Presumably MacDive asks you what tank you are using?
Yep. This is from the mobile app.
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Presumably MacDive asks you what tank you are using?
Thanks, I've been lucky to have pretty decent air consumption from the start, and it has only gotten better.
Obviously you’re forming gills since you’re apparently hardly breathing!
To be fair, I'm moving around a lot less weight than you do. Warm water in the Great Lakes is the dead of winter here. This weekend will be the first time I've worn my shortie locally since like November.
Hah! The dive iI posted above had a bottom temp of 43F and I had on approx 100lbs of gear (doubles, lead, regs, drysuit with thick undies, etc). That 0.54/cft min was the lowest mine has ever been. It was a very relaxed dive.
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?
What is the basis for expressing RMV as a surface value?
RMV is the result of a medical test that requires the patient to be at rest. I want a real world calculation that is based on moderate u/w work.
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.