SAC rate...Highs and lows

[mudguppy]

To anyone who knows about Surface Air Consumption rates...

What is a very low sac rate and what is a very high sac rate?

I did some diving over the weekend....8 on sat....and they all figured out at around an 8-10 sac rate. I had heard that if I had a 9 sac rate it was very low and that mine couldn't be that low unless I was dead, or just laying on the bottom, which I wasn't. (and I don't think I'm dead yet....well... not physically anyway ).
I figured, and re-figured and still come up with the same thing. I even tried putting all the dives together (being repetitive dives) and figuring that way from the total air used. The water, by the way, was 88 - 89 degrees F. and maximum depth was 30ft. (also diving at 4300 ft. elevation).

Does SSI, PADI and the others all figure it the same way?

I used:

PSI divided by bottom time x 34 (being fresh water)
_________________________
Depth + 34


Help me out and let me know if I am missing something.
Much appreciated!

 

[ERP]

Your computing SAC in terms of PSI/minute which isn't very useful for comparisons unless you specify what sort of tank your using.

100 psi on an AL80 isn't the same as 100psi on a Steel LP104

Personally I usually calculate it in terms of cuft /minute, since this is independant of tank size and pressure

 

[GDI]

:doctor:
Yeh it looks like you are using PSI/min. you may want to switch that to FT3/min.
first what size tank are you using? An 80al only has about 37.5 psi/ft3@3000 psi fill. Keep in mind that an 80AL is actually rounded up from about 77.6 ft3. Not only is ft3 better when you need to compare cylinders but it will be more accurate given your RMV or lung volumn with each breath.. you will need to average out over several dives
try this (and there are others ways)

1. Air start - Air end = air used
3000 -1500 = 1500 psi used

2. dive was to 66 ft seawater for 30 minutes

depth must be in atm absolute (depth/33)+1 = ata (seawater) or (depth/34)+1 = ata ( freshwater)

3. air used is 1500 psi/30 min = 50 psi/min @66 ft

4. SAC is measured at the surface: therefore: 50psi/min divided by 3 ata = 16.7 psi/min @surface

5. Ok now convert to ft3/min:Simply take the 16.7 psi/min and divid that by the 37.5 psi/ft3 of cylinder size. invert and multiply and the psi cancel 16.7/37.5 = 0.44 ft3/min

The issue is what if I don't get a full fill, that bis why you average this out over several dives. Your lungs will always take the same amount of air in (respective to the density as you get deeper) on each and every breath. the cylinder does not care about density.
Check out some other ways to do it as well

 

[Drew Sailbum]

You need to work in ft3/min (or l/min for you metric folks).

In ft3/min, you can note that one of Cayman's best trained free divers has a SAC of about .26 ft3/min while scuba diving - very low. Conversely, I've seen folks with SAC rates as shockingly high as 1.4 ft3/min - just call him Hoover.

 

[landlocked]

ERP once bubbled...
.... what sort of tank your using.
....

She was using an Aluminum 80.

 

[Big-t-2538]

mudguppy once bubbled...
To anyone who knows about Surface Air Consumption rates...

What is a very low sac rate and what is a very high sac rate?

Does SSI, PADI and the others all figure it the same way?

I used:

PSI divided by bottom time x 34 (being fresh water) _________________________
Depth + 34
Help me out and let me know if I am missing something.
Much appreciated!

You're not converting for what kind of tank you are using. Also, you never convert to cubic feet per minute. You need a sac-rate calculator, then a conversion for the tank to get cfm

 

[ABQdiver]

GDI once bubbled...
:doctor:
Yeh it looks like you are using PSI/min. you may want to switch that to FT3/min.
first what size tank are you using? An 80al only has about 37.5 psi/ft3@3000 psi fill. Keep in mind that an 80AL is actually rounded up from about 77.6 ft3. Not only is ft3 better when you need to compare cylinders but it will be more accurate given your RMV or lung volumn with each breath.. you will need to average out over several dives
try this (and there are others ways)

1. Air start - Air end = air used
3000 -1500 = 1500 psi used

2. dive was to 66 ft seawater for 30 minutes

depth must be in atm absolute (depth/33)+1 = ata (seawater) or (depth/34)+1 = ata ( freshwater)

