SAC Rate, What is Normal?

[GoBlue!]

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?

If you used 100psi in 5min at the surface, yes. If you used 100psi in 5min at 33ft, then you'd divide by 2 (since your 100psi at 2 ATA is equal to 50psi at surface), etc.

Jim

 

[Charlie99]

I believe there are 0.0283 liters/cu ft, so just multiply the SAC in cu ft/min by 0.0283 and you're all set.

Jim

You need to move your decimal point a bit also.

How about we compromise on 28.3 liters / cu ft


(You only need to think about the weight of water to get the decimal point right. 1 liter is about 1 kg. 1 cu ft of fresh water weighs 62.4 pounds. 1kg is about 2.2 pounds. Yep, it all matches up.)

For the level of accuracy involved in SAC or RMV; 0.4cfm=12lpm 0.5cfm = 14 lpm, 0.75cfm = 21 lpm.

 

[eponym]

uhhh, your decimal point's a little off. think about what you wrote a sec, and you'll give yourself a big grin.

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

 

[Web Monkey]

I beleive DAN has raw dive data. I wonder if they can calculate the stats?

Terry

 

[GoBlue!]

You need to move your decimal point a bit also.

How about we compromise on 28.3 liters / cu ft

Touchee! You're absolutely correct. I gave the conversion for cu ft to cu meters....forgot to go from cu meters to liters. Uggh.

Thanks for the correction. I should've thought about it too, that 0.03 liters made no sense for that 1 ft^3 imaginary box I was envisioning in front of me.

Jim

 

[eponym]

Here's a measurement conversions site I've found useful more than once. They can handle more measurements than you'd believe. Check out the terms for pressure in their list of measurements.

 

[SuPrBuGmAn]

.38 on todays dive.

Different formula from what I'm used to...

Normally go by ((pressure/time)x33(34 for freshwater))/(depth + 33(34 for freshwater))

I believe that only works for a standard 80/3kpsi tank though... I'm sure they all have there faults.

 

[eponym]

.38 on todays dive.
Different formula from what I'm used to...
Normally go by ((pressure/time)x33(34 for freshwater))/(depth + 33(34 for freshwater))
I believe that only works for a standard 80/3kpsi tank though... I'm sure they all have their faults.

Your formula is correct. Since it measures psi use, it works for any size tank but you can't compare rates from different tanks. If you use 15 psi per minute from an 80, you'll use about 24 psi per minute from a 50-cu.ft. tank.

Knowing your cu.foot (or liter) rate is useful if you have to switch to a different-sized tank and want to estimate how long it will last you.

-Bryan

 

[ScubaCrossing]

0.5 is my average. avg. temp 55 degrees dry suit 24#s of weight. normaly chasing students...

 

[Charlie99]

.38 on todays dive.

Different formula from what I'm used to...

Normally go by ((pressure/time)x33(34 for freshwater))/(depth + 33(34 for freshwater))

I believe that only works for a standard 80/3kpsi tank though... I'm sure they all have there faults.

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.