SAC Rate, What is Normal?

[Diver0001]

Or assume he's used the decimal separator favored by most countries, the comma.

You've got it. The Dutch use a comma where the decimal is supposed to go. I forgot to change it when I copied it. Sorry for any confusion.

R..

 

[GoBlue!]

You've got it. The Dutch use a comma where the decimal is supposed to go. I forgot to change it when I copied it. Sorry for any confusion.

R..

Ah! Of course. My apologies.

Jim

 

[frankster1138]

The formula is:

SCR = (1-P2/P1) x Vf/Aa/Tm

Where
P1 = pressure in psi at beginning of test dive
P2 = pressure in psi at end of test dive
Vf = actual volume of air in tank in cu. ft.
Aa = pressure in absolute atmospheres
Tm = time of dive in minutes

I was confused for a second there! This formula does not work when using a tank that is not full.

P1 should read 'PSI of tank when full'
P2 should read 'P1 - used PSI'

I got worried when I ran the numbers for some old dives until I saw what was going on.

 

[The Kraken]

My main question is still about the actual volume of gas in the tank.
Is the "77.4 CF @ 3000 psi / 80 CF Tank" a given for the computation?

 

[Charlie99]

My main question is still about the actual volume of gas in the tank.
Is the "77.4 CF @ 3000 psi / 80 CF Tank" a given for the computation?

If by "80 CF Tank" you mean the very common Luxfer S080, then yes.

http://www.luxfercylinders.com/products/scuba/specifications/us_imperial.shtml

 

[Charlie99]

So it follows that an 80 CF tank @ 3000 psi = .027 cf per psi
Therefore if I use up 100 psi in 5 minutes then my SAC =

(100/5)x .027 psi or 20 psi/min x .027 which equals a SAC of .54?

Don't forget to factor in your average depth.
Divide your cfm/min or psi/min by the avg absolute pressure in atm to normalize it to surface.

Multiply actual used CFM by 33'/(avg depth + 33').

(Yes, you metric guys can stop laughing. It IS much simpler to add 10 meters, then drop a zero).

 

[Rick Murchison]

0.3 - 1.3 usually...
depending on what I'm doing.
Average planning numbers for a recreational dive in good conditions proven to work with a reasonable margin of safety over the years are 0.55 for bottom gas and 0.45 for deco.
Last dive was 0.42 - fairly relaxed, warm, low effort.
Rick

 

[Kim]

When I checked my Vyper and converted the liters out to cubic feet I found it was 0.54 - not the best - but actually I was very pleased as I always considered myself a bit of a hoover! It's nice to see in the poll that it's fairly average - I feel better now!!! Recently in Hawaii I managed to dive an AL80 to one minute of NDL on a 32 meter wreck - I figure I'm beginning to approach the ballpark....

 

[The Kraken]

Charlie,
You wrote "Don't forget to factor in your average depth."
But isn't the SAC an index based upon one's consumption at the SURFACE?

 

[SuPrBuGmAn]

I don't see how you can get 0.38 with that formula unless you are measuring time in fractional hours. Per your formula, breathing 3000psi in 60 minutes at 33feet results in 25psi/min.

Your formula results in a PSI/MIN result which is dependent upon the tank size. Typical results with an AL80 (77.4 cu ft at 3000psi) will be in the 12 to 40 PSI to minute range.

To convert PSI/MIN with AL80 to cubic feet per minute, divide the PSI/MIN by 38.75... in other words 0.5cfm = 19psi/min, 38psi/min=1.0cfm, etc. (when calculating in my head, I just use either 38 or 40 -- for purposes of SAC that's close enough)


The psi/min to cfm conversion factor of 38 is simply 3000psi/77.4cfm = 38.75. Plug in the numbers for your tank if it isn't an AL80.

I got the .38 from the formula provided in this thread. I just added the formula that I normally use afterwards. Read into the post a little further...