SAC Rate - help defining variables

[adza]

Hi,

I'm in the process of trying to work out my SAC rate, however I'm stumped on a formula. I know the first thing I need to figure out is the volume, which should be worked out by the following formula:

vol = ((Start Pressure-End Pressure)/Working Pressure) *Tank Volume


I know what the Start and End Pressures are, and what the Tank Volume is, but can someone please explain to me what is meant by "working pressure"? (Sorry if it seems like a simple question - it's got me stumped).

Thanks!

Adza

 

[The Kraken]

The working pressure of the tank is the pressure at which the tank achieves it rated volume.

This pressure is stamped somewhere on the tank.

For example, I dive E8-119's. They hold 119 cubic feet of gas at their rated (working) pressure of 3442 psi.

I think most aluminum 80's are rated at 3000 psi.

This is the formula I use for calculating the SAC rate:

(((B-E)/B)x((B/W)xV)/T)/((D/33)+1)

Where:
B = Beginning Pressure
E = Ending Pressure
W = Working (Rated) Pressure
V = Nominal Volume of Tank
T = Time of Dive in Minutes
D = Average Depth of Dive in Feet

the K

 

[Tobagoman]

The Kraken is absolutely correct. The working pressure is needed to determine what the actual volume of gas in the cylinder you are using is when you start your dive.

Calculating SAC rate this way will give you an answer in cubic feet per minute of gas used. This rate then can be used for any type or volume of cylinder that you dive with. If you were to only calculate the SAC in psi/min then it would only pertain to that type and volume of cylinder that you were using to determine the answer to the calculation.

 

[dbulmer]

Metric equivalent.


Start pressure - End pressure = Gas consumed in Bar
Gas consumed * tank size = no of litres consumed
no of litres consumed / duration of dive = litres consumed per minute
litres consumed per minute / ata = surface consumption rate

(Ata is (depth/10)+1 so for 20metres the ATA is (20/10)+1=3 )

 

[Ice9]

hahaha! So numbers like 1.03 are not good, right?

 

[The Kraken]

hahaha! So numbers like 1.03 are not good, right?

Depends upon how hard the diver is working!!!

the K

 

[Puffer Fish]

hahaha! So numbers like 1.03 are not good, right?

Average, peak.... depends...

 

[donacheson]

The working pressure of the tank is the pressure at which the tank achieves it rated volume.

This pressure is stamped somewhere on the tank.

For example, I dive E8-119's. They hold 119 cubic feet of gas at their rated (working) pressure of 3442 psi.

I think most aluminum 80's are rated at 3000 psi.

This is the formula I use for calculating the SAC rate:

(((B-E)/B)x((B/W)xV)/T)/((D/33)+1)

Where:
B = Beginning Pressure
E = Ending Pressure
W = Working (Rated) Pressure
V = Nominal Volume of Tank
T = Time of Dive in Minutes
D = Average Depth of Dive in Feet

the K

That's right, but unnecesarily complicated. ((B-E)/W)V/T/((D+33)+1) yields the same result.

There may be a bit of confusion about V - Europeans sometimes rate tanks by water volume; e.g., an Al80 is an 11 liter tank. In the formulae given above, V is the volume of free (1 atm, room temperature) air that has been squeezed into the tank at rated pressure. Luxfer rates their Al80 for 77.4 cubic feet of free air.

 

[jeckyll]

I know I'm lazy, but I really like
http://www.spearfishing.org/bruces_tips/java/sac.html

 

[TomP]

I know this isn't useful but i'm a bit anal about terminology. A term should be clear, precise and unambiguous which is why i indulged myself with a pointless arguement over 'working pressure'. Other than retaining gas, i'm unclear as to whether a tank works at all, and if it does i expect it would work just as well at any pressure less than working pressure [above too but i don't want to go there].