 
tchil01 once bubbled...
It does give me a readout of my depth every 30 seconds as well as my beginning and ending PSI. Is calculating average depth of a dive as simple as taking the average of those logged readings?

Yep... but for an hour long dive, it might be a pain in the butt! :)

Just figure out the average depth, convert it to atmopheres absolute. Figure out your air usage in ft3 and divide by time, then by average ATA and you've got your number.


Also, does anyone have a better program that will interface with the Oceanic computers?

Sorry... can't help there...
 
Just get a new computer... :wink:

Hmmm, maybe computers do rot your brain?. (I know the person who usually posts this comment will appreciate it, you know who you are...)
 
tchil01 once bubbled...
OK, this is a pretty simple question, but I am having a brain cramp today :confused:

I have an Oceanic Datmax Pro Plus. Oceanic’s download program Oceanlog is pretty anemic, to say the least. One of the things it does not do is calculate SAC for a dive. It does give me a readout of my depth every 30 seconds as well as my beginning and ending PSI. Is calculating average depth of a dive as simple as taking the average of those logged readings?



In principle adding up all those depths and dividing by the number of readings will give you AN average depth but you have to look at your measurements to get an idea how meaningful the average depth is that you get.

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.

an easier way to find your SAC rate is by swimming at a fixed depth for a fixed time and note your pressure drop and calculate your SAC that way.
 
Sheck33,

The computer will follow the depth variations and your gas consuption rate varies with depth, so the numbers compute a valid average SAC for the dive using beginning and ending tank pressure. Using average depth to compute SAC is quick, easy, and automatically built into the Suunto Dive Manager software. Over a number of dives you will get a good idea of how your average SAC rate for a dive varies. When I have measured my SAC at a constant depth the results fall into the same range as the SACs computed based on average depth.

The Suunto Dive Manager allows the data to be exported into a file that is readable by a spreadsheet program. Once this is done you're free to easily compute anything you like. Can the Oceanic software do this? If so it would be easy to compute the average depth and the SAC.

Ralph
 
tchil01 once bubbled...
OK, this is a pretty simple question, but I am having a brain cramp today :confused:

I have an Oceanic Datmax Pro Plus. Oceanic’s download program Oceanlog is pretty anemic, to say the least. One of the things it does not do is calculate SAC for a dive. It does give me a readout of my depth every 30 seconds as well as my beginning and ending PSI. Is calculating average depth of a dive as simple as taking the average of those logged readings?

Also, does anyone have a better program that will interface with the Oceanic computers?

Thanks

Ty

If what you want is to compute your "surface air
consumption rate", you have enough data to do it.
However, in general it's not as easy dividing
your total air consumption by your average average
depth and total dive time.

Please allow me a bit of calculus:

Your average air consumption rate for a dive , R,
in psi/min, is given by the integral

R = (1/T) INT(0,T) {[33/(33+z(t))] [dp(t)/dt] dt}

where T = duration of the dive
"INT(0,T) denotes the integral from the
beginning of the dive (t=0) to the end
(t=T)
t = time in minutes
z(t) = dive depth (in feet) at time t
p(t) = tank presure at time t
"d" denotes the derivative

Unfortunately, to obtain R it is NOT, in general, a simple calculation of dividing total air consumption by average depth and total time. However, in the special case where one spends one's entire dive at a constant depth Z for a period of time T and consumes P psi from one's tank, then and only then does R=[33/(33+Z)]P/T

With your 30 second data, you can approximate the
integral with:

R = (1/0.5N) SUMi(i=1,N) {[33/(33+zi)] [pi-pi-1)}

where SUMi(i=1,N){...} = sum of N sets of values
{...}
zi = depth (in feet) at the ith 30 second
interval
pi-pi-1 = change in pressure at the end of
the ith 30 second interval

My apologies for the mathematics, but it's important:

(1)to be able to accurately compute average air consumption rate, R, and to realize that R is simply computed with the equation R=[33/(33+Z)]P/T ONLY when one's depth is constant throughout the entire dive, and

(2) to realize that you can accurately approximate R with 30 second data regardless of your dive profile.
 
However - it doesn't appear to be that complicated to me. Let's plug some actual numbers and see how it works out, ok?
For my example, I'm going to spend 10 minutes at 99 FSW, 10 at 66 FSW and 10 at 33 FSW. I note that I've used 2250 psi. Now, that's an average depth (whether you sample every 30 seconds, one second or one minute makes no difference for this profile) of 66 FSW, or 3 ATM... 2250/3 = 750; 750/30 = 25. So, using a straight average and dividing by the average ATM and the total dive time, I come up with 25 psi/min SAC.
Now, if my SAC is in fact 25 psi/min, I would expect to use 1000 psi during my 10 minutes at 99 FSW (4X25X10); 750 psi during my 10 minutes at 66 FSW (3X25X10), and 500 psi during my 10 minutes at 33 FSW (2X25X10)... 1000+750+500=2250... just what I did use, so the 25 psi/min SAC is correct.
Now, why do I need to use calculus again?
Rick
 
You beat me to it Rick, :)

I've been playing with the calculations. Donacheson’s formulas are correct, but just as Rick demonstrated, using the average depth provides a quick and easy computation of average SAC. My example used only two depths to show using average depths derives average SAC. I've been seeking a clean proof of this but haven't yet succeeded. The simple demonstration must suffice for the moment unless someone else has a more comprehensive derivation.

Ralph
 
donacheson is correct, the integral given would be the way to exactly calculate one's total air consumption and with the dive duration it will yield an average air consumption.

What i was getting at in my earlier post is the fact that adding an x-number of depths divided by that x-number of depths does not necessarily result in an average depth that is useable for calculating a SAC rate. If you want to avoid doing the exact math it is easier to stay at the fixed depth for a fixed time to find a SAC rate, done over many dives this will yield a useable SAC rate, unless your computer does it all for you, but it is good to know how to get to those numbers and KNOW what is going on, turn on the brain, computers can screw up and still give (wrong) numbers, if i do a 30 min 30 foot dive and my computer tells me to decompress for 26 min at 10 foot it helps to know that those numbers are probably not realistic for a first dive :D
 
OK...
I'll assume y'all are right and my simple pea-brain method isn't. Now, kind sirs, give me an example - with numbers please - where my method won't work.
Thanks.
Rick
 
https://www.shearwater.com/products/perdix-ai/

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