Real time SAC? How? (Ridiculous SAC)

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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/peregrine/

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