SAC Rates

[Mike Boswell]

Here is the data for a typical dive I did three weeks ago on Roatan:

7/5/2010 Roatan 77.4 3000 65.3 2988 1363 26 84 0.33 0.359

Cyl: AL80
Cap (Cu Ft): 77.4
Rated Pressure (psi): 3,000
Dive Time (min): 65.3
Start Pressure (psi): 2988
End Pressure (psi): 1363
Avg Depth (fsw): 26
Min Temp (F): 84

My SAC calculation: 0.359

SAC = {(77.4 x (2988-1363)/3000)/65.3}/{1+(26/33)}

Suunto's SAC calculation (corrected to STP): 0.33

 

[peterbj7]

Scuba Math is more art than science ... it's the process of "rounding off" to be able to do quick calculations in your head ... or, as I tell my student, the art of "close enough"

That's just what it is, but that's also a definition of "practical arithmetic", the ability to do in your head calculations whose answer is precise enough for the current purpose. Useful for checking something to see that it has to be wrong, even if at that stage you don't know what the correct answer should be. My niece, shortly before leaving high school to attend university to read statistics (of all subjects!), pressed the [divide] key on her calculator instead of [subtract], and duly wrote down the wildly incorrect answer because she had never learned to do "quick and dirty" mental arithmetic - doesn't seem to be taught these days, for some inexplicable reason.

 

[Rhone Man]

It works real well when you need to explain to your spouse why you need that new scuba toy too ... but then you always round down when you're calculating for her how much it's gonna cost ...

... Bob (Grateful Diver)

Careful.... they learn that trick and flip it around on you pretty quick when they go out shopping.

 

[sam1]

I know there are a zillion variables involved and every diver will experience different rates on almost every dive....can I expect it to improve more over time?

I have been calculating my SAC rate on nearly every dive since I started diving nearly five years ago. Your post/question prompted me to put my data into a graph (attached). I have distinguished between "warm water" dives (with 3mm wetsuit---mostly in the Caribbean) and cold water dives (with a drysuit---in New England). I have added trend lines for each. As you can see by the scatter, as you wrote, there are many factors that influence the SAC rate. As you can also see, I got better with age (well, experience anyway).
********

Having problems uploading the file!

 

[jridg]

Really, as has been said before - take enough air to do what you want and come back.

Yes, I compute SAC - it is actually trending lower lately from 0.7 avg to around 0.6 avg. One hard swim during the dive can blow that entire idea away.

So, what to do? Plan for slightly higher than your average so that if you have to exert yourself, you won't be in trouble!

Does it REALLY matter? Not a great deal in my mind. You learn what you can do, you try to relax, you take deep, slow beaths, don't play any 'games' with breathing and you'll be fine. I almost always come up with lots of air remaining - you know what? I prefer it that way.

My 2 psi....

 

[peterbj7]

If all you're doing are easy recreational dives, no, it probably doesn't matter a whole lot. But once you start to push the envelope it suddenly becomes one of the most important factors to concern yourself with.

 

[sam1]

I know there are a zillion variables involved and every diver will experience different rates on almost every dive....can I expect it to improve more over time?

I have been calculating my SAC rate on nearly every dive since I started diving nearly five years ago. Your post/question prompted me to put my data into a graph (attached). I have distinguished between "warm water" dives (with 3mm wetsuit---mostly in the Caribbean) and cold water dives (with a drysuit---in New England). I have added trend lines for each. As you can see by the scatter, as you wrote, there are many factors that influence the SAC rate. As you can also see, I got better with age (well, experience anyway).

(Reposting with attachment.)

 

[11x]

is there a magical sac calculator out there for us math impared? my last dive dive #4 in my diving career i dove a aluminum 100 with a starting pressure of 3400 psi and dove to 31 ft in 72ish water. i came up with 500 psi after 51 miunites of constant swimming.(no resting) i am 6'1" and 230lbs.

 

[bygolly]

is there a magical sac calculator out there for us math impared? my last dive dive #4 in my diving career i dove a aluminum 100 with a starting pressure of 3400 psi and dove to 31 ft in 72ish water. i came up with 500 psi after 51 miunites of constant swimming.(no resting) i am 6'1" and 230lbs.

SAC Calculator

 

[11x]

many thanks