SAC Calculations

[brutus_scuba]

Alrighty I've seen people talka bout their SAC on here, and I know that it's a number to show the rate at which youre breathing and that people who sip the air out of their cylinder have a .3x o .4x SAC and that a lot of people are in the .5x to .7x range, right?

but how do you calculate your SAC, I was never tought to do so in my BOW class so did my instructor leave something out or is it something ya just pick up or what?

 

[ppo2_diver]

Most basic open water classes don't cover SAC rates. A good instructor will include this calculation for any recreational deep diver specialty. It is one of the things that I cover in my deep class. I have different SAC rates depending on the task at hand. I have a SAC rate for a normal dive, a SAC rate for deco, and a SAC rate if I'll be working hard at something (fighting current, etc.)

To calculate your SAC rate, swim at a constant depth for any specific amount of time. Like 10 minutes. Write down your starting tank pressure. After your 10 minutes is up, write down the tank pressure. Subtract the ending pressure from the beginning pressure to get the PSI used.

The formula to calculate it is:
(psi used / tank working pressure in psi) x total tank capacity
-----------------------------------------------------------
(depth in feet + 33) / 10

Once you do the formula above, dive that number by the number of minutes you were at the specific depth. I mentioned 10 minutes above.

Here is an example:
I use 370 psi in doubled 71 cu ft. steel LP tanks. 142 total tank capacity with a working pressure of 2475. I swam around at 50 feet for 10 minutes.

((370 / 2475) x 142)
-------------------- / 10
(50 / 33) / 33

This results in:
(21.2)
------ / 10 = .84 cubic feet per minute SAC rate
2.5

 

[theskull]

It is not often taught. It is often referred to in a decent AOW course, and should be considered in Deep Diver class. Divemasters learn the math but don't always get the importance. You must learn it as part of a tech-diving class.

Here ya go:

SAC Rate
Surface Air Consumption Rate

1. Determine how many PSI per cubic foot.
Example: 80 cu. ft. cylinder rated at 3000 psi.
3000 / 80 = 37.5 psi per cu. ft.

2. Determine how many cu. ft. used on dive.
Example: 3000 – 500 = 2500 psi / 37.5 = 66.7 cu. ft. used.

3. Determine how many cu. ft. used per minute.
Example: 66.7 / 30 minutes = 2.22 cu. ft. per minute.

4. Determine equivalent rate at surface.
Example: 53 ft. ave. depth / 33 = 1.6 ATM + 1 = 2.6 ATA.
2.22 (consumption) / 2.6 (ATA) = .85 SAC rate.

Basic standards:
.5 or less is excellent
1 is average
1.5 or more is generally a beginner

Factors affecting SAC rate:
Level of exertion
Comfort (temperature, anxiety, etc.)
Drysuit (consumes air)
Physical attributes—body size, lung size, conditioning
Correct weighting (frequent buoyancy corrections consume air)

Dive safe and have fun,

theskull

 

[The Kraken]

Brutus,

You've been given some good information here.

But keep in mind that one's SAC rate is a HISTORICAL figure that can be used in PLANNING before the dive.

For example, a diver diving an AL80 with 500psi residual pressure at the end of the dive will get the approximate dive times at these respective depths:

SAC rate .45 cfm = 57min @ 50', 50min @ 60', 46min @ 70'
SAC rate .50 cfm = 51min @ 50', 45min @ 60', 41min @ 70'
SAC rate .60 cfm = 42min @ 50', 38min @ 60', 34min @ 70'
. . . and so on.

The true SAC rate of a diver will vary at any time during any dive because of the effects that pressure has upon one's consumption of gas at varying depths, that's why it is generally calculated based on the average depth of the dive.

There is another rate of consumption that is more valuable during the dive, and that's one's RMV. The RMV is generally expressed as the PSI used per minute. This calculation reflects the amount of gas you're using RIGHT NOW.

The diver's RMV will vary with depth and exertion, etc.

But let us just say the diver is quite calm, relaxed and is having a good dive.

Just for kicks and giggles let's say the diver's customary RMV for a dive at his current depth is 125 psi. A quick check of the SPG/computer and time keeping device will let the diver easily determine if he is on track with his consumption. Any increase could indicate a potential problem.

A little simple math will also allow the diver to determine an approximate remaining dive time based on the current data.

the K

 

[Packhorse]

Or in metric

air used/((average depth in meters/10 )+1)/ minuites dived = SCR in litres per minuite

air used =start tank pressure in Bar - end tank pressure = bar used * volume of tank in litres (or WC rating of tank)


((average depth in meters/10 )+1) this calcutales ambiant pressure. eg at 20m amb pressure = 3 bar . at 35m pressure = 4.5 bar at 0m pressure is 1 bar

Its so much easier to use metric. To be honest i dont know why so many people still use PSI and feet.

 

[bperrybap]

Most basic open water classes don't cover SAC rates. A good instructor will include this calculation for any recreational deep diver specialty. It is one of the things that I cover in my deep class. I have different SAC rates depending on the task at hand. I have a SAC rate for a normal dive, a SAC rate for deco, and a SAC rate if I'll be working hard at something (fighting current, etc.)

To calculate your SAC rate, swim at a constant depth for any specific amount of time. Like 10 minutes. Write down your starting tank pressure. After your 10 minutes is up, write down the tank pressure. Subtract the ending pressure from the beginning pressure to get the PSI used.

