SAC calculation

[thethumper]

Ok..... I'm gonna go out on a limb here & ask... I have been diving for about 12 years & still dive tables (no computer). How do you calculate SAC? I know it's probably a rudimentary skill, however, it's one I don't posses. I dive overhead using the rule of 3rds. When I hit 1/3, it's time to turn back.

Feeling less than adequate.;-0

[Uncle Pug]

thethumper once bubbled...
Ok..... I'm gonna go out on a limb here & ask... I have been diving for about 12 years & still dive tables (no computer). How do you calculate SAC? I know it's probably a rudimentary skill, however, it's one I don't posses. I dive overhead using the rule of 3rds. When I hit 1/3, it's time to turn back.

Feeling less than adequate.;-0
Takes a lot to make an admission like that... and so I'm not going to be unkind.

SAC is your surface air consumption... the amount of air you use at 1 ATA (atmosphere absolute.) With that figured in cf per minute for both working and resting rates you will be able to do gas planning... converting back to psi for use during the dive itself with whatever size cylinder you are using.

Basically you need to keep track of the gas you use over a period of time at a specific depth while at rest and while swimming. I would advise you to take some classes but in the mean time you can burn a few tanks and get a good idea of your SAC rates.

Swim for 20 minutes at a depth of 34ffw and note how much gas you have used. You will need to figure your cylinders PSI/cf ratio... if for instance you are using double LP104s then you can figure that every 100psi represents 8cf. If you need help with this PM me.

Take the cf used and divide it by 2 since you were at 2ATA and then divide that number by 20 since you were there for 20 minutes.

This number would be your working SAC. Do the same just floating around for 20 minutes at 34ffw and you can calculate your resting SAC (used for figuring deco gas requirements.)

How you use this SAC figure in gas management is another lesson.

Now that I have attempted to answer your question... let me be frank... you really shouldn't be diving overhead without the proper training. If you do not know how to calculate your SAC then you have not had the proper training. Scuba Board is not a substitute for proper training.

While I admire you (seriously) for being able to make this admission of inadequacy I will not continue to admire you if you persist in the folly of diving overhead without proper training.

[ERP]

It's fairly simple.

First calculate how much air you used in the dive, from the ratio of pressure/volume.

vol = ((Start Pressure-End Pressure)/Working Pressure) *Tank Volume

OK so Now you know how much air you used, divide it by the time you dived and you have a consumption rate.

CR = vol/time

You now need to adjust this to take into account the compression of the gas at depth, just divide the number by the number of atmospheres absolute your diving at


ATA = 1 + (average depth/33) -- Assuming ft
or
ATA = 1 + (average depth/10) -- Assuming meters

CR/ATA = SAC

I'm sure someone will correct the blaring error I almost certainly made in that.

[Ontario Diver]

Just a simple calculation....

put on a tank, go to 33 ft depth (2 atm)

Note your starting pressure (P1)
hang out relaxed for static SAC (like a deco or safety stop), swim around for dynamic SAC

after 10 min, note your ending pressure (P2)

Take P1 and Subtract P2. Gives you the presure that you breathed down. Divide that number by 10 (for 10 minutes) and by 2 for 2 atm and you'll have your sac. (Surface Air Consumption in terms of PSI)

For example, if I dive to 33 feet on my AL 80 with a starting pressure of 3000 and after 10 minutes my pressure is 2376. then I used 3000-2376 = 624 psi over 10 min at two atm. divide by 10 min and two atm and I get about 31 psi per minute that I use on an al 80.

This is good if you never change tanks.

SACCF - Surface Air Consumption - Cubic Feet can be more useful.
take your psi and tank size and figure it out in terms of cubic feet. For example my al 80 really has 77 cf in it at 3000 so

77 SACCF
----------- = -------------
3000 31

SACCF = .79 cubic feet per minute of air I use on the surface.

The way I can use this is to calculate the amt of air I use at depth

For example, to spend 20 min at 100 ft (5 atm)

air consumption = SACCF * min * atm
= .79 *20 *5
= 80 cubic feet of air

So to follow rule of thirds I'll need 80 * 3 cf of air - so the double 120s are going to be needed!


Hope that this helps ( I just finished the course !)

[Ontario Diver]

Geeze ain't we all fast :D

[thethumper]

guys..... And UP, why wasn't this tought in PADI Cavern Class? Was I given "Improper" training? Maybe I should have gone w/NACD after all.

[Rooster1]

thethumper once bubbled...
guys..... And UP, why wasn't this tought in PADI Cavern Class? Was I given "Improper" training? Maybe I should have gone w/NACD after all.

