SAC Rates

[The Kraken]

NW,
Not to belabor the point, but doesn't the SAC rate (Surface Air Consumption) express in cubic feet per minute the volume of air used at the surface?

Pressure is not a measure of volume, it's a measure of pressure. One does not consume 16 psi of air at the surface - one would consume 1 cubic foot of air at an ambient pressure of 16 psi.

When breathing down a tank, the diver consumes a volume of gas (cubic feet) resulting in a measurable difference in pressure.

 

[MXGratefulDiver]

NW,
Not to belabor the point, but doesn't the SAC rate (Surface Air Consumption) express in cubic feet per minute the volume of air used at the surface?

Pressure is not a measure of volume, it's a measure of pressure. One does not consume 16 psi of air at the surface - one would consume 1 cubic foot of air at an ambient pressure of 16 psi.

When breathing down a tank, the diver consumes a volume of gas (cubic feet) resulting in a measurable difference in pressure.

Pressure is not a measure of volume ... but pressure "differential" over a measured period of time is. That is what I stated.

The reason this is important is that when you are diving, the only information you have to tell you how quickly you are consuming the air in your cylinder, and what amount of air is remaining, is your pressure gauge. So knowing the pressure differential (i.e. the rate at which that pressure gauge is going to decrease) helps you determine things like turn pressure and rock bottom. This is why knowing your SAC is important. In other words, during a dive you are not reading the cubic feet of air remaining in your cylinder, you are reading the pressure, in PSI, of air remaining.

And keep in mind that SAC is based on a specific cylinder, and changes with cylinders of different sizes and working pressures.

Knowing RMV lets you determine, based on your dive plan, whether you have enough gas for the dive you're planning to do ... since cylinders are rated according to the amount of cubic feet of gas they will hold. Also, knowing your RMV ... and the size and working pressure of your cylinder ... lets you determine your SAC for a given cylinder - even if you initially determined it on a cylinder of a different working pressure or size.

Both of these numbers are important gas management tools, even though they are equivalent values expressed in different units. They are useful for different aspects of your dive planning and execution.

... Bob (Grateful Diver)

 

[The Kraken]

NW,
I guess what is confusing me is this section of you post . . .

"To be accurate ... SAC is the amount you breathe ... in PSI (pressure differential) ... in one minute on the surface. It's specific to the cylinder you're breathing from.

RMV is the equivalent value, expressed in cubic feet or liters per minute (volume) ... in one minute on the surface. It's independent of the cylinder you're breathing from."

You're saying exactly the same thing but giving it two different names. But with respect to the RMV, the amount of gas you breathe from a cylinder is dependent upon the cylinder because you can't consume 120 cubic feet of gas from an 80 cubic foot tank.

SAC rate is not expressed as say "32 psi per minute", it's expressed as cubic feet of air per minute, just as you describe in your definition of "RMV". All of the people in the world breathing at the surface don't change the ambient of air pressure, it remains relatively constant (for all intents and purposes that we discuss here).

The SAC rate determines how many minutes you can dive a tank of a given volume at a specific ATA. For example, a diver has a SAC rate of .4. The diver consumes .4 cubic feet of air at the surface. At 66 feet (3 ATA) the diver still has a SAC of .4, but will consume 1.2 cubic feet of gas. So based on his SAC rate the diver, allowing a reserve of 500 psi, can dive an aluminum 80 (3000 psi working pressure at full fill of 77.4 cubic feet) for approximately 54 minutes.

All things being equal and in the best of all possible worlds, a diver's SAC rate wouldn't change, just the depth at which s/he is diving.

The diver's SAC rate doesn't change at depth, but the volume of gas consumed does because of the increase in ATA.

I think all that's happening here is an attempt to give a description or name to the process by which a diver creates a correlation between the change in tank pressure to the volume of gas consumed at a specific depth.

 

[MXGratefulDiver]

NW,
You're saying exactly the same thing but giving it two different names.

Correct ... but knowing both values is useful for different aspects of dive planning and execution, for reasons I've already explained.

But with respect to the RMV, the amount of gas you breathe from a cylinder is dependent upon the cylinder because you can't consume 120 cubic feet of gas from an 80 cubic foot tank.

Incorrect ... and I've already explained why. In fact, SAC is dependent on the cylinder you breathe, because your baseline (cubic feet per psi of pressure) depends on the volume and working pressure of your cylinder. RMV is independent of the cylinder you breathe, because it is a measure of volume, and the volume remains the same regardless of the size and working pressure of the cylinder.

If you want Kraken, I have a multi-page writeup on gas management that explains it. PM me if you'd like to see the whole thing, and I'll e-mail it to you.

SAC rate is not expressed as say "32 psi per minute",

Actually, it is.

it's expressed as cubic feet of air per minute, just as you describe in your definition of "RMV".

No ... cubic feet per minute is RMV.

All of the people in the world breathing at the surface don't change the ambient of air pressure, it remains relatively constant (for all intents and purposes that we discuss here).

The SAC rate determines how many minutes you can dive a tank of a given volume at a specific ATA. For example, a diver has a SAC rate of .4. The diver consumes .4 cubic feet of air at the surface. At 66 feet (3 ATA) the diver still has a SAC of .4, but will consume 1.2 cubic feet of gas. So based on his SAC rate the diver, allowing a reserve of 500 psi, can dive an aluminum 80 (3000 psi working pressure at full fill of 77.4 cubic feet) for approximately 54 minutes.

All things being equal and in the best of all possible worlds, a diver's SAC rate wouldn't change, just the depth at which s/he is diving.

The diver's SAC rate doesn't change at depth, but the volume of gas consumed does because of the increase in ATA.