The formula to calculate it is:
(psi used / tank working pressure in psi) x total tank capacity
-----------------------------------------------------------
(depth in feet + 33) / 10

Once you do the formula above, dive that number by the number of minutes you were at the specific depth. I mentioned 10 minutes above.

Here is an example:
I use 370 psi in doubled 71 cu ft. steel LP tanks. 142 total tank capacity with a working pressure of 2475. I swam around at 50 feet for 10 minutes.

((370 / 2475) x 142)
-------------------- / 10
(50 / 33) / 33

This results in:
(21.2)
------ / 10 = .84 cubic feet per minute SAC rate
2.5

The formulas above are confusing and incorrect. (probably typos)
Yes, the final answer is correct but the descriptive text showing
the formulas has errors in it.

(Examples above show 3 ways to calculate ATA:
(Depth + 33)
(Depth / 33) / 33
(Final caculation wich is correct is not shown) but is (Depth +33) / 33

thescull's description is pretty good.

The thing to remember is that people tend to be a bit sloppy when
talking about "SAC" rates. They tend to leave off units which is actually
very imporant. They also some times report SAC rates in terms of
pressure and sometimes in terms of volume.
For example, the units of SAC rates that people report could be:
BAR/MIN
PSI/MIN
Cubic Ft/MIN
Liters/Min

You may also see another acronym of "RMV",
Respiratory Minute Volume. This is a surface air usage base on volume
not pressure. You get values with units such as:
- Cubic Feet/Min
- Litres/Min

One thing to keep in mind when using these consumption numbers is that
pressure/min values such as PSI/min cannot be compared when
the tanks are not the same. i.e. 25 PSI/Min on a AL80 is not the same as 25 PSI on a steel 100. This can even be the case for different 80 cubic ft tanks!
That is why for comparison purposes, it is best to use consumption
numbers that are based on volume such as RMV values.
When using RMV values, you can compare values regardless of
tank size or pressure rating.

--- bill

 

[Fastmarc]

Hi. Just a couple of questions.
In 'theskull' formula, what unit would the SAC rates be?
Also, what formula would you use to calculate your average depth?

 

[bradshsi]

theskull formula gives SAC in cubic ft/min

To find your average depth you can add all your depths x the time at that depth and then divide the result by your total time.

For example you dive to 80 ft for 10 minutes, ascend to 60 ft and remain there for 30 minutes, then ascend to 15 ft for 5 minutes before surfacing (total dive time is 45 minutes).

The sum of all depths x times is then = (80 x 10) + (60 x 30) + (15 x 5) = 2645

Average depth = 2645 / 45 = 58.8 ft

Alternatively buy a dive computer that calculates average depth for your dive.

 

[theskull]

theskull formula gives SAC in cubic ft/min

To find your average depth you can add all your depths x the time at that depth and then divide the result by your total time.

For example you dive to 80 ft for 10 minutes, ascend to 60 ft and remain there for 30 minutes, then ascend to 15 ft for 5 minutes before surfacing (total dive time is 45 minutes).

The sum of all depths x times is then = (80 x 10) + (60 x 30) + (15 x 5) = 2645

Average depth = 2645 / 45 = 58.8 ft

Alternatively buy a dive computer that calculates average depth for your dive.

Buy the computer, fer sure fer sure. At the end of the dive it will give you time and average depth (and, if air integrated, the number of psi you used as well!)

Or descend to a depth (33 ft. makes it really easy), take a time and psi reading, dive at that depth for a set amount of time and take an ending time and psi reading. This will get you close enough, but not as good an average as one figured over the entire dive, since a complete dive with ascents and descents will also include your consumption of air in your BC to correct for buoyancy at depth and any air you may waste by being excited during the initial descent (where many people pant and consume a LOT of air).

theskull

 

[meekal]

theskull formula gives SAC in cubic ft/min
Alternatively buy a dive computer that calculates average depth for your dive.

if your SAC is for your personal fullfillment (read: recreational diving) let the 'puter do the work for ya:

from my SUUNTO HELP FILE:
===================================================
SAC calculation method
To calculate Surface Air Consumption (SAC), Suunto Dive Manager first determines the average depth of the dive. It is the average of all depth reading shown on the depth graph in the dive log entry.

It next determines the total amount of air used on the dive. Dive Manager uses information you provide about cylinder size and about pressure gauge readings. Enter this information on the Air consumption tab in the properties window. Dive Manager then calculates the total volume of air used for the dive.

Dive Manager then determines the volume of gas used per minute and adjusts the result to sea level from the dive's average depth. Finally, it adjusts the result from ambient water temperature to zero degrees Celsius.

The result is a volume of gas per minute at Standard Temperature and Pressure (STP). Because Dive Manager uses volumes at STP, you can directly compare SAC numbers from one dive to another, regardless of depth and water temperature. Dive Manager reports SAC in metric or imperial units. Metric units are Standard Liters per Minute (SLM), and imperial units are Standard Cubic Feet per Minute (SCFM).
===================================================

if for a 'serious' dive (read: cave, deep dive, etc) follow your training.

you can drop by http://piddlefish.servepics.com/scuba/DiveGraphs.html to see my dive graphs. you will immediately see the direct correlation between depth and SAC.