It is taught in Padi AOW which is comes before Cavern Class

[Uncle Pug]

thethumper once bubbled...
Maybe I should have gone w/NACD after all.
...you need to git yourself into a GUE class. SAC is fundamentals stuff. You're only 90 miles from High Springs!


btw... did you take care of that little web-page thingy we talked about?

[scubafanatic]

Surface Air Consumption Rate

The compressibility of gasses is also an important consideration for divers due to its affect on how long a diver can stay underwater. Scuba regulators are designed to deliver air to a diver at the same pressure as the surrounding water pressure, at ambient pressure. That means that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amount of air as two breaths at the surface. Obviously then, a tank will only last half as long at 33 feet as it would at the surface. A tank that would last 1 hour at the surface would only last 1/3 as long, or 20 minutes, at a depth of 66 feet, etc.

It can be beneficial to be able to estimate how long a scuba tank might last at a given depth when dive planning. To determine this, it is first necessary to determine a divers Surface Air Consumption (SAC) rate. For example, if you are diving at 33 feet, and use 500 lbs of air in 10 minutes, it is easy to determine that you are using 50 lbs per minute. This is only true for this depth however. How much air would you use at 66 feet, or 99 feet?

The first thing we must do is calculate SAC rate. In the preceding example, a diver using 50 pounds per minute at a depth, would use 25 pounds per minute at the surface. His surface air consumption rate is 25 pounds per minute. With our SAC number of 25, it is easy to calculate our consumption rate for depths of 33, 66, 99 feet etc. We know we are under 3 times the pressure at 66 feet, so we would use 3 times as much air, or 75 pounds per minute at this depth.

The process becomes slightly more complex if depth consumption rate (DCR) is determined at a depth that is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use a formula that is simply an adaptation of Boyle's Law to determine our SAC rate:

SAC Rate = (DCR x 33) / (Depth + 33)

Let's look at an example. Suppose you did a 50 foot dive for 25 minutes and used 1700 pounds of air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula we get:

SAC Rate = (68 x 33) / (50+33)

or: SAC Rate = 2244/83 or about 27 pounds per minute.

We can then turn the equation around to determine our DCR for any depth.

DCR = SAC Rate x (Depth + 33)/33

Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of air at a depth of 75 feet.

Dropping our numbers into the equation we get: DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81

This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into the 2000 pounds, we see this amount of air would last 24.4 minutes.

It is important to note that SAC Rate takes into account the assumption that you are exerting the same amount of energy at any given depth, and you are using the same size tank as you used when calculating your DCR.

For example, under strenuous diving conditions, you can consume air 4 times faster than when sitting still taking pictures. Also it is obvious that a 50 cubic foot tank would not last as long as an 80 cubic foot tank, even if they were both filled to the same pressure.

[thethumper]

in AOW. I went through PADI AOW & I wasn't tought SAC. Had I been I wouldn't be asking this question now.

UP.... GUE won't have anything to do w/me.... I'm shunned 'cause I smoke. The messed up thing about it is that who ever I've dove w/in the past 10-12 years, I've always outbreathed. My buddy is an air hog. I like alot of the DIR/GUE ways, but because I smoke, I can't participate in all the Reindeer games. As far as the site thingy, I'm glad you said something, I'll work on that today while I'm @ work.

[O-ring]

Shoot, just grab a couple cans of Copenhagen and your smoking problem is solved. If AG or JJ gives you any crap about that just spit tobacco juice at them...or turn their air off.

[roakey]

thethumper once puffed...
The messed up thing about it is that who ever I've dove w/in the past 10-12 years, I've always outbreathed.
This is quite common. Current thinking is that because you lock up so much blood with CO from the smoke you inhale that your body learns how to deal with less O2 than a non-smoker.

Unlike a fit person's lesser lower SAC, this is not a sign of fitness, it's quite the opposite. It's an indication of the body's struggle to deal with an unhealthy situation.

Roak

[thethumper]

roakey once bubbled...

This is quite common. Current thinking is that because you lock up so much blood with CO from the smoke you inhale that your body learns how to deal with less O2 than a non-smoker.

Unlike a fit person's lesser lower SAC, this is not a sign of fitness, it's quite the opposite. It's an indication of the body's struggle to deal with an unhealthy situation.

Roak

Whatcha sayin' Roakż If I stop smokin' I'll become an air hog?

[zboss]

thethumper once bubbled...
in AOW. I went through PADI AOW & I wasn't tought SAC. Had I been I wouldn't be asking this question now.

