Any idea's on why my SAC is all over the place?

[rottielover]

Hi all...

The wife and I are leaving for Coz in just over a week now...

I've been playing around with Suunto's Dive Manager software, as well as scubabase, trying to calculate my SAC rates from my prior 8 dives... I don't wanna be the one to ruin the day of diving cause I hoover....

I only have data from my 4 OW cert dives and my 2 PPB dives in the computer, my other 2 dives are written in my log book at home (I'm going to enter them tonight to see if the numbers still hash).

Anyway, here is what I got, [I can't explain it, especially Peak Dive 1]

Dives in order, all on AL80:

OWcert1 SAC=0.76 SCFM 25ft 22 min Start 3000psi End 1800psi
OWcert2 SAC=0.40 SCFM 22ft 22 min Start 1800psi End 800psi
OWcert3 SAC=0.55 SCFM 24ft 49 min Start 3000psi End 1100psi
OWcert4 SAC=0.56 SCFM 44ft 40 min Start 3000psi End 900psi

PeakPB1 SAC=0.17 SCFM 20ft 19 min Start 1100psi End 500psi
PeakPB2 SAC=0.68 SCFM 27ft 40 min Start 3000psi End 1000psi

I took the SAC number directly out of Suunto Dive manager.

Scubabase calculates SAC in Cu. Ft. / Min and I got:

OW1 33.96
OW2 27.27
OW3 24.14
OW4 29.87
PPB1 19.66
PPB2 31.13


Any tips/tricks that I haven't been taught yet? My instructors all said that my wife and I seem to have an excellent sense for bouyancy, which is all find a good, but it doesn't seem to be helping my air consumption.

Or maybe I just have those pre-trip jitters?

 

[Blackwood]

During your OWcert dives, you were demonstrating certain skills some of the time, and, no doubt, sitting around on the bottom waiting for others to do the same the rest of the time.

Conditions (current, temperature, dive goals, etc.) will effect how much you breathe.

I'm a bit confused about the above data...

OWcert1 SAC=0.76 SCFM 25ft 22 min Start 3000psi End 1800psi
OWcert2 SAC=0.40 SCFM 22ft 22 min Start 1800psi End 800psi
OWcert3 SAC=0.55 SCFM 24ft 49 min Start 3000psi End 1100psi
OWcert4 SAC=0.56 SCFM 44ft 40 min Start 3000psi End 900psi

PeakPB1 SAC=0.17 SCFM 20ft 19 min Start 1100psi End 500psi
PeakPB2 SAC=0.68 SCFM 27ft 40 min Start 3000psi End 1000psi

I'm suspect those listed depths are max... If they are average, I get different SAC Rates (substantially different on OWdive2 and PPB1).

OW1 .81
OW2 .71
OW3 .58
OW4 .59
PB1 .51
PB2 .72

Check to see if those are maximum depths. If they are, fill in the average depths and I'll re-run the numbers.

Scubabase calculates SAC in Cu. Ft. / Min and I got:

OW1 33.96
OW2 27.27
OW3 24.14
OW4 29.87
PPB1 19.66
PPB2 31.13

Something is very wrong there. You are clearly not breathing air anywhere close to that quickly. If you were, you'd not have been able to log that much bottom time.

I imagine you put the data in wrong.

 

[JimC]

The second list looks like PSI / Minute

 

[The Chairman]

I would refer you to our article on INAC, but it is still being written!

It's Not a Competition!

SACs are great to figure out what size tank you are going to dive, but they should NOT be used to try to increase your bottom time. The only way to "ruin" a dive is to come up with a headache because you tried skip breathing or to come up with too little air.

However, there are several things you can do to increase your bottom time.

First, learn how to relax. Fold your hands (stop sculling) and stop swimming so hard. Learn to mimic an amoeba and swim as little as possible. It's a HUGE ocean: don't try and see it all in one day.

Second, get your trim under control. Your head should be slightly down as you scuba. You should be able to stop and keep that attitude with NO EFFORT. This means adjusting the position of your tanks and weights to achieve the desired attitude. Make sure you don't have TOO much weight either. The more weight you have, the more air in your BC and the greater your drag!

Third, start doing laps in the pool! That's right: laps. I swim five days a week... three in just my swim suit, and twice in an all out kick fest with fins, mask and snorkle! Even when I do weights, I still get a few laps in. This is THE best way to increase your bottom time and your comfort level in the water!

 

[Blackwood]

The second list looks like PSI / Minute

Order of magnitude, yes. You are probably right, but the numbers still seem off.

 

[rottielover]

I knew I didn't trust scubabase for some reason!

I think they mis-labeled the SAC rate field in the program. It says Cu. Ft /Min , but that doesn't jive....


I'm not in competition with anyone, I just don't want to be the first one to turn pressure all the time...


Your correct, those are max depths, I'm assuming that the program is caculating the Avg. Depth, because it doesn't ask you to enter it.


At this point I'm planning on sticking with Suunto's Dive Manager software, as there is something wrong with the copy of Scubabase I have (perhapse there is a patch I don't know about).


Time for me to dust off the calculator, pad, and pen and start working the numbers.....

 

[SuPrBuGmAn]

I think you are using the ((pressure/time)*33)/(depth + 33) SAC equation correct? Alot of people on here use a different equation that works to get your SAC no matter the cylinder size...

Were the dive profiles the same? If some were at depth the entire dive followed by an ascent to the surface, they probably wouldn't compare well to a dive that you followed the countour of an the ocean floor or a large wreck shallower as the dive went on.

Dives in freshwater will also vary from saltwater dives somewhat due to a slight pressure difference.

Comfort during the dives is another factor, you might breath more or less at night than during the day. More during current, more during low visibility, more if you were spearfishing, ect...

 

[rottielover]

I think you are using the ((pressure/time)*33)/(depth + 33) SAC equation correct?

I'm not exactly sure what calc these two programs are doing.... What's the calculation that works no matter the cylinder size?

 

[Diver Dennis]

Make sure you have the right cylinder size entered and working pressure of 3000 PSI.

 

[Diver Dennis]

The program will read the actual starting pressure of the cylinder.