Sac

[jimmyboy]

How do you figure out your surface air consumption?

 

[WellBelowH2O]

How do you figure out your surface air consumption?

Check this link out: Click

 

[The Kraken]

Derivation of SAC based on actual average of dive depth:

(((Pb-Pe)/Pb)x((Pb/Pw)xV)/T)/((D/33)+1)

Where:
Pb = Beginning pressure of tank
Pe = Ending pressure of tank
Pw = Nominal working pressure of tank
V = Nominal volume of tank
T = Dive time in minutes
D = Average depth of dive measured in feet

 

[CBulla]

You can't beat what Kracken said.. he's totally right on.

I try to do a SAC every 10 or so dives.. its interesting to see where you stand after about 40...50...100 dives.... I also don't trust myself sometimes and i go HERE to double check my math.

 

[BabyDuck]

kraken, i saw the thread and had to read it to make sure you responded - i was gonna pm jimmyboy to pm you if you hadn't found it! ;x

 

[rab]

Derivation of SAC based on actual average of dive depth:

(((Pb-Pe)/Pb)x((Pb/Pw)xV)/T)/((D/33)+1)

Where:
Pb = Beginning pressure of tank
Pe = Ending pressure of tank
Pw = Nominal working pressure of tank
V = Nominal volume of tank
T = Dive time in minutes
D = Average depth of dive measured in feet

I don't know why you have the extra Pb terms in there. Just simplify it to be:
(((Pb-Pe)/Pw) x V) / (T x (D/33 + 1))

Where:
((Pb-Pe)/Pw) = Fraction of a nominally full tank
(D/33 + 1) = Correction for compression due to depth
33ft = 1ata pressure change; use 10m for metric

Technically, you've determined your Respiratory Minute Volume (RMV) which should be expressed as unit volume per minute (i.e., cu.ft/min or L/min). Some will point out a SAC expressed in pressure per minute (e.g., 25psi/min), but that is dependent on the size of the tank.

-Rob

 

[ScubaSarus]

I actually like to average my under water consumption by logging the dives and not concentrate on what my consumption would be on the surface. Its a different way of me doing things thats all.

 

[DI_Guy]

I used to calculate my SAC using Kraken's formula and then I bought the SCUBASE Dive Log Software... I downloaded all my dives into the software program just for comparison to my dive log book. It uses the dive profile from your computer in its calculation and it comes out pretty close to Kraken's formula. It's amazing how your SAC increases during dives where you have to work or fin hard.

DI_Guy

 

[jbd]

NAUI has a SAC rate calculator that works similar to slide rule although it is circular. Costs less than $10 IIRC. Simple and quick to use and takes up very little space.

 

[kennethw]

If 'lazy', use the Sunnto Cobra and download the log into their Dive software, it automatically calculates it for you....

But on a serious note, do your 'readings' under 3 circumstances:

1. At rest;
2. Moderate exercise (normal finning); and,
3. Heavy exertion (fin like crazy).

To a certain extent, (3) is the most important for gas management. If you plan for heavy exertion for most of your dive, you 'should' never run out of air.

As for the formula, I am a little lazy and do direct calcuations, I use:

(PbV - PeV) x (T x (D/10 + 1)) where D is MSW.

Pw is mainly used when you need to calculate the tanks to be used for gas management. Absolute volume used is absolute volume used....