UP.... GUE won't have anything to do w/me.... I'm shunned 'cause I smoke. The messed up thing about it is that who ever I've dove w/in the past 10-12 years, I've always outbreathed. My buddy is an air hog. I like alot of the DIR/GUE ways, but because I smoke, I can't participate in all the Reindeer games. As far as the site thingy, I'm glad you said something, I'll work on that today while I'm @ work.

So just adopt the GUE methodology and say you are a GUE diver... they have no patent on that technique. SO what if they don't issue you a c-card it's not like you can't go diving on your own with just a PADI cert...

Let the flames begin!

[thethumper]

zboss once bubbled...


So just adopt the GUE methodology and say you are a GUE diver... they have no patent on that technique. SO what if they don't issue you a c-card it's not like you can't go diving on your own with just a PADI cert...

Let the flames begin!

After hearing what I've heard from other cavers & tech divers in Jax that are not GUE but do practice DIR skills & set ups, I'm not sure I would want to be let anyone know I was a GUE diver. They seem to get pretty well slammed by alot of the Hard Core techies. But I've not been to a class, nor do I know any of them personaly. But they sure get alot of flack. I AM NOT A SHEEP and I belive there is room for deviation. But this is about SAC, not DIR/GUE. When I get the resolve to quit smoking I will. And it's lookin' like it's around the corner.

[zboss]

thethumper once bubbled...


After hearing what I've heard from other cavers & tech divers in Jax that are not GUE but do practice DIR skills & set ups, I'm not sure I would want to be let anyone know I was a GUE diver. They seem to get pretty well slammed by alot of the Hard Core techies. But I've not been to a class, nor do I know any of them personaly. But they sure get alot of flack. I AM NOT A SHEEP and I belive there is room for deviation. But this is about SAC, not DIR/GUE. When I get the resolve to quit smoking I will. And it's lookin' like it's around the corner.

Good for you man! On one hand I am happy that there are additional training opportunites available from a variety of diving organizations but on the other hand if GUE is all about a diving philosophy... well SCUBA is a young sport and the equipment, knowledge, and techniques are still evolving. I would think that as diving medicine gets better, training will become more refined and focused in on the actual issues both physically, sociologically, and physcologically.

One could also draw a parallel between amature and professional athletics... there are not very many olympians that smoke or are not physically/emotionally healthy but then again most also operate in perfect condictions using incredible equipment - so who is the better athlete: the one that can perform at capacity in less than perfect conditions or the one that can only compete in perfect conditions.

Sorry to get off the subject of SAC... maybe I am just defending your position about smoking; my sister and brother both smoke... he goes easy on gas, but she tears through a tank. In either case I am not entirely sure it is just not that air consumption is tied to physical fitness and smoking is associated with poor physical conditioning.

[O-ring]

They seem to get pretty well slammed by alot of the Hard Core techies.
When you are on top you always have to take the flak from the others scrambling up the hill and sniping at your heels.

Smoking is a controversial topic to be sure. On the one hand, people have the right to do whatever they want. On the other hand, in a sport where healthy lung function is so important why would you do anything to jeopardize it?

[thethumper]

O-ring once bubbled...

When you are on top you always have to take the flak from the others scrambling up the hill and sniping at your heels.

Smoking is a controversial topic to be sure. On the one hand, people have the right to do whatever they want. On the other hand, in a sport where healthy lung function is so important why would you do anything to jeopardize it?

Because I started smoking when I was 12. That was 20 years ago. If I knew then what I know now, would I have started???
No, but that's the same answer any smoker will give you. If I could stop smoking RIGHT THIS SECOND I would. It's worse that coc***... Uh....we won't go into that. That was when I was more stupider. Everone has a vice of some form. I smoke. I don't drink, I don't do drugs.... I just wanna dive the best I can dive.

[O-ring]

I was thinking you could use the Copenhagen to not smoke for the duration of the class and get by :D

I dive with some people who smoke and I have family members who smoke....I agree with you, it is tough. We won't solve that debate today, so let's stop while we can.


Everone has a vice of some form.
Agreed...mine is diving! Oh, and mexican food...

[Soggy]

O-ring once bubbled...
Agreed...mine is diving! Oh, and mexican food...

Sounds like a dangerous combination!

[O-ring]

Sounds like a dangerous combination!
...more so with a drysuit since, uhhh, breaking wind from the mexican food can cause runaway ascents due to inflation. Gotta keep the dump open and away from any aquatic life that you don't want to kill.. Not to mention the stale smell when you finally surface and unzip that sucker...

[roakey]

thethumper once puffed...
Whatcha sayin' Roakż If I stop smokin' I'll become an air hog?
No, I'm saying that your implication that because you have a better SAC than other divers and are better/more relaxed/more experienced or whatever than them and therefore GUE should entertain you is wrong. Your low SAC is due to your body being all hosed up.

