How to calculate SAC rate?

[letterboy]

Not sure if anyone has seen/heard about this type of program but a few weeks ago i saw a guy with a program on his laptop that computed the data using the cubic feet per minute. IMO entering numbers in a computer prog is easier than trying to figure it out long hand. I can't remember the name of it so i am sure this isn't much help but if anyone has seen a prog like it. . . . . . .

John

 

[String]

A 3 line java script could do it, the forumla is very simple and on page 1 of this thread.

Most dive logging software does it (including SDM)

 

[eponym]

John -

I'm good enough at numbers that I can get a rough figure in my head after a dive. It helps to think in terms of atmospheres instead of feet. But since my consumption rate's pretty stable, figuring it out is not as big a kick as it used to be.

But I really replied to your post just so I could say what a great sigline that is. Welcome to SB!

- Bryan

 

[scubadoguk]

all the figures ....try metric SO much easier

 

[letterboy]

Thanks Bryan I find that this is so true as i have begun to expand my diving horizons, Beyond maryland quarries and into PA. Quarries lol

John

 

[GoBlue!]

Mine does.... I have to set it before I get in the water (it can't autodetect salinity) but I can set it to fresh or salt water. Using 33 for sw and 34 for fw does make a difference when planning some dives especially on a rebreather.

OK, admittedly I don't know the first thing about rebreathers, but could someone explain to me why one would need to use 1atm=33ft w/ sw & 1atm=34ft w/ fw? I don't see how the LINEAR depth will make any difference physiologically, as all is based on pressure.

Jim

 

[H2Andy]

GoBlue, unless I am missing something (and it may very well be the case), you would
want to use 34 feet for fresh water since it takes more fresh water to create one
atmosphere of pressure. thus, you would be able to get a more accurate dive plan.

i'm not sure how linear "depth" is relevant here (to me, linear means how far back into a system/wreck you go, not how deep you are).

 

[tndiveinstruct1]

OK, admittedly I don't know the first thing about rebreathers, but could someone explain to me why one would need to use 1atm=33ft w/ sw & 1atm=34ft w/ fw? I don't see how the LINEAR depth will make any difference physiologically, as all is based on pressure.

Jim

Jim,

Here's the reason. A change of 1 ft in saltwater causes a .445 psi increase whereas a change of 1 ft in freshwater causes a .432 psi increase in pressure. So if we multiply .445 x 33, we find that the pressure is 14.685 psi. This is close enough to 1 ATM for me. While it is slight, it does need to be accounted for. The reason for the increase is the dissolved minerals in the saltwater.

Hope this helps...

Randy Cain

 

[wxyz]

Here is the SSI formula:

(PSI used / bottom time) 33*
---------------------------
(Max depth + 33*)

I haven't seen "Max depth" used before. However if I plug in "Average depth" into the above formula, I end up coming out with the same SACs as other formulas I'm familiar with.

I think there's a big (important) difference between SACs calculated with Max vs. Avg depth

can someone please comment more specifically on SSI's formula use of "Max depth"?

 

[GoBlue!]

GoBlue, unless I am missing something (and it may very well be the case), you would want to use 34 feet for fresh water ...

Yep, that's what I said.... 1 atm=34 ft for fw.

i'm not sure how linear "depth" is relevant here (to me, linear means how far back into a system/wreck you go, not how deep you are).

Linear depth, for the purposes of this post, is the perpendicular distance you are to the surface.

A change of 1 ft in saltwater causes a .445 psi increase whereas a change of 1 ft in freshwater causes a .432 psi increase in pressure. So if we multiply .445 x 33, we find that the pressure is 14.685 psi.

Yep, I understand this. The question, really, is what constant the deco modelers use for converting pressure to depth. I.e., does the RGBM model (for example) care whether you are in fresh or saltwater?

Where's BRW....

Jim