SAC vs RMV, revisited

[Bob DBF]

Which is one of the fundamental flaws with the Imperial unit convention (cu.ft/min).

It isn't so bad when one gets used to it. And before you get into how much better metric is, I already know it is easier in the work where I did use it. My problem is that I have a physical awareness of imperial measurements that I haven't picked up with metric.

Everything was supposed to convert to metric here in the US back when I was a kid, but it never happened. Now we have a mish mash of conflicting measurement that will not be resolved in my lifetime.

 

[BlueTrin]

Wouldn't it be .5 cuft/min/atm which would work out to 2cuft/min at depth?

Edit: I think this is confusing because cu.ft is not a quantity of gas but a volume.

(sorry I was editing while others replied)

 

[Bob DBF]

This is why we will express it as 2 cu.ft/min/atm: when the pressure increases the volume will decrease.

At 2cuft/min/atm, one shouldn't use less volume of gas at depth than one does at the surface. If one uses 2cuft/min at the surface, one will not use less at depth. It's probably time to clean up terms and definitions, so the math will work.

 

[Storker]

Wouldn't it be .5 cuft/min/atm which would work out to 2cuft/min at depth?

In the example the person was breathing 0.5 cu.ft/min but he was at 100ft, which is assumed to be 4 atm.

So the equivalent volume of gas at the surface would be 2 cu.ft/min.

This is the fundamental problem. Volume never is a useful metric for gas amount. Unless you specify pressure, that is. Because then PV=nRT kicks in, and we're all good.

Y'all's propensity for giving gas amounts in cu.ft. with no regard for pressure, assuming implicitly that the cu.ft are at surface pressure does not help. At all.

 

[BlueTrin]

At 2cuft/min/atm, one shouldn't use less volume of gas at depth than one does at the surface. If one uses 2cuft/min at the surface, one will not use less at depth. It's probably time to clean up terms and definitions, so the math will work.

Yea I realised you are right: the problem is that we should use (unit for qty of gas)/min/atm.

You could use moles or weight of gas, or define a new unit which is the quantity of gas at 1 atm in either 1L or 1cu.ft (irrespective of the gas, so you don’t have problems with different mixes) ?

 

[Storker]

You could use moles or weight of gas, or define a new unit which is the quantity of gas at 1 atm in either 1L or 1cu.ft (irrespective of the gas, so you don’t have problems with different mixes) ?

Or, keeping it simple, gas volume at surface pressure? This "at surface pressure" thing is where the Imperial convention fails.

I don't measure my gas consumption in volume per time. I measure my gas consumption in surface volume per time. Noticable difference.

 

[NAM001]

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.

Yes sac and RMV are often interchanged. Where SAC and RMV is the same on the surface. as for the K factor is tnat not what we use the T value for. T beiong in metric how many liters to a bar, or imperial how many psi is a cuft. ,,,,,,,,,,,,,,,Imperial ,,,, AL 80 T=40 for metric the tank volume is the T factor. For the most part I dont care which term they use as they more than often than not push buttons of techies about the specifics rather than the general concept regarding what they are being used for in a post.

 

[BlueTrin]

Or, keeping it simple, gas volume at surface pressure? This "at surface pressure" thing is where the Imperial convention fails.

I don't measure my gas consumption in volume per time. I measure my gas consumption in surface volume per time. Noticable difference.

But do you call it litres as well ?

 

[Steelyeyes]

I'm using a Scubapro G2. I'm guessing it calculates RMV but only once you tell it which sized tank you were diving. The manual is not the most clear on the planet. It would be nice to be able to tell it that Tank 1 is an AL 80 and see the number in real time instead of for bragging rights afterward.

 

[Storker]

But do you call it litres as well ?

As in surface liters, yes. PV = nRT.