Calculating Sac Rate in Excel

[MAK52580]

Hello everyone. I had a question and thought that it might be best posed to tek divers.

I keep a dive log, in addition to my regular log in an Excel spreadsheet. This makes it easy tracking and is also a good reference in case I ever lose my physical log.

I have been trying to enter an Excel formula that will automatically calculate my SAC rate based on the time, depth, amount of psi, etc. and have been unable to make it work. I was wondering if any of you kept this information in an Excel spreadsheet and could give me the formula in order to make this work. Any help you can provide would be great. Thanks.

Marc

 

[fgray1]

But I do have a program that has SAC and a bunch of other good tools to use for dive calculations. If you want it I'll send it to you.
It's a small zip file.

Fred

 

[MAK52580]

Fred,

That would be great. I have been looking for something like that for a long time. My email addresses are: mkokosky@crdf.org and mak52580@yahoo.com Thanks again.

Marc

 

[Capt Jim Wyatt]

SAC Rate calculation

If you have even rudimentary skills in Excel you should be able to set up a spreadsheet for this.

 

[Spectre]

If your logging max depth, the SAC rate calculation you come out with will be errored on the low side, and therefore is basically useless for any sort of dive planning. You'll need average depth to truely get an accurate reading.

 

[Capt Jim Wyatt]

Spectre once bubbled...
If your logging max depth, the SAC rate calculation you come out with will be errored on the low side, and therefore is basically useless for any sort of dive planning. You'll need average depth to truely get an accurate reading.

If you use max depth your SAC rate will come out higher and is a more conservative number.

The calculations should be done at your max depth and not include ascents/descents.

Get to depth & check time & pressure .. do your dive .. once you come back to the ascent line check time & pressure again. Use those variables to calculate the SAC rate.

Clearly you need to do this procedure over several dives to get the average SAC rate --

:bidea:

 

[fgray1]

You use it as a base line for dive planning not a set number.
There are so many veriables to deal with that you need to be conservative in your planning.
To do anything eelse is a plan for disaster.

Fred

 

[NEWreckDiver]

I have put together an Excel spreadsheet with every formula I can think of. All you need to do is to is fill in the variables.

You are welcome to try it if you wish. Feel free to e-mail me if you want it.

 

[JimC]

You need a few things:

Constants (normaly):
Rated tank volume in cf: A1
Rated tank PSI: A2
Depth to 1 Atomosphere: A3

Variabls:
PSI In: B1
PSI Out: B2
Time: B3
Depth: B4

Sac Forumla: (GasUsed(psi) / Time(min) * RatededVolume(cf/psi)) / Depth(atmospheres)
Sac Forumla: = ((B1 - B2) / B3 * ($A$1 / $A$2)) / ((B4/$A$3)+1)

Say your on an AL80, doing a 66 foot dive in salt water for 60 minutes. Startign with 2900psi and ending with 900psi:

= (2900-900) / 60 * (80/3000) / ((66/33)+1)
= 2000 / 60 * 0.026 / 3
= 0.3 cf/min

 

[Spectre]

Capt Jim Wyatt once bubbled...
If you use max depth your SAC rate will come out higher and is a more conservative number.

Not true.

Take a 40 minute dive with a max depth of 99, average depth of 66 using 3/4 of a 100 cuft bottle.

99 = 4 ATM. 4*40 = 160 surface minutes. 75/160 = .47 cuft/min

66 = 3 ATM. 3*40 = 120 surface minutes.
75/120 = .63 cuft/min

----------------
However yes, one should make every attempt to calculate two SAC rates, your at rest and at stress rates. You want to use your at stress rates for gas management planning. If you are going to break your dive up into two parts, doing your at depth separately then your ascent and decent, you should _still_ use your minimum depth during the at depth, if not the more accurate average depth. Using max depth mathematically guarantees to result in a lower number then reality.