SAC vs RMV, revisited

[tursiops]

ScubaBoard and the diving literature is full of references to these two terms, without any agreement as to what they mean.

One ScubaBoard thread from a few years ago that tried to sort this out, unsuccessfully, is
SAC vs RMV - What is standard?.

The problem is there is no authority as to what these terms mean. The NOAA Dive manual, the Navy Dive Manual, PADI, TDI, all use these terms differently, or not at all.

For sure, without any argument, the way the calculation is done is in three parts, with a possible fourth part:
(1) How much gas has been used (start pressure - end pressure=P); this can be in psi or bar.
(2) How long did that take? Now you can calculate pressure-drop/minute (P/min) averaged over the dive.
(3) What was the average depth (D) of the dive? This allows the varying-depth dive to be translated to the surface, thus giving a surface air consumption (SAC) or surface consumption rate (SCR), depending on your agency/country/training/references. The translation is:
SAC =psi/min / (D/33 + 1) (imperial)
SAC = bar/min / (D/10 + 1) (metric)
The SAC (pressure/min) is totally dependent on the size tank you are using, so SAC is valuable only if you keep using the same size tank. Most folks want to transcribe their SAC to something that doesn't depend on the tank size. This is sometimes called Respiratory Minute Volume (RMV) or sometimes it is still called SAC ("based on volume") as opposed to what we calculated previously which was "based on pressure."

The relationship between SAC (pressure/min) and RMV (volume/min)) is:
RMV (volume/minute) = SAC (pressure/min) * k (volume/pressure).

The factor k varies by what tank you have.
For an AL80, which actually holds (imperial) 77.4 cuft at 3000 psi, the factor k = 77.4/3000 = 0.026 (cuft/psi). Multiply your AL 80 SAC (for example, 20 psi/minute) by k and get RMV = 20 * 0.026 = 0.52 cuft/min.
Metric is easier. If your SAC is 1.4 bar/minute, k for the tank is (say) 11 liters/bar, so we get RMV = 1.4 * 11 = 15.4 liters/min.

Note that because the SAC has been "brought to the surface" that the RMV is also a surface value.

Be careful just plucking formulaes out of publications and web pages and using them; they may not be telling you want you want to know. Be sure that you distinguish between pressure/minute (dependent on the tank) and volume per minute (independent of the tank). Be sure your values have been translated to the surface, so depth is not a factor. And be sure it is YOUR tank (from which you got your SAC) that you are calculating the k-factor for, so you can get the RMV.

Why bring this up again? Shearwater now provides SAC in psi/minute or bar/minute as one of the things it displays. See page 52 or the Teric manual, for example. Naïve users may not appreciate how both useful and possibly confusing this is. It is also possible that Shearwater may actually be helping stabilize the definitions in use! That alone would be nice.

 

[Diver below 83]

Thanks for sharing. Always helpful to have new info.

 

[ChuckP]

deleted - tired eyes

 

[hammet]

Caution.... Shearwater has confirmed there is an issue with the SAC rate code that causes it to display incorrect values. A fix will be available with the next update.

Teric SAC rate confusion

I agree it is extremely valuable in conserving air as you. I've gotten my average rmv down from .5 to about .4 just by glancing at my wrist every once in a while.

 

[saxman242]

What is the basis for expressing RMV as a surface value? RMV, outside of diving, is essentially the actual air consumed (tidal volume time frequency basically) and the SAC rate should be the surface corrected value, hence it's name, no?

 

[tursiops]

What is the basis for expressing RMV as a surface value? RMV, outside of diving, is essentially the actual air consumed (tidal volume time frequency basically) and the SAC rate should be the surface corrected value, hence it's name, no?

This is part of the evolution in shifting definitions. You need to be clear as to whether you are expressing RMV at depth or at the surface. Just as you need to be clear as to whether you are expressing SAC in pressure or volume units, and whether you are giving your numbers in metric or imperial units. If I say my SAC is 20, what do I mean? 20 psi/min, or 20 liters/min? Since there is no agreement on these terms, we have to say the units along with the numbers.

 

[kotik]

What is the basis for expressing RMV as a surface value? RMV, outside of diving, is essentially the actual air consumed (tidal volume time frequency basically) and the SAC rate should be the surface corrected value, hence it's name, no?

If you use RMV for dive planning, your surface value is the most sensible number to keep track of. Knowing that, you can convert it for any depth.

For example: say your RMV at the surface is .5 cubic feet / minute. If you dive at an average of 33 feet, the ambient pressure is double that of the surface, so gas usage doubles too. You would then consume 1 cubic feet per minute.

Depending on tank size, that can be converted to SAC. Say you have an aluminum 100, which holds 100 cubic feet at 3000 psi. On the surface you would breathe that down in 200 minutes. So your SAC with that particular tank is 3000 psi/200 minutes = 15 psi per minute. On a dive at 33 feet average depth, tank pressure would drop 30 psi every minute and would be totally depleted in 100 minutes.

 

[scubadada]

Hi @tursiops

Exactly how I use the terms ^^

 

[kotik]

Hi @tursiops

Exactly how I use the terms ^^

It's how I use them too, but I can't remember the original source of which definition I decided to accept for SAC.

RMV is easy, it has a clear definition in the US Navy Dive Manual, and it has "volume" right there in the term.

The only book I find offhand that defines SAC though is a TDI manual, and it gives the volume/time usage, not pressure/time.

It's understandable that computer manufacturers choose the usage discussed here. They have to call it something, and SAC is a recognized term that is "sometimes" used to mean pressure/time, which is what the computer can monitor.

 

[saxman242]

If you use RMV for dive planning, your surface value is the most sensible number to keep track of. Knowing that, you can convert it for any depth.

For example: say your RMV at the surface is .5 cubic feet / minute. If you dive at an average of 33 feet, the ambient pressure is double that of the surface, so gas usage doubles too. You would then consume 1 cubic feet per minute.

Depending on tank size, that can be converted to SAC. Say you have an aluminum 100, which holds 100 cubic feet at 3000 psi. On the surface you would breathe that down in 200 minutes. So your SAC with that particular tank is 3000 psi/200 minutes = 15 psi per minute. On a dive at 33 feet average depth, tank pressure would drop 30 psi every minute and would be totally depleted in 100 minutes.

I 100% agree that surface corrected values are needed for dive planning, and that one must differentiate between actual air consumption at depth and if the value is brought to the surface. I just find the movement of RMV by default to be a surface corrected value bizarre. RMV is a measured value outside of the diving world meant to measure actual air interchange. It seems like it would make a lot more sense to use this value as it is used elsewhere, for actual air consumption, and use SAC as the surface corrected value. Obviously I'm not going to change the way the values are used in the diving world, so it's a moot point.