meaning of SAC?

[Diveral]

Surface Consumption Rate (SCR) is measured in PSI/min and is basically the same thing as SAC but measured in different units.

 

[Cave Diver]

You get your SAC rate by swimming a known distance, at say, 40 feet or so. You take your guage pressure, swim as you normally would something like 50 meters, and that tells you how much air you consumed under "normal" conditions..

I was always taught to get SAC rate by swimming at a prescribed depth for a given time, such as 10 minutes @ 30'

SAC = Gas Consumed/(Depth in ATA x Time)

ATA = (FSW/33)+1
ATA = (30/33)+1 = 1.91

Assuming 700 psi used then SAC = 700/(1.91 x 10) = 36.6psig per minute used.

Respitory Minute Volume (RMV) represents the volume of gas consumed rather then the psi.

RMV = SAC/(Cylinder Working Pressure/Cylinder Volume)

A diver using 700 psi on an AL80 (CWP= 3000psi, CV = 80cft) would have an RMV of:

RMV= 36.6/(3000/80)
RMV= 36.6/(37.5)
RMV= .976 cubic feet per minute

Once you know your SAC for a given cylinder and your RMV for a given cylinder, you can use the following formula to calculate your SAC rate for any given cylinder.

SAC = (CWP/CV) x RMV

Example: A diver with an RMV of .976 wishes to use an AL100cft cylinder

SAC = (3000/100) x .976 = 29.28

Since the SAC rate is based on cylinder volume, knowing your RMV is important for gas management planning when using different cylinders!

Comparing SAC rates is useless if divers are using different size cylinders.

 

[The Kraken]

Since the SAC rate changes based on cylinder volume, knowing your RMV is important for gas management planning when using different cylinders!

With all due respect, the SAC rate does not change based upon cylinder volume. All things being equal the only thing that changes is the amount of time one has based upon the volume of the tank.

If one has a SAC rate of 1 cubic foot per minute and is sitting at the surface breathing down a tank, then it would take the person 80 minutes to breathe down an 80 cubic foot tank and 120 minutes to breathe down a 120 cubic foot tank. The diver's SAC rate hasn't changed, merely the amount of available gas to be consumed.

As the diver reaches greater depths, all things being equal, his SAC rate won't change, but the volume of gas that is used per minute does, but that is because of the increased pressure at that depth. So if a diver has a SAC rate of 1.0 at the surface he will consume 5 cf of gas at a depth of 5 ATA but his SAC rate is still 1.0.

SAC rate changes only with the diver's exertion/anxiety levels.

A SAC rate is best determined by the average depth of the dive because a dive isn't made at a constant depth. Now, given the majority of the dive may be made at a specific depth, one has to descend to and ascend from that depth. These depths, too, should be included in the computation of your SAC rate.

the K

 

[Cave Diver]

Since the SAC rate changes based on cylinder volume, knowing your RMV is important for gas management planning when using different cylinders!

With all due respect, the SAC rate does not change based upon cylinder volume. All things being equal the only thing that changes is the amount of time one has based upon the volume of the tank.

Post edited for the use of the word "change." That was an inaccurate portrayal of the point I was making.

 

[Notso_Ken]

Since the SAC rate changes based on cylinder volume, knowing your RMV is important for gas management planning when using different cylinders!

With all due respect, the SAC rate does not change based upon cylinder volume. All things being equal the only thing that changes is the amount of time one has based upon the volume of the tank.


the K

If you will notice in the original message, the writer learned SAC and RMV the same way I did. SAC is in units of pressure, RMV is the corresponding measure in units of volume. Therefore, in that reference system, yes, SAC DOES change depending on cylinder volume. My SAC rate is different if I'm using an AL80 rather than some other size tank. My RMV is the same regardless of tank size.

Ken

 

[Daylonious]

I was always taught to get SAC rate by swimming at a prescribed depth for a given time, such as 10 minutes @ 30'

SAC = Gas Consumed/(Depth in ATA x Time)

ATA = (FSW/33)+1
ATA = (30/33)+1 = 1.91

You're right - I left out that part. It's start at a constant depth, swim for 5 min, then note pressure used using a normal swim kick.

See, TOLDYA someone would explain it better than me.. lol

D.

 

[The Kraken]

If you will notice in the original message, the writer learned SAC and RMV the same way I did. SAC is in units of pressure, RMV is the corresponding measure in units of volume. Therefore, in that reference system, yes, SAC DOES change depending on cylinder volume. My SAC rate is different if I'm using an AL80 rather than some other size tank. My RMV is the same regardless of tank size.

Ken

If you were to go to a respiratory/cardio specialist it would be very possible that they do some tests to determine the condition of your respiratory system.

One of the first tests done is to determine one's "Surface Air Consumption" rate. This is determined by having the subject breathe through a hose that measures the volume of air consumed per minute, not the pressure.

Pressure is a measure of force. Cubic feet and liters are measures of volume.

Think about it, if you take a device, at the surface, that simply measures the amount of air you have consumed in one minute, how do you determine how many psi of pressure you have reduced in the earth's atmosphere?

Oh well, a rose by any other name . . .

the K

 

[Cave Diver]

Think about it, if you take a device, at the surface, that simply measures the amount of air you have consumed in one minute, how do you determine how many psi of pressure you have reduced in the earth's atmosphere?

Oh well, a rose by any other name . . .

the K

If you breathe off of a SCUBA cylinder, how do you measure the amount of air you consume? You do so by noting how many psi it drops. Then you convert that amount of psi to volume based on the known capacity of the cylinder you're breathing off.

Determining the number of PSI that you use on the surface vs. depth is the SAC.

When you convert it to volume, its RMV.

Please explain how you figure out the amount of air that you have used out of your SCUBA cylinder in a 10 minute period if you don't measure it in PSI/Bar?

 

[joe rock]

If you breathe off of a SCUBA cylinder, how do you measure the amount of air you consume? You do so by noting how many psi it drops. Then you convert that amount of psi to volume based on the known capacity of the cylinder you're breathing off.

Determining the number of PSI that you use on the surface vs. depth is the SAC.

When you convert it to volume, its RMV.

Please explain how you figure out the amount of air that you have used out of your SCUBA cylinder in a 10 minute period if you don't measure it in PSI/Bar?

Isn't the real issue of this debate whether or not the consumption RATE is dependent on cylinder size?

I have to agree with Kracken, there are LOTS of variables with SAC or RMV. But it's my understanding the size of the tank will determine how MUCH gas you have to consume, but not the RATE of consumption. Or am I missing something very simple? (which isn't hard for me )

JR

 

[The Kraken]

Nope, Joe, you're not missing anything.

the K