SAC Rate

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

BradMM

ScubaBoard Supporter
ScubaBoard Supporter
Messages
271
Reaction score
33
Location
Texas
# of dives
200 - 499
Using the formula from TDI's Intro to Tech manual, p. 46...
[(psi used/working pressure) x cylinder capacity] / {[(depth/33)+1] x minutes}

If I go from an AL80 tank with a working pressure of 3000 to a steel HP80 with a working pressure of 3400 and everything else remaining the same, mathematically, my SAC rate goes down because of reducing the numerator based on the higher working pressure of the HP tank.

I'm not a Tech diver but this is instructive for me. SAC rate is not simply "What is your rate of consumption" but more importantly "How long will my gas last?" That's the question a Tech diver would ask, correct? I've always calculated simply my consumption rate and compared to the tank capacity.
 

rhwestfall

Woof!
ScubaBoard Sponsor
Messages
17,880
Reaction score
22,463
Location
"La Grande Ile"
# of dives
200 - 499
you normalize it to RMV (ft^3/min) to use as a cross platform (different tanks) measure....
 

Basking Ridge Diver

Contributor
Messages
1,968
Reaction score
1,012
Location
New Jersey
# of dives
200 - 499
If I go from an AL80 tank with a working pressure of 3000 to a steel HP80 with a working pressure of 3400 and everything else remaining the same, mathematically, my SAC rate goes down because of reducing the numerator based on the higher working pressure of the HP tank.

So just a reminder - AL80 tank is 77.4 cu ft cylinder capacity not 80 cu ft cylinder capacity.
I dont use HP 80's - but depending on your tank - it could be 82 cu ft cyclinder capacity.

Make sure you are using the correct values to get what you are looking for. :)
 

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba instructor
Messages
12,709
Reaction score
11,095
Location
U.S. East Coast
# of dives
2500 - 4999
Are you expressing SAC in psi/minute or cuft/min? If the latter, it will not change from tank to tank, which is the point of using SAC in cuft/min.

Provide the numbers you are using and I can give you a better answer.

There is no difference between "What is your rate of consumption" and "How long will my gas last?" except for factoring in the tank you are using and your depth. The first number might be (example) 0.5 cuft/minute; in a 100 cuft tank that means 200 minutes at the surface, 100 minutes at 33 ft, etc.
 

tbone1004

Technical Instructor
ScubaBoard Supporter
Scuba instructor
Messages
19,034
Reaction score
11,644
Location
Greenville, South Carolina, United States
# of dives
I'm a Fish!
@BradMM if you are using an AL80 vs a HP80, your psi/min consumption will be different because they have a different tank factory. I.e. 100psi in an AL80 is 100*77.4/3000=2.58cf. In a HP80 100*80/3442=2.34cf

It all nets out to the same CFM, but you have to remember that 100psi does not equal the same volume in each tank because they are different sizes
 

BradMM

ScubaBoard Supporter
ScubaBoard Supporter
Messages
271
Reaction score
33
Location
Texas
# of dives
200 - 499
Basking, yes, I used the actual water capacity for the tanks.

turniops, using this formula, with the same data for psi used, depth and minutes, simply by changing the tank causes me to go from 0.53 for the HP80 to 0.65 for the AL80. I assume the units are cu ft/min. I need to go now but I'm going to work back through the formula to see if that's the units it provides. Thanks!
 

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba instructor
Messages
12,709
Reaction score
11,095
Location
U.S. East Coast
# of dives
2500 - 4999
turniops, using this formula, with the same data for psi used, depth and minutes, simply by changing the tank causes me to go from 0.53 for the HP80 to 0.65 for the AL80. I assume the units are cu ft/min. I need to go now but I'm going to work back through the formula to see if that's the units it provides. Thanks!
I think what you may be missing is that the "psi used" will be more in the HP80, because the AL80 has 3000 psi per 77.4 cuft, or 38.8 psi per cuft, whereas the HP80 has 3442psi per 80 cuft, or 43 psi/cu ft. So at a SAC of 0.6 cuft/min, you'd use up 23.3 cuft/min (at the surface) in the AL80, but you'd use 25.8 cuft/min in the HP80 (there are more psi in the HP80 to use).

That is an increase of 25.8/23.3 = 11% in "psi/min" for the exact same SAC (in cuft/min), for those two tanks.

You may also be missing that 3400 is not the right working pressure for the HP80; you need to use the 3442 at which the 80 cuft is rated.

I don't see how you get from 0.53 to 0.65. It appears you may have used the wrong pressure...3100 instead of 3400 (or better, 3442) for the HP80 when you did your calculation.
 

BradMM

ScubaBoard Supporter
ScubaBoard Supporter
Messages
271
Reaction score
33
Location
Texas
# of dives
200 - 499
Last first... yes, I used 3400 and not the actual capacity of 3442. Thanks for pointing that out.

I never considered the fact that my pressure gauge reading on one tank will not reflect the actual gas consumed based on the different tank being used. I just recently bought the HP80 so I'm still learning but that appears to be where things got off track.

Thanks a bunch, that will keep me busy for a while!
 

BradMM

ScubaBoard Supporter
ScubaBoard Supporter
Messages
271
Reaction score
33
Location
Texas
# of dives
200 - 499
Interestingly, on the same page, the metric version of the formula does not include working pressure.

(bars used x cylinder capacity) / {[(depth/10) + 1] x minutes}
 

NAM001

Contributor
Messages
5,002
Reaction score
1,318
Location
the moon
# of dives
500 - 999
Im a bit confused by this thread. Is this really7 SAC or RMV OOORRRR tank factor this guy is talking about. There seems t be a mix in there.

Tank factor is full tank pressure devided by its capacity. so if you are using a HYP 3000 psi tank that is a 10 liter tank then then you have apx 200atm times 10liter volumn or 2000 liters in the tank when full. 2000 liters at 3000 pai is about 1.5 psi per liter if it were cuft then it would be. perhaps 3000 psi for 77 cu ft and that would be 3000 / 77 or about 40 psi per cu ft. My lp 95 2640/95 is tank factor or about 28 psi per cuft. Each tank type has a different tank factor. ou grab a tank and you remember the factor. so when you see a 500 psi drop youcan equate it to abount used in liters or cu ft.
 
https://www.shearwater.com/products/perdix-ai/
http://cavediveflorida.com/Rum_House.htm

Top Bottom