Trying to figure out my SAC rate

[SteveTW]

I'm new to diving and bored with this cold winter so I thought I'd try to figure out my SAC rate. What I did was to take my last 7 dives, they were multi-level dives, and work with them. The deepest I dived was 72'. The shallowest dive being 53'. So I crunched the numbers and then averaged the 7 SAC rates and came up with an average of 23.6 psi per minute. Again, these were multi-level dives. Did I do it right? And is that a very high rate?
Thank's for any help.
Steve

 

[elan]

Steve, You did not provide which tank you were using. Assuming you were using Al80 it gives approx 0.6 cuf/m, not bad - an average rate. IT is always more diffucult to determine it when you do multilevel dives so it is recommended that you do it on a flat shallow profile besides it depends upon the environment a lot. But you did good imho. Keep in mind taht some of that air is your BCD SAC rate if you inflate it.

 

[Bubbletrubble]

Deepest or shallowest dive is completely irrelevant information.
Did you have a record of average depth to help you calculate SAC for each dive?
What kind of tank did you use for your dives?
Did you use the same size tank for all 7 dives?
23.6 psi is in the range of a normal/average SAC rate, I guess. However, psi per minute may not be as interesting volume/unit time (cu ft/min).

 

[Sloth]

SAC is generally expressed in cubic feet since PSI is dependent on tank type.

Here is a SAC calculator to use SAC Calculator or you can use a tank factor scuba math to convert between PSI and cuft.

Tank factor math for single aluminum 80 to convert PSI to cuft SAC = PSI per minute ÷ 100 * 2.5 (alum 80 tank factor) reverse it to convert cuft to PSI. PSI = cuft per minute * 100 ÷ 2.5.

Additionally dont get to hung up on an exact number. Round it up and go with that so like elan pointed out you could average out at .6 cuft per min. So you could reasonable plan your dives using a SAC of .6 for gas calculation but you might want to bump it a little for conservatism.

Next big thing is actually using that info on a dive. Determine your PSI average for ever 5 mins. So going off your numbers and using an aluminum 80 I would calculate 125 psi per every 5 minutes at 1 ATA or at the surface. Then I would look at PSI used every 5 mins per each ATA so something like this:

1 ATA 125 PSI every 5
2 ATA 250 PSI every 5
3 ATA 375 PSI every 5
4 ATA 500 PSI every 5

Then on your dive figure in your head what your average depth has been for the latest 5 min interval, look at your gauge and see if you have used the predicted amount of gas since the last check.

Personally I calculate a SAC range such as .6 to .7 and then track on a dive were I am in the range. Underwater its all ballpark anyway.

(hope my math is right! I suck at math)

 

[Bubbletrubble]

In case you were wondering, here's one way to calculate SAC/RMV in cuft/min units:

(Total capacity of tank in cu ft/tank service pressure in psi)*(psi start - psi end)/(avg depth in fsw/33+1)/(duration of dive in min) = SAC/RMV in cuft/min units

Example calculation...
Tank = AL80
Total tank capacity = 77.4 cuft
Tank service pressure = 3000 psi
PSI start = 2900 psi
PSI end = 800 psi
Avg depth = 39.8 fsw
Duration of dive = 56 min
(77.4/3000)(2900-800)/(39.8/33+1)/56 = 0.44 cuft/min

 

[SteveTW]

Thanks for all the replies. Actually I always use my HP 100's. Being pretty new AND oldthese are the tanks I bought as soon as I got my OW cert.
Thanks again,
Steve

 

[SteveTW]

"Bubbletrubble", no, I didn't record my average depth. I don't have a cable to plug my dive computer into my CPU. I just took my last 7 dives out of my dive log. Always the same tanks, HP 100's, single of course. All of these dives were done in a quarry, by the way.

"Sloth", thanks for the SAC calculator. I'll try to use it but then again I don't know my average depth off hand. Hated math in school and still do. Thank God for calculators and the likes .

Thanks again,
Steve

 

[gcbryan]

"Bubbletrubble", no, I didn't record my average depth. I don't have a cable to plug my dive computer into my CPU. I just took my last 7 dives out of my dive log. Always the same tanks, HP 100's, single of course. All of these dives were done in a quarry, by the way.

"Sloth", thanks for the SAC calculator. I'll try to use it but then again I don't know my average depth off hand. Hated math in school and still do. Thank God for calculators and the likes .

Thanks again,
Steve

You should be able to come pretty close to the average depth if you just run the dive through in your head. On a multi-level shore dive in a lot of cases the average will be close to half of your max depth if you spent some time at depth and then slowly followed the slope back up and spent some time in the shallows.

If you do the SAC calculations just based on your deepest depth but if you actually spent much of the dive much shallower then your SAC rate calculation will come out much lower than it was in reality...in other words it will appear as if you could spend much more time at depth than you are actually able to.

Think about your 7 dives and decide if you spent most of your time at depth or most in the shallows or equal time at all depths. Come up with a reasonable average depth and then do the calculations.

I did this before I had a computer with average depth info and the numbers turned out to be very close to those numbers I got later with the newer computer and the actual numbers.

 

[mpgunner]

I use my dive computer output to check my SAC rate for a specific depth. Watching and improving your SAC rate is a good thing to do.

 

[Bronson]

To SteveTW, i find it interesting that a OW diver would be thinking about SAC. Excellent. I think i was diving 15 years before i was thinking of SAC and now i use it to plan dives. Slouth above gave an interesting web page to calculate SAC (thanks Slouth). I dive in metric-land these days and have forgotten all about psi but i leanred to do it this way:

- Dive to 10 meters (33 feet) which is 2 bar.
- Dive relaxed and breath down 10 bars on the gauge
- use a 12 cf tank with 200 bars air (about 2900 psi) 2400 cf of air
- calculate the usage and divide by 2 bars

Seemed easy for me to do. Came up to 11 cf/minute