I think all that's happening here is an attempt to give a description or name to the process by which a diver creates a correlation between the change in tank pressure to the volume of gas consumed at a specific depth.

You are misunderstanding some very basic principles of gas management ... which is understandable seeing's how the major agencies don't teach this stuff.

Perhaps to understand the importance of making a distinction between SAC and RMV you will consider the following example.

You are planning a dive ... you will be using an LP95 with a working pressure of 2640 psi, and have a starting pressure in the cylinder of 2,500 psi. You have an RMV of 0.5 cubic feet per minute.

Your dive is to surface swim from shore to a buoy and drop down to a depth of 30 fsw. You'll follow the contour straight out to a depth of 55 fsw where you'll run into a walll. Assume that you timed it perfectly and there's no current. You'll follow the wall at an average depth of 60 fsw for 10 minutes, then you'll ascend to a depth of 35 fsw and follow the contours back till you reach a boat where you know there's an octopus den ... approximately 6 minutes. You'll hang around the boat for about 5 minutes, then do a slow diagonal toward your entry point till you reach safety stop depth ... approximately 3 minutes. You'll swim along at 15 feet for your 3 minute safety stop, then turn directly toward shore. About a minute later you'll stand up and exit the water.

- How much gas will you need to do this dive?
- What pressure will be in your cylinder when you turn the dive at the top of the wall?
- What pressure will be in the cylinder at the end of the dive?

In order to answer those questions, you will need to know both your SAC ... in psi per minute, and RMV ... in cubic feet per minute. You will also need to know the baseline of your cylinder.

BTW - this is very similar to a question my AOW students get on their exam. It involves nothing more than understanding the basic concepts of SAC, RMV, and baseline ... and some 8th grade arithmetic.

... Bob (Grateful Diver)

 

[The Kraken]

OK, I see where you're going with this.
You're looking at the differences in indicated pressure drop per minute based upon the current depth at which you're diving . . .

But still the Surface Air Consumption rate is expressed as the volume of air that one breathes at the surface in the period of one minute right? And volume is expressed in cubic feet, right?

 

[Rick Murchison]

I wouldn't get too wrapped around the axle over whether you choose to express SAC in CF/min or PSI/min. Bob is correct that the generally accepted expression is in PSI/min, as that's usually the way it's introduced in OW training.
"RMV," however, is borrowed from the medical community, where it is generally expressed in Liters/min, which in Scuba in the States isn't particularly useful, and so we in Scuba usually use CF/m. Still, there's room for confusion.
Personally, I prefer the term SCR (Surface Consumption Rate) - a purely Scuba term, with the generally accepted expression in CF/min.
Rick

 

[MXGratefulDiver]

That's correct, Rick ... which is why I said in my initial post that it's really a nit.

The salient point being that someone who wants to plan their gas usage for a given dive needs to understand the relationship between the psi reading on their gauge and the volume of gas in their cylinder. Since the only thing you have to go on while diving is the pressure reading on your gauge, knowing your consumption rate in CFM won't do you a lot of good when relating your air consumption to what your pressure gauge is telling you. You also need to know how that gauge reading relates to actual volume of gas in your cylinder at any given time.

As I tell my students, don't get too wrapped up in the terminology ... but there is a direct correlation between CFM and psi ... and that relationship depends on the size and working pressure of your cylinder. Using separate terminology to show the equivalence in that relationship makes gas planning and management much easier to teach.

... Bob (Grateful Diver)

 

[The Kraken]

NW,
I am in complete accordance !!!

"knowing your consumption rate in CFM won't do you a lot of good when relating your air consumption to what your pressure gauge is telling you"

Please forgive my misunderstanding. I'm a bit anal about descriptives . . .
It's not that I don't understand the physics, I was just having a little bit of a problem relating a measure of force in an application where a measure of volume is the norm.

But, yes, knowing the relationship of tank pressure to volume is of paramount importance in diving.

. . . sorry!! :bonk:

the K

 

[Uncle Pug]

The salient point being that someone who wants to plan their gas usage for a given dive needs to understand the relationship between the psi reading on their gauge and the volume of gas in their cylinder. Since the only thing you have to go on while diving is the pressure reading on your gauge, knowing your consumption rate in CFM won't do you a lot of good when relating your air consumption to what your pressure gauge is telling you. You also need to know how that gauge reading relates to actual volume of gas in your cylinder at any given time.

You should know how many cf each 100 psi represents for the cylinder you are diving... and if you would dive the same size cylinders all the time you wouldn't have the extra confusion. Standardize!

Now in my case I know that for each 100 psi in my 104 I have 4cf and each 50 psi is 2cf. I don't pick nits smaller than that. I also know that figuring .5cf/min SAC (my post so I can use whatever term I like ) gives me margin since my SAC is a bit lower than that.

It is a piece of cake for me to figure on the fly how many minutes I have left at whatever depth and how many psi I need for the time I want to spend above 2ATA.

 

[JustinW]

Just because the 102s have a 3500 psi capacity you can't use it for the starting pressure unless the tank was filled to capacity. Psi used is SPG starting pressure minus SPG ending pressure. If you started with 3000 psi (a typical fill pressure around here) you are about 14.43 cu ft short of a max fill. This being the case your SAC would be even lower.

OK, at first this confused me, but now I don't think it has. OK, I computed my SAC rate based upon the PSI that i used, I just used the total capacity to compute how many PSI/cubic foot the tank holds. So even if I started with a tank that was only half full (say 1600) and still breathed down my 1071 psi, that wouldn't make any difference compared to if the tank was full.

my computer gives me differential pressure, and I used that as the data for gas used. I still did this right didn't I?