SAC rate calculation...

[The Kraken]

DA,
You're much more advanced in this than I, but I would think that SAC (Surface Air Consumption) would deal with volume and not pressure. One consumes in volume, not pressure. That is, if I'm sitting on my sofa, I will breathe 1 cubic foot of air per minute, actually, it's more like .37 cf.

If you were in a water tight compartment in a ship that had been sunk, you would be concerned about the volume of air you consumed, not its pressure.

At the surface, the pressure is a given and we concentrate on how many cubic feet of air we consume per minute. With a tank under pressure, we are concerned about how many cubic feet of air we are going to consume at a given ATA.

One can extrapolate a psi equivalent to a cubic foot of gas at a given depth and somewhat determine how much breathing gas/time is left.

But to me, SAC remains a measure of the volume of air consumed, not the number of pounds of pressure used.

JMO

 

[fmw625]

DA, I didn't realize that there was only 77 cu. ft. of air in an AL80. I just assumed that the 80 meant 80 cu. ft. Looks like it should be called an AL77.
Is the importance of the psi consumed per min. based on what is your rock bottom pressure? ie the pressure which you turn the dive.

 

[wcl]

Hi,

As I was taught, SAC is PSIG(of a tank)/min breathing in 1atm ambient pressure.

The pro of this unit choice and definition is that PSIG of a tank is what you measure and monitor directly on your SPG. Thus, calculations using SAC yield answers directly measurable in the field.

The con is that it is one step derived from the more fundamental underlying process (RMV, the volume of air breathed per minute at 1 atm). SAC, as defined, varies per tank; RMV does not.

You can refine both definitions with other situational conditions (physical activity levels, temperature, etc.), and both vary with experience and physical condition at the moment. These are harder to quantify reliably and repeatably, but should be kept in mind as qualitative adjustment factors when using SAC/RMV calculations.

The beauty of defnitions is that they can be anything you decide to define them to be; the drawback is that everyone needs to agree what that defnition is to use it to communicate.

In all cases where it matters, be explicit in all terms. I always took off points grading physics exams when students failed to specify assumptions, definitions, and units!

Cheers,
Walter

DA,
You're much more advanced in this than I, but I would think that SAC (Surface Air Consumption) would deal with volume and not pressure. One consumes in volume, not pressure. That is, if I'm sitting on my sofa, I will breathe 1 cubic foot of air per minute, actually, it's more like .37 cf.

If you were in a water tight compartment in a ship that had been sunk, you would be concerned about the volume of air you consumed, not its pressure.

At the surface, the pressure is a given and we concentrate on how many cubic feet of air we consume per minute. With a tank under pressure, we are concerned about how many cubic feet of air we are going to consume at a given ATA.

One can extrapolate a psi equivalent to a cubic foot of gas at a given depth and somewhat determine how much breathing gas/time is left.

But to me, SAC remains a measure of the volume of air consumed, not the number of pounds of pressure used.

JMO

 

[fmw625]

Hi,

As I was taught, SAC is PSIG(of a tank)/min breathing in 1atm ambient pressure.

The pro of this unit choice and definition is that PSIG of a tank is what you measure and monitor directly on your SPG. Thus, calculations using SAC yield answers directly measurable in the field.

The con is that it is one step derived from the more fundamental underlying process (RMV, the volume of air breathed per minute at 1 atm). SAC, as defined, varies per tank; RMV does not.

You can refine both definitions with other situational conditions (physical activity levels, temperature, etc.), and both vary with experience and physical condition at the moment. These are harder to quantify reliably and repeatably, but should be kept in mind as qualitative adjustment factors when using SAC/RMV calculations.

The beauty of defnitions is that they can be anything you decide to define them to be; the drawback is that everyone needs to agree what that defnition is to use it to communicate.

In all cases where it matters, be explicit in all terms. I always took off points grading physics exams when students failed to specify assumptions, definitions, and units!

Cheers,
Walter

Walter,

In other words, if I know my RMV (cu.ft./min), I should be able to figure my SAC (PSIG/min) for any volume of tank that I happen to be using while diving? :icon5:

 

[DA Aquamaster]

Pretty much. As long as you know what volume a tank holds at what pressure, you can always figure how many PSI in that tank will equal however many cu ft and vice versa and then use the numbers any way you like. What counts is to stick with a process you are comfortabel with and that produces usable numbers.

In practice, I think many experienced divers who do a lot of gas planning use the reading off the SPG only as a means to determine consumption in cu ft, then work all the rest of the equations in cu ft and finally convert the cu ft remaining back to Psig to note turn pressures, etc.

 

[DA Aquamaster]

DA,
You're much more advanced in this than I, but I would think that SAC (Surface Air Consumption) would deal with volume and not pressure. One consumes in volume, not pressure. That is, if I'm sitting on my sofa, I will breathe 1 cubic foot of air per minute, actually, it's more like .37 cf.

The principle problem with SAC is that it is (traditionally) in Psig and is consequently tank dependent. In the distant past you could buy SAC rate calculators that were essentially circular slide rules where you could enter psi used, depth and time to determine your SAC rate and then subsequently set the SAC rate and input the depth and time to determine the air that would be used. This actually worked really slick in the days when there was only one or 2 types of tanks in common use (AL80's and Steel 72's) and where only one air supply was normally used on a dive.

It does not however work nearly as well when a diver may be using 2 or 3 different types of tanks on a single dive. (backgas, travel gas, deco gas, etc.) As you point out, what we are really concerned with is volume and working in cu ft has the advantage that what constitues a cu ft of gas does not change depending on what type of tank it is in as it does if you are using Psig as a unit of volume.

I still have a couple of SAC rate calculators that I will use to crunch the numbers for my SAC rate when I periodically check my consumption rate, but I immmediately convert this to an RMV.

The last tech course I took was a TDI course and all the gas planning was done in cu ft but the instructor and the book kept referring to air consumption in cu ft as a SAC when he/it would have probably been more correct in calling it an RMV. So obviously the definiton is getting a little blurred as SAC seems to be becoming commonly regarded now as a proper term for surface air consumption in cubic feet.