How are you computing your SAC?

[The Kraken]

You can record the incremental times of you dive, ie, start time, arrive time at bottom, bottom time in minutes, ascent time/depths, total them up and calculate the mean.
For example: 60' dive
0 to 60' descent - 1 min. =30 avg
60' depth / 40 min
60 to 15' ascent - 2 min = 37 avg
15' safety stop - 3 min

So you got: (30+2400+75+45)/46 min
Comes out to an average dive depth of 55.4 feet for 46 minutes.

Sort of did this on the fly so please overlook any glaring mathematical errors.

the K

 

[Sideband]

If you can't hover, you can't fly





Join Date: May 2004
Location: Alpharetta, GA
Posts: 2,425 You can record the incremental times of you dive, ie, start time, arrive time at bottom, bottom time in minutes, ascent time/depths, total them up and calculate the mean.
For example: 60' dive
0 to 60' descent - 1 min. =30 avg
60' depth / 40 min
60 to 15' ascent - 2 min = 37 avg
15' safety stop - 3 min

So you got: (30+2400+75+45)/46 min
Comes out to an average dive depth of 55.4 feet for 46 minutes.

Sort of did this on the fly so please overlook any glaring mathematical errors.


the K

Why not just use the 40 minutes at 60'? Read your pressure once there and just before ascent.
Joe

 

[The Kraken]

Sideband, that's when I revert to the good ole Citizen Hyper Auqualand !!!!! Too interested in what's going on around me to be recording that many numbers . . .

You're very right about the tank temp decreasing. We dive a lake here in the Atlanta area that in the summer reaches surface temps of 80 degrees. Drop down to the 80/90' level and the temps are in the mid 40's. BIG drop in pressure indication.

the K

 

[Sideband]

lol. You did see the first post. After I got all of that in I saw your last post and thought I'd refrain from sounding like I just wanted to argue and go with the easier one. You did see where I said you were technically right though.

For anyone that is lost, I wrote a much longer reply to a different post Kraken made then pulled it right away, but not before he had a chance to read it.

 

[The Kraken]

Sideband,
You know your way is a more practical way of gauging your SAC, but being involved in QA/QC I'm statistically retentive . . . .

The main thing is that the diver know the basics of how to determine her/his SAC to select the proper tank/s for a particular dive.

There's really no reason that I split hairs so finely other than it just suits me to do so.

There, I've said it !!!! I've come out !!!! I'm a statistic GEEK !!!

 

[fmw625]

Sorta like figuring the area in a graph and taking the mean. Thanks for the information. I can see that a computer would be handy for figuring SAC.

 

[sharpenu]

The problem is that no one does a purely level dive. That was the point of this thread. I used to think I was staying pretty level, certainly within 5 feet or so. But as soon as I got a computer that gave me a profile to look at, I realized I was up and down throughout the dive. Even though I thought I was pretty level in the water column, my depth varied by up to 10 feet. Over an hour's dive, this error becomes magnified.

I was surprised at just how far off I was.

 

[Sideband]

You might try this:
Pick some 10X10 area of bottom and study it for 10 minutes. Record tank pressure at beginning and end of the ten minute period. Then do what ever you normally would but pay attention to your depth. Try to realize when you are going up or down. How much is your depth changing and for how long? After the dive, unless you were bobbing from buoyancy control issues, you should have a very good idea of what your approximate average depth was (compare to a computer if you can). Use the first ten minute period and fixed depth to figure your SAC. Calculate it again using your estimated average depth for the rest of the dive and the remaining dive time. How close did you get? The second SAC will probably be higher since you were moving and not confined to the 10 foot square.

Don't get too absorbed in having to know your exact SAC rate because you never will. It will vary a bit on every dive. For dive planning always round up and give yourself a margin for error in your favor.

Joe