SAC Rate

[RaginCajun]

Can anyone help refresh my memory on how to calculate SAC rate?

 

[undrwater]

How 'bout this?

http://www.spearfishing.org/bruces_tips/java/sac.html

 

[Packhorse]

Volume of air consumed/ average ambiant pressure in ata / minutes

in metric if you suck thru 180bar from a 10l tank at 20 meters in 30 minutes then

180*10=1800 (volume of air consumed)
20 meters = 3 ata of pressure (1 ata for every 10 meters plus 1 ata at sea level)

1800/3 = 600 / 30min= 20 litres per minute.

Same for imperial. Work out cubic foot consumed on a dive. Work out average ambiant pressure 0ft =1ata,33ft=2ata,66ft=3 ata99ft=4ata.

 

[spankey]

Working Formula: (TV x GP) / (T x AP)

TV = Tank Volume
GP = Gas Used
T = Time
AP = Absolute Pressure

Dont forget if you at altitude you need to calculate LAP

 

[RaginCajun]

Thanks guys!!!!!!!!!!!

 

[rawls]

Can anyone help refresh my memory on how to calculate SAC rate?

While you're at it, go ahead and spend one dive and calculate 3 SAC's...One without exertion, one with moderate exertion, and one with heavy exertion...

 

[TwoBitTxn]

Working Formula: (TV x GP) / (T x AP)

TV = Tank Volume
GP = Gas Used
T = Time
AP = Absolute Pressure

Dont forget if you at altitude you need to calculate LAP

This doesn't work. You have the wrong function between TV and GP

It should be (TV/GP)/ (Tx AP)

The numerator for an aluminum 80 at 3000 psi is about 39 (rounding and considering a full aluminum 80 is only 77 ft3 or so.

TwoBit

 

[Packhorse]

The whole formula is poorly defined anyway.
What is Tank Volume? is it volume when empty or is it volume when fill eg 80cf
What is GP? is it in psi or cubic foot?
What is time? minutes or hours?
What is Absolute Pressure? Tank pressure at start? or at end? or pressure used? or max depth pressure?
If you know the formula then you will know the answers to the questions. But the OP dosent know the formula or the answers so I`d say it would just be all blablabla to him.
Using Volume of air consumed/ average ambiant pressure in ata / minutes
the hardest part is working out volume of air consumed.
Assuming you started with a 80cf(a true 80cf) tank rated at 3000psi then every 1 psi =80/3000= .0266 cf
So for every psi you use out of that tank multiply that by .0266 and this will be your volume of air consumed. 1000psi * .0266 = 26.6 cf

So 2000psi froma 80cf tank on a 66ft(3ata) average dive with for 30 min would be
53.2 cf/ 3 /30 =.59cfm

 

[Firefyter]

This doesn't work. You have the wrong function between TV and GP

It should be (TV/GP)/ (Tx AP)

The numerator for an aluminum 80 at 3000 psi is about 39 (rounding and considering a full aluminum 80 is only 77 ft3 or so.

TwoBit

This still doesn't work.... This whole equation boils down to total ft³ consumed divided by time, with the end figure divided by pressure at depth.

It should be {[PSI used x (Tank volume/Tank Pressure rating)] / Time}/AP

Example: You make a dive at 33 feet for 60 minutes with an AL80, using 2500 psi. The figures work out like this:

{[2500 x (77/3000)] / 60} / 2
{[2500 x (.02566666)] / 60} / 2
{64.1666666 / 60} / 2
1.0694444 / 2
.534722 ft³ per minute SAC

I used to be a geek

 

[Packhorse]

What do you mean "used to be"