3. air used is 1500 psi/30 min = 50 psi/min @66 ft

4. SAC is measured at the surface: therefore: 50psi/min divided by 3 ata = 16.7 psi/min @surface

5. Ok now convert to ft3/min:Simply take the 16.7 psi/min and divid that by the 37.5 psi/ft3 of cylinder size. invert and multiply and the psi cancel 16.7/37.5 = 0.44 ft3/min

The issue is what if I don't get a full fill, that bis why you average this out over several dives. Your lungs will always take the same amount of air in (respective to the density as you get deeper) on each and every breath. the cylinder does not care about density.
Check out some other ways to do it as well

That's one of the best explanations I've seen yet. The whole SAC process has been buggin me since I started diving.

Does anyone have the psi/ft3 for other popular tanks as well?
Another question: Do you figure the rate of air consumption at the max depth for your dive or the average? It looks like the above calcs are for the max depth, which with should change with different ata....

 

[mudguppy]

I was using an alum. 80. Maybe this is a lot more trickier than I thought.

 

[Rick Murchison]

mudguppy once bubbled...
all figured out at around an 8-10 sac rate. I had heard that if I had a 9 sac rate it was very low and that mine couldn't be that low unless I was dead, or just laying on the bottom, which I wasn't. (and I don't think I'm dead yet....well... not physically anyway ).
I figured, and re-figured and still come up with the same thing. I even tried putting all the dives together (being repetitive dives) and figuring that way from the total air used. The water, by the way, was 88 - 89 degrees F. and maximum depth was 30ft. (also diving at 4300 ft. elevation).

Whew!
You do want to complicate the math a bit... From your raw data I'm assuming you're using an AL 80 and that you were using about 19 psi/minute at 30'. If that is the case then...
We have to correct everything for the 4300' altitude, then recorrect to sea level to get a "standard" SAC rate.
If you were diving at 30' in fresh water at sea level, then 19 psi/min would indeed be a SAC rate of 10 psi/min, or about .26 CFM, which is indeed very low - exceptional, I'd say. At 4300', however, the average atmospheric pressure is about 12.5 vice 14.7 psi, so your equivalent depth was about 25' vice 30' for calculating SCR at 1 ATM. (This is not to be confused with your equivalent depth for decompression purposes, which was 36' vice 30') With this reduced pressure in mind, your SAC rate (corrected) is 11 psi/min vice 10, or .28 CFM - still remarkably low.
Now, for some real fun, let's calculate the SAC based on the actual surface pressure at 4300'... Your AL 80, which at sea level holds 77.4 CF at 3000 psi (38.75 psi/CF) now holds 91 CF based on the new value for 1 ATM of 12.5 psi, or 32.96 psi/CF. Your depth, 30', is now 2.04 ATM vice the 1.88 it would be if you were diving with the surface at sea level, and the 19 psi/min becomes a new SCR of 9.3 psi/min or .28 CFM again - but realize the .28 CF has only 92% of the gas the .28 CF we calculated before for sea level has in it.
Clear?
For comparison, my resting doing nothing SCR is about .35 CFM, my normal about .45 - .48. For planning I normally use .5 to 1.0 depending on anticipated work load and as high as 2.0 under extreme circumstances, though I try to avoid extreme exertion and the highest I've actually logged was 1.51 - and I never want to see that again.
Bottom line - if your figures are accurate you have an exceptionally low SCR (Surface Consumption Rate) - among the lowest I've ever seen. But I have seen SCR's in your range before (always in women, by the way), so you aren't dead!
Rick

 

[GDI]

ABQdiver once bubbled...


That's one of the best explanations I've seen yet. The whole SAC process has been buggin me since I started diving.

Does anyone have the psi/ft3 for other popular tanks as well?
Another question: Do you figure the rate of air consumption at the max depth for your dive or the average? It looks like the above calcs are for the max depth, which with should change with different ata....

:

doctor:
you can easily calculate the tank size for any tank. Just take the PSI and divide it by the ft3
example : (all at 3000psi filled full)
Al tanks
50 ft3 = 60psi/ft3
63ft3 = 47.6 psi/ft3
80ft3 =37.5psi/ft3

You simply take the fill pressure and divide by the volume. A higher fill pressure will effectively increase the volume. It will not change your SAC rate. You can also calculate to the average depth of your dive