SAC Calculation

[Da Wolf]

Last night while checking into my past dives I have logged on my dive program, I noticed that it is generating a SAC rate for each of my dives (which I know most dive computers do now). But it is generating the SAC rate in cu Ft/min rather than PSI/min. I am using a Pro Plus 2. Is there a setting in the Oceanic Program that can convert those numbers to PSI/min rather than the current cu Ft/min? Or is there a formula I can enter into my Excel sheet that will convert those numbers? I find it rewarding to see a graph of my SAC rate decreasing the more I dive, and would like to see more "normal" SAC rate numbers (PSI/min rather than cu Ft/min).

Thanks

 

[fisheater]

It's my understanding that ft3/min is the "standard" (at least for those of us in countries that refuse to go metric).

The reason is that it's useful over a wide range of tank sizes. PSI/min is only good on whatever tank size the calculation was based.

For example, if I were to go through 100 psi/min on a 100 cu ft. tank, I would eat through a 20 cu ft. pony bottle at 500 psi/min.

 

[Jax]

Surface Air Consumption rate is a Volume / time.

You need to know the volume at surface, to convert to the volume at depth, and then multiply by a tank-specific Tank Factor to figure your pressure usage.

 

[beertiki]

Divide the starting psi of the tank by the cu feet of the tank. Multiply that number by the SAC you have.

Ex - an 80 cf tank at 3000 psi = 37.5 psi per cf

 

[Blackwood]

Divide the [-]starting[/-] rated psi of the tank by the cu feet of the tank.

Fixed.

 

[rob@diveinutila.com]

Volume (liters or cuft) per min is kinda weird at first but it will translate into any cylinder size whereas a psi/bar per minute is only valid for tanks of the same size. So unless your computer knows what size your cylinder is... Also standard European and US cylinder sizes are differ slightly (but probably not enough to be too noticeable for what we need).

 

[Insta-Gator]

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

 

[RTee]

I think it is a matter of getting used to it (cft/min) because when you do it greatly facilitate calculating your gas requirement based on the anticipated depth for a given dive notwithstanding what tank you have at your disposal. I used to calculate it handraulically but now I get it automatically from my suunto desktop software once I have entered starting and end pressure (for a specified cylinder).

 

[Insta-Gator]

Question: If your gauge (SPG - P=pressure) only tells you your pressure, what good is knowing your RMV (volume) while underwater? Do you smart folk do the conversion while at depth to determine if you have enough volume remaining? If I know my SCR and multiply that by ATA I know my PSI consumption at depth and can clearly use the SPG for valuable information.
Maybe y'all are just smarter than I am.

 

[Kern]

Sounds like we must be.

Got an 80, SPG he say 3000psi. At 1500psi must have 40cft. Must use 8cft every 300psi. Oh shyt, only got 1000psi left & rock bottom was 27cft, gota go.