Real time SAC? How? (Ridiculous SAC)

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!

130bar for 2x12s gives a gas used of 3,120Lt therefore 23.28Lt/min, not the 21.7Lt/min Subsurface displayed.

If you’re constantly changing your configuration you’re not giving yourself the chance of becoming familiar, and comfortable, with one setup.

You are totally right: 130×24÷2,31÷58 = 23,2870577698 litres/minute

So....
1) How the heck is Subsurface calculating the SAC?
2) well... that makes things a little worse
 
What is it you don't trust?

I find a lot of electronic calculations in consumer products are off--how many times has the temperature reading on your computer been different from other divers or a temperature reading from a more accurate source? The gas mileage calculation a car displays when driving on long trips is another example. I don't know if it sampling rate, sensors, or what, but the information displayed is usually in the ballpark, but always off when you compute 100% known values and input by hand. I haven't really looked at Shearwater's calculation, maybe it is better. When you are doing something like SAC rate I find it is always important to do the math by hand over several samplings.
 
You are totally right: 130×24÷2,31÷58 = 23,2870577698 litres/minute

So....
1) How the heck is Subsurface calculating the SAC?
2) well... that makes things a little worse
Which becomes even worse if we calculate the average deep manually, being (21+1)/2 = 11 meters rather than 13.1.... making the SAC even higher.
 
Which becomes even worse if we calculate the average deep manually, being (21+1)/2 = 11 meters rather than 13.1.... making the SAC even higher.
That is not likely to be your average depth but rather the average of your deepest and your shallowest depth. I am assuming 21m is your max depth, and 1m is the shallowest you can measure.
 
You are totally right: 130×24÷2,31÷58 = 23,2870577698 litres/minute

So....
1) How the heck is Subsurface calculating the SAC?
2) well... that makes things a little worse

I don't know anything about subsurface, but I THINK the screenshot you put up says you used 2,900 litres, working backwards, assuming you used twin 12.2 litre tanks , I get 119 bar used ..... not 130 bar above. [ 2900 / (12.2 x 2) ] .... thanks Ill go have a look at sub surface looks good
 
@estresao I don't see the problem. Your Subsurface screen shot shows you used 2900.7 liters, and had an average depth of 13.1m. So your SAC = (2900.7)/(2.31)/58 =21.7 lt/min, just as it says.
 
Subsurface apart..... My doubles are 12L x 2, thefefore 24L each bar. I ve started having 200 bars and got off the water having 70 bars, so I've consumed 200-70=130 bars, times 24L each equals 3.120L of gas consumed instead of what Subsurface indicated.

Diving time was 58 minutes, so I"ve consumed 3.120L / 58minites = 53,7931034483 litres per minute.

Average deep 13.1 so average pressufe was 2.31 bars, so SAC is 53,7931034483 / 2.31, which is 23,2870577698 litres/minute
 
What no-one seems to have commented is that using doubles will always make you have a higher SAC due to extra drag of the tanks the way they are no longer as streamlined and the extra regs. If you are also using a drysuit, then more drag and additional use of air to fill suit, especially if as per the profile you go up a bit and dump air and then down and add more.

Personally, my SAC goes from 10-10.5 on a single in a wetsuit to 12.5 using a single and pony and extra bits on a 40+m dive to 13.5 when using a drysuit on the same deep dive.
 
You are totally right: 130×24÷2,31÷58 = 23,2870577698 litres/minute

So....
1) How the heck is Subsurface calculating the SAC?
2) well... that makes things a little worse


Subsurface is incorporating real gas law into their equations. They are not calculating using the Ideal gas law. Thus come up with different, but more precise numbers.

See: When real gas corrections matter – The Theoretical Diver
 
https://www.shearwater.com/products/swift/

Back
Top Bottom