How are you computing your SAC?

[The Kraken]

Ooooooooooooooo - weeeeeeeeeeeeee - ooooooooooooooo . . . :eyemouth:

 

[Charlie99]

Scubase uses the following SAC formulas:

US Units: Pressure = psi, Volume = cu.ft., Depth = ft, SAC = psi/min or
cu.ft./min
Metric Units: Pressure = bar, Volume = l, Depth = m, SAC = bar/min or l/min

Constants:

US: AtmDepth = 33 ft
AtmPressure = 14.6959409 psi
Metric:
AtmDepth = 10 m
AtmPressure = 1.0132502 bar

We have noticed that many dive computer software SAC calculations differ
from Scubase slightly. For example, some simplify the formula by assuming 1
bar = 1 atm. Some also neglect to add the atmospheric pressure to the tank
gauge pressure for WorkPressure.

Alex, I don't believe that the manufacturers have simplified the formula by assuming that 1 bar = 1atm, but instead the difference is that they use the more widely accepted definition of 10msw being exactly 1 bar, not the 10 m = 1atm definition that you have chosen.

 

[akscubainst]

IMO if you want to really know what your SAC rate is, then you have to do a dive to specifically calulate it.

The last time I went out to calc my SAC we took all of our gear including deco bottles and made a dive to 120 when we got to 120, we noted tank pressure and ran for 20 minutes then noted the amount of gas used and from there we could do the calcs. This gave us a SAC for just basic diving not working or deco those would require 2 more dives.

120's isn't the best depth since it doesn't divide by 33 evenly but it worked.

The nice thing about doing it deep is that it uses a significant amount of air (I used about 1000psi in double 100's) doing the same dive at 33' would have only used 430psi.

Dave

 

[RIDIVER501]

IMO if you want to really know what your SAC rate is, then you have to do a dive to specifically calulate it.


Dave

Dave, Take a look at the spread sheet I posted earlier in the thread. on the 1st page you can put your real dive stats in for your last 5 dive to get a rough running avg of your SAC rate.

I just plugged in my last five dives and came up with a .39 avg.
Do the dives, plug in the numbers, get your results.

 

[Charlie99]

Dave, Take a look at the spread sheet I posted earlier in the thread. on the 1st page you can put your real dive stats in for your last 5 dive to get a rough running avg of your SAC rate.

I just plugged in my last five dives and came up with a .39 avg.
Do the dives, plug in the numbers, get your results.

Unfortunately, a lot of us don't have computers that display AVERAGE depth of the dive, which is what is needed to get your program to work.

To calculate SAC using the junky Oceanic software I need to download the data, print it to a file, import into Excel, sum up all of the sample depths and average them. After doing that about 10 times, I found that eyeballing the profile gave an average depth accurate to less than 5'.

Based on those numbers plugged into the excel spread sheet I have provided I came up with a SAC rate of .56 as well

I don't understand how you can come up with a number, since Sharpenu didn't post his average depth.

Perhaps this is a good example of what Sharpenu was commenting on.

 

[RIDIVER501]

Unfortunately, a lot of us don't have computers that display AVERAGE depth of the dive, which is what is needed to get your program to work.

you need the computer to tell you that? You don't know that you spent the bulk of your dive at x' depth and use that as a best guess?

There is a difference between an educated guess on a data to give you a resonable idea of what you did and picking any old number to give yourself the "Best"/most desireable answer.

come on. I did 2 dives last night and know where the bulk of my time was spent on both those dives. I don't need to plug my vytec into a computer for it to tell me where my avg depth was for the dive.

The spread sheet I put together will use what ever number you put into it. You are making the assumption it has to be a certain number. it doesn't.

if you has the super-whammy-dyne computer that would give you avg depth, you probably wouldn't need the spread sheet I have provided because you SWD computer would probably give you SAC rate too.

 

[Charlie99]

you need the computer to tell you that? You don't know that you spent the bulk of your dive at x' depth and use that as a best guess?

There is a difference between an educated guess on a data to give you a resonable idea of what you did and picking any old number to give yourself the "Best"/most desireable answer.

come on. I did 2 dives last night and know where the bulk of my time was spent on both those dives. I don't need to plug my vytec into a computer for it to tell me where my avg depth was for the dive.

The spread sheet I put together will use what ever number you put into it. You are making the assumption it has to be a certain number. it doesn't.

if you has the super-whammy-dyne computer that would give you avg depth, you probably wouldn't need the spread sheet I have provided because you SWD computer would probably give you SAC rate too.

I generally have a pretty good idea of my average depth, but I didn't see any average depth or info on dive profile posted by Sharpenu.

So how did you figure out the average depth number for Sharpenu's dive?

 

[Alex777]

<< Alex, I don't believe that the manufacturers have simplified the formula by assuming that 1 bar = 1atm, but instead the difference is that they use the more widely accepted definition of 10msw being exactly 1 bar, not the 10 m = 1atm definition that you have chosen. >>

I was just quoting the author of Scubase (also an "Alex") at that point. All I can tell you from my own calculations is this:

There is a slight difference betwenn how Suunto Dive Manager and Scubase do the calculation. Scubase adds 14.7 psi to the tank working pressure to account for atmospheric pressure at the surface; SDM does not.

None of this matters much unless you are anally-retentive about the details, which I guess I am.

 

[Charlie99]

There is a slight difference betwenn how Suunto Dive Manager and Scubase do the calculation. Scubase adds 14.7 psi to the tank working pressure to account for atmospheric pressure at the surface; SDM does not.

Assuming both starting and ending pressures are measured the same way, whether it's PSI-Gauge or PSI-absolute makes no difference whatsoever in psi used. Gauge vs absolute only comes into the calculation of what the internal volume of the tank is. I haven't verified it, but my first guess is that the working pressure is in GAUGE pressure, and the extra 1atm should not be added.

According to my scoring, it's Suunto 2, Subase 0

Charlie

Heh, if you are going to write software, you (meaning the other Alex) might as well do the right calculations, even though it's 0.5% (psi) or 1.3% (bar vs atm) errors on something that easily varies 10% under supposedly identical conditions, and by a factor or 2 or 3 when you change the workload.

 

[Sideband]

Ah, my misunderstanding . . . apologies!

the K

No apology needed. BTW, when did you become staff? Don't know how I missed that unless I just wasn't paying attention. Congrats, although maybe very belated.

Joe