SAC Calculation

[Pullmyfinger]

Here's a link to a thread I started awhile ago:

http://www.scubaboard.com/forums/basic-scuba-discussions/364228-air-consumption-rate-planners.html

I used my surface air consumption rate, and used the usual formulas to calculate my depth air consumption rate for different depths.

I listed my gas consumption in cu.ft./min. and in psi/min. there are tables for tanks that I use or my dive buddies might use.

Naturally, you can just do the math and figure out your gas consumption for any depth, but I figured a table would be helpful. Just for an at a glance idea, mainly to compare different tanks.

I used .7 as an average.

Anyway, they are attachments in my first post of the above linked thread.

Disclaimer: I was never suggesting in any way that anyone dive without a SPG, and I am not doing so now.

-Mitch

 

[LouieLouie]

Keep in mind that the relation between gas volume and pressure is increasingly non-linear beyond 3300 PSI or so. PSI/min is a useless measure for SAC if using high pressure tanks.

 

[MXGratefulDiver]

Last night while checking into my past dives I have logged on my dive program, I noticed that it is generating a SAC rate for each of my dives (which I know most dive computers do now). But it is generating the SAC rate in cu Ft/min rather than PSI/min. I am using a Pro Plus 2. Is there a setting in the Oceanic Program that can convert those numbers to PSI/min rather than the current cu Ft/min? Or is there a formula I can enter into my Excel sheet that will convert those numbers? I find it rewarding to see a graph of my SAC rate decreasing the more I dive, and would like to see more "normal" SAC rate numbers (PSI/min rather than cu Ft/min).

Thanks

In my gas management article I describe the relationship between gas consumption as a function of pressure vs as a function of volume. For purposes of discussion, I refer to the former as SAC and the latter as RMV.

Simply put, gas consumption rate as a function of volume (RMV) is useful for dive planning ... because you know the volume of air in your cylinder.

Gas consumption rate as a function of pressure (SAC) is useful for dive execution ... because the indicator you have available for how much gas you have is your pressure gauge.

Both are expressions of the same thing, simply expressed in different ways ... both are useful. Converting from one to the other is simple. The equations are included in the above linked article ...

... Bob (Grateful Diver)

 

[Peter Guy]

Me, I find doing gas consumption figures has another important function -- it lets me know how narced I am!

If I'm nearing 100 feet and try to do the calculations, it sometimes goes like this:

Hmm -- Depth -- 100 feet -- that's 4 ATAs -- tank -- 130 -- Tank Factor 3.8 -- OK -- hmmm -- psi -- 2793 -- OK -- 2 minutes later -- psi 2593 -- Uh, uh -- why did I want to know that?

For those of you who don't think you are narced -- try doing the SAC/RMV calculations in your head every few minutes as you get deeper and deeper!

 

[dmoore19]

As a little note of humor. Last month I completed my AOW certification. One of the things that was done in an attempt to show the affect of Narcosis included a combination lock. Every one in the class prior to the dive was given a combination lock, similar to the ones we had on our lockers when I was in high school. We were given the combination and told to open the lock. We were timed on how long it took us to open it. When we were on the deep dive, sitting at 100' we were given the lock to open it and were timed again. The times were recorded. Every one of us opened the lock quicker at 100' than we did on the surface. I don't think the exersize displayed the effect that was desired.

 

[Peter Guy]

When we were on the deep dive, sitting at 100' we were given the lock to open it and were timed again.

Was that a metaphorical use of the word or an actual description?

 

[dmoore19]

Was that a metaphorical use of the word or an actual description?

We were tucked in between 2 coral heads in a sand chute to get out of the current. (Cozumel). Fingers in the sand to hold position.

 

[RTee]

Me, I find doing gas consumption figures has another important function -- it lets me know how narced I am!

If I'm nearing 100 feet and try to do the calculations, it sometimes goes like this:

Hmm -- Depth -- 100 feet -- that's 4 ATAs -- tank -- 130 -- Tank Factor 3.8 -- OK -- hmmm -- psi -- 2793 -- OK -- 2 minutes later -- psi 2593 -- Uh, uh -- why did I want to know that?

For those of you who don't think you are narced -- try doing the SAC/RMV calculations in your head every few minutes as you get deeper and deeper!

Peter,

I think a lot of divers would think they are already narced even before starting the dive by just attempting to do those calculations at the surface let alone at 100 ft.....

 

[James R]

Easy SAC calculator page: SAC Calculator

 

[Da Wolf]

Thanks to everyone for all of the usefully information. Think I will stick to the cu ft.min reading instead of trying to convert to psi where mistakes can be made. Thanks again for all the helpful information.