How are you computing your SAC?

I know how to compute it, but since I have started downloading the information from my AI computer into Scubase, I realize how far off I am. I did a dive yesterday where I did a dive of 33 minutes to a max of 75 feet. I used 1500psi of a 3500psi 120 cuft tank. That works out to a SAC of .47cuft/min.

Because I did not spend the entire dive at 75 feet, my computer shows a SAC of .60 cuft/min. Before I started using this computer, I was showing an average SAC of .45. Since I began using the AI computer, my SAC is up to an average of .56.

Food for thought. If you are figuring your SAC and are getting low numbers, they may not be an accurate reflection of your true gas consumption.

I have not really bothered to do it manually. I just dumbly rely on my computer...

Based on a thread from last Year (2004) . I put together this excel spread sheet.
The sheet is protected so the calculations can't be messed with. there are 3 work books.
WB1 helps you find an avg sac rate.
WB2 helps you find a theoretic duration of gas supply at depth
WB3 I just put in there as a one dive checker.

calculation takes into account:
different size tanks
variable starting pressure
and depth.

temperatures were not factored in.

let me know what you think

sharpenu:

I know how to compute it, but since I have started downloading the information from my AI computer into Scubase, I realize how far off I am. I did a dive yesterday where I did a dive of 33 minutes to a max of 75 feet. I used 1500psi of a 3500psi 120 cuft tank. That works out to a SAC of .47cuft/min.

Because I did not spend the entire dive at 75 feet, my computer shows a SAC of .60 cuft/min. Before I started using this computer, I was showing an average SAC of .45. Since I began using the AI computer, my SAC is up to an average of .56.

Food for thought. If you are figuring your SAC and are getting low numbers, they may not be an accurate reflection of your true gas consumption.

Click to expand...

Based on those numbers plugged into the excel spread sheet I have provided I came up with a SAC rate of .56 as well

I have been downloading dive data from my Suunto Cobra into Suunto Dive Manager and tracking my SAC there (in cu. ft./min.). Recently, I exported the same data into Scubase and noticed that its opinion of my SAC differs from SDM. Apparently, SDM does not add 1 ata of pressure to the working pressure ("Tank Work" in Scubase), and Scubase does. The differences are small, but I'm curious - which approach is correct?

sharpenu:

I know how to compute it, but since I have started downloading the information from my AI computer into Scubase, I realize how far off I am. I did a dive yesterday where I did a dive of 33 minutes to a max of 75 feet. I used 1500psi of a 3500psi 120 cuft tank. That works out to a SAC of .47cuft/min.

Because I did not spend the entire dive at 75 feet, my computer shows a SAC of .60 cuft/min. Before I started using this computer, I was showing an average SAC of .45. Since I began using the AI computer, my SAC is up to an average of .56.

Food for thought. If you are figuring your SAC and are getting low numbers, they may not be an accurate reflection of your true gas consumption.

Click to expand...

It sounds like you've been computing your SAC based on the assumption that you're doing a square profile, and using your max depth. What you need to use is your average depth ... because even if you are doing a square profile, descent and ascent times will skew the results if you base it on max depth.

(1) To find SAC, take total gas consumption. Divide by total minutes. In your case, 1,500/33 = 45.45 psi per minute.

(2) Calculate your average depth in ATA ... (depth (fsw) divided by 33) + 1 ... and divide your psi per minute by the ATA. From the numbers you provided above, I would estimate that your average depth was 52 fsw ... 52/33 + 1 = 2.57 ATA

(3) Calculate your cylinder's baseline ... volume divided by working pressure ... in your case, that would be 120/3500 = .034 cubic feet per psi ... multiply (2) by the baseline.

That will give you your air consumption, in cubic feet, at surface pressure ... e.g. Surface Air Consumption.

So in your case ... based on the numbers you provided ...

45.45 psi per minute actual consumption/2.57 ATA = 17.68 psi per minute surface consumption x .034 cubic feet per psi = .60 cubic feet per minute

... Bob (Grateful Diver)

Alex777:

I have been downloading dive data from my Suunto Cobra into Suunto Dive Manager and tracking my SAC there (in cu. ft./min.). Recently, I exported the same data into Scubase and noticed that its opinion of my SAC differs from SDM. Apparently, SDM does not add 1 ata of pressure to the working pressure ("Tank Work" in Scubase), and Scubase does. The differences are small, but I'm curious - which approach is correct?

Click to expand...

One thing to consider, the computers take a average over the whole dive, in the first 2-3 minutes, your SAC is going to be very high because of the temperature of your tank cools of rapidly in the water, thus deceasing the pressure inside your tank, it is not uncommon for you to drop 300-400 psi within this time frame just from rapid cooling of the aluminum tank.

FWIW, the deeper the average depth you use in your calculation, the 'better' the SAC number you'll get.

One of the dives for my AOW this weekend was to determine SAC rates for both "resting" and "working". The working one wasn't much fun chasing after my instructor at 33+/- feet for 5 minutes, but provided some interesting results. We recorded our SPG's before and after the 5 minute dash and afterward calculated our "working" SAC. Converted to RMV, mine ended being almost 2!! (CF/Min @ 1ATA) I had always figured a "working/stress" RMV of 1, but I guess I can really suck it down when I need to!

Alex777,
That's interesting about SDM. I usally plug in my numbers there to get my real world RMV. I'll have to double check it's calculations tonight, but you're correct that it should add 1 ATM. Meaning at 33 feet your'e actually breathing at 2 ATA.

Thanks,
Jason

The following was posted on another forum by the Scubase author. The last sentence describes the situation with Suunto Dive Manager:

"A few people e-mailed me questions about Surface Air Consumption (SAC)
calculations in Scubase. I hope it is of some interest to the members of
this group, too:
There are two kinds of SAC that Scubase can calculate: Pressure/min or
Volume/min.

Pressure/min depends on Air Used, Average Depth and Bottom Time.

Volume/min also depends on Tank Size, specified either as Full volume at a
given (working) pressure or Empty (water volume) at 1 atm.

Scubase uses the following SAC formulas:

US Units: Pressure = psi, Volume = cu.ft., Depth = ft, SAC = psi/min or
cu.ft./min
Metric Units: Pressure = bar, Volume = l, Depth = m, SAC = bar/min or l/min

Constants:

US: AtmDepth = 33 ft
AtmPressure = 14.6959409 psi
Metric:
AtmDepth = 10 m
AtmPressure = 1.0132502 bar

Pressure/minute = ((GasUsed / BottomTime) * AtmDepth) / (AvgDepth +
AtmDepth)

Volume/minute:

If Tank Volume Empty:
TankPressure = AtmPressure
If Tank Volume Full:
TankPressure = WorkPressure + AtmPressure

Volume/minute = ((((GasUsed * TankVolume) / TankPressure) / BottomTime) *
AtmDepth) / (AvgDepth + AtmDepth)

We have noticed that many dive computer software SAC calculations differ
from Scubase slightly. For example, some simplify the formula by assuming 1
bar = 1 atm. Some also neglect to add the atmospheric pressure to the tank
gauge pressure for WorkPressure.

Regards,

Alex Khain
Scubase Dive Log
www.scubase.net"