SAC Rate

[rakkis]

O2 does not drive the breathing reflex. The amount of dissolved CO2 in your blood does. So an increased pO2 should not noticeably decrease your air consumption.

 

[mikerault]

I didn't say it increased it...I said it decreased it.

Mike

 

[rakkis]

My brain and my hands weren't talking to each other. I made a correction to my post.

 

[mikerault]

Interesting.... I looked at several of my dives at various depths to look for a trend.

DEPTH SAC
106 .51
88 .57
82 .41
63 .48
50 .48
48 .45
40 .50
29 .46
26 .55
15 .50

There did seem to higher rates the shallower I got, and all dives were from the past year, so my physical fitness shouldn't have changed that much.

Looking at the numbers, something odd happened on the 88 foot and 106 foot dive to drive your SAC up, the values fall well out of +/- one standard deviation from the other numbers to that point (+/- .047), eliminating them as outlayers shows the same trend as I see in my data.

 

[arman68]

At last I think I understand how to calculate SAC properly... It has taken me a while, and involved reading this whole thread and others, as well a quite a few other documents. What has made it difficult and confusing is the difference between metric and imperial: from what I understand the SAC rate values are different for imperial and metric, and are calculated differently. So, here it is:

[B]Metric[/B]: SAC rate is in [I]litre/minute[/I]

SAC = ( tank_volume_in_litre x used_air_in_bar )
    / ( minutes x avg_absolute_pressure_in_bar )

avg_absolute_pressure_in_bar = ( avg_dept_in_meter / 10 ) + 1

[B]Imperial[/B]: SAC rate is in [I]cubic feet/minute[/I]

SAC = cubic_feet_of_air_used
    / ( minutes x avg_absolute_pressure_in_bar )

cubic_feet_of_air_used = tank_size_in_cf x used_air_in_psi
                       / tank_rating_in_psi
avg_absolute_pressure_in_bar = ( avg_depth_in_feet / 33 ) + 1

SAC rate can be converted from metric to imperial easily as we know that
1 cf = 28.32 l
Converting tank sizes is much more difficult, so if you are used to metric SAC, and have to use imperial measures, the best way to handle it is to calculate imperial SAC and then convert it to metric (and vice versa).

Example:

45 minutes dive at average depth of 13m
I used 170 bar of 12l tank

SAC = (12x170) / (45x(13/10)+1)) = 2040 / (45 x 2.3) = 19.7 l/min

We convert to imperial:
SAC = 19.7 / 28.32 = 0.7 cf/min



I hope I am correct. Could someone more knowledgeable please confirm? Once done, I think those formulas have their place in a sticky.

 

[Blackwood]

...

There are many ways to write the formula. As noted in another thread (http://www.scubaboard.com/showthread.php?t=169209):

...because my mind likes to group things a certain way, I express it as follows:

{Vrated/(Prated*Pambient)} * {(PSIo-PSIf}/T]

The first {} is tank information... how much volume you get from a full tank at a given ambient pressure (which, for SAC purposes, is your average pressure depth).

e.g. HP100 -> 100CF from 3443PSI at 1ATA

The second {} is dive information.

e.g. 3443PSI/60minutes if you empty (hoover) your HP100 in an hour

Together, that's a (S)AC rate of about one-and-two-thirds CFM.

 

[creamofwheat]

SAC...tricky thing, that.

I have been as low as 0.37 or so, on the dives i did in Cozumel (drift, and warm water...not much effort required on my part), but I dive the cold waters of the PNW, and I seem to average around 0.45 ish. When I get really cold, my SAC goes through the roof...it's been as high as 0.8 on dives where I have gotten too cold. And I am only a 110lb female!! That's a true air hog for ya.

 

[ianw2]

In my opinion you need to work on developing excellent bouyancy skills, learn to move efficiently in the water with no flapping arms or extra leg, trunk or body movement, develop a cleaner and more streamlined gear configuration to reduce the power required to move at a given speed and develop an efficient breathing pattern consisting of breathing deeply and pausing slightly at the top of the inhale to maximize gas exchange.

Thanks to the paper sent to me by Bob Bailey, I’ve been calculating my SAC and RMV for my dives to date (less the 7 dives in Loretto, BCS, Mexico that my Sherwood Wisdom failed to log for download). Since I had absolutely no idea what SAC or RMV meant, I could not have been working specifically to decrease those values. Even so, I see some very interesting trends and can draw some very interesting conclusions from my data regarding activity, stress, water temp, etc.

Dive No. SAC RMV
1 41 1.07
2 29 0.75
3 40 1.05
4 37 0.96
5 59 1.53
6 36 0.93
7 39 1.01
8 35 0.91
9 31 0.80
10 39 1.02
11 30 0.77
12 28 0.74
13 64 1.67
14 53 1.37
15 36 0.94
16 27 0.69
17 32 0.83
25 21 0.54
26 31 0.81
27 28 0.73
28 27 0.70
29 28 0.74

Dive 5 was a Navigation Class dive where I was swimming my lengths as fast as possible (BAD IDEA!!!)

Dive 13 was a Rescue Class Dive practicing some pretty tough skills.

Dive 14 was a sucky dive with huge surge, very low vis and worrying about my wife and our dive partners.

Dives 18 - 24 were the Lost Mexico Dives.

Dive 26 was a Wreck Class Dive on the Valiant at Avalon Harbor. Deepish (for me)…lots of "issues"…

OK, So…my numbers are not indicative of a long-time diver with tremendous experience and skill.

Things just "clicked" in Mexico. Being able to dive in nice, warm water doing three dives per day for a week really helped with learning; buoyancy control issues came into control much better, weighting anxiety went away, etc.

Now I know what I want to concentrate on, I can work on improving my diving. The numbers will be an interesting way to track my progress.

BTW - I quit smoking earlier this year. Does anyone have any numerical data (not anecdotal data) on the effect of quitting smoking on SAC and RMV?

Cheers, Bob! And thanks again for providing me with another tool to help me dive better!

 

[djanni]

I never calculated SAC until Oceanlog 2.0 which does it for you. Yet before that I did take a lot of interest in bottom time. Monitoring depths, starting psi ending psi and etc.

Early on I could get 35-45 minutes on an AL80 at depths around 50'-60'. At 25'-30' add about 10 minutes. Actually my bottom time seems to be all over the place and I'm sure that's for all the reasons these very experienced divers are talking about.

Then, at about 90 total dives, I bought a camera and I've taken it on almost every dive since then. Another 90. Interesting thing happened. I added about 10 minutes to all my dives and for all practical purposes, that's regardless of depth. According to Oceanlog my SAC ranges from .47 to .60. Again, depends on all those various factors I guess.

The point to this is, I think my bottom time or SAC improved as soon as I stopped thinking about it so much and started concentrating on the photo ops.

My wife... well she doesn't breath underwater. Whatever my remaining psi is upon surfacing (and I try to plan that for 500) she'll have and additional 300 to 600 psi.

Anyway.... get a camera.

 

[mikerault]

IANW2....your plot of points looks too good, no outlayers what so ever in the plot of SAC/depth all trending upwards with depth, in fact is is a completely linear plot, are you sure you are calculating this right? The equation for a line through your data points is:

y = 0.026x - 0.002

And no point deviates by more than +/- .014 from the calculated value, the fit is too good, you might want to check your calculations. Plus a SAC of 1.67 at 64 feet is pretty bad.

Mike