GUE doesn't want students whose bodies are all hosed up.

Roak

[Uncle Pug]

You can take the DIRf and still be a smoker... not a good thing... but you can.

The reason I suggest the GUE course is that starting with DIRf I think you would not only gain an understanding of SAC but also learn how to use it in gas planning.

Once you take a DIRf my guess is that you will have all the motivation you need to quite smoking so that you can go on with GUE training.

It would be a win/win for you.


btw: if as you say, "I AM NOT A SHEEP and I belive there is room for deviation." then deviating from what the other "cavers & tech divers in Jax" think shouldn't be a problem. ;)

[thethumper]

roakey once bubbled...

No, I'm saying that your implication that because you have a better SAC than other divers and are better/more relaxed/more experienced or whatever than them and therefore GUE should entertain you is wrong. Your low SAC is due to your body being all hosed up.

GUE doesn't want students whose bodies are all hosed up.

Roak

If what you says is true of GUE for smokers, why not go ahead & exclude people w/I don't know lets say high colesterol...that hoses up your heart, or people who are over 6' tall. After all they can't get into spaces that shorter people can. Or how about people who are 25, 50 or 100# over weight. I've seen several GUE guys that fit that discription... Who else can we come up with.

What I'm saying is What difference does it make if you smoke, or your over weight or over tall or have high colesterol as long as your bettering youself & your sport?

Ellitism theology should have gone the way of Hi***!!!!!!

[O-ring]

...but, these are all correctable conditions. Overweight people need to consume less calories and exercise. People with high cholesterol need to eat a low cholesterol diet and exercise (I had high cholesterol a couple years ago and did this). People who are over 6' tall need to have their legs shortened...ok, kidding here, but that is something one cannot help.

IMHO, bettering yourself should start with basic health and not with scuba related stuff...move onto that once one has all the basics of bettering covered.

[thethumper]

:banging:

It's still discrimination.

[O-ring]

It's still discrimination.
GUE is the non-smoking restaurant of the dive certification agencies. ;)

God my mother hates non-smoking restaurants :)

[Spectre]

Ok... first off... SAC rate calculations are all math based... you _can_ learn the math on the Internet. learning how to actually use the numbers you calculate as a basis for dive planning can also be learned outside of a scuba class .

Now as far as the DIR-f goes, I don't completely understand the non-smoking [i]requirement. A recommendation is fine, but a prerequisite for the course seems a little extreme. There are many things just as bad, or worse, than smoking. What's next? "Meat eaters need not apply"?

It just seems out of place IMHO.

Yes... smoking is bad. I don't think you'll find one [minimally intellegent] smoker that doesn't accept the fact that it's bad for you... How many smokers do you know that started smoking in their 30s, or 40s, when they were old enough to truely understand the risks, and seen the effects? I don't know any smokers that started smoking after they were 19. However its shown that getting off nicotine is as hard, if not harder, then heroin! I've never been addicted to heroin, so I don't know for sure, but I'm sure it has to do with the really really hard part about quitting smoking.... which has to do with it's _everywhere_. It's nearly impossible to shelter yourself from seeing or smelling smoking while you try to quit.

Oh... O-Ring. Copenhagen won't work... it gets nasty when you spit into your reg, and your buddy'll probably kick your butt after a few s-drills!

[thethumper]

Spectre once bubbled...
Ok... first off... SAC rate calculations are all math based... you _can_ learn the math on the Internet. learning how to actually use the numbers you calculate as a basis for dive planning can also be learned outside of a scuba class .

Now as far as the DIR-f goes, I don't completely understand the non-smoking [i]requirement. A recommendation is fine, but a prerequisite for the course seems a little extreme. There are many things just as bad, or worse, than smoking. What's next? "Meat eaters need not apply"?

It just seems out of place IMHO.

Yes... smoking is bad. I don't think you'll find one [minimally intellegent] smoker that doesn't accept the fact that it's bad for you... How many smokers do you know that started smoking in their 30s, or 40s, when they were old enough to truely understand the risks, and seen the effects? I don't know any smokers that started smoking after they were 19. However it's shown that getting off nicotine is as hard, if not harder, then heroin! I've never been addicted to heroin, so I don't know for sure, but I'm sure it has to do with the really really hard part about quitting smoking.... which has to do with it's _everywhere_. It's nearly impossible to shelter yourself from seeing or smelling smoking while you try to quit.

Oh... O-Ring. Copenhagen won't work... it gets nasty when you spit into your reg, and your buddy'll probably kick your butt after a few s-drills!

thethumper<---------------takes hat off to Spectre.