SAC vs RMV, revisited

[Dan]

Are you sure? We use litres per 100 km.

That was a typo & had been corrected by @Storker in post #114. I fix it for good now for future readers.

 

[Ukmc]

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.

Thank you for this post, it helped me to clarify the confusion that I had from reading various sources and some using RMV and SAC interchangeably but with different units.
It took me some searching to understand that DiveLog reports RMV for what I thought was a SAC rate (cu ft/min ) which has been normalized for a size of the tank.
I understand that AL80 has 77.6 cu ft for 3000 psi working pressure.
Recently I used AL100s for the first time. Does it have 97 cu ft volume for a 3300 cu ft pressure? I can not find this info through online searches but if I assume the same 1.03 coefficient...
Thank you.

 

[Colliam7]

Recently I used AL100s for the first time. Does it have 97 cu ft volume for a 3300 cu ft pressure? I can not find this info through online searches but if I assume the same 1.03 coefficient.

The Huron Scuba cylinder chart (SCUBA Cylinder Specifications – Huron Scuba, Snorkel & Adventure Travel Inc. PADI 5 star IDC in Ann Arbor, MI) indicates that a Luxfer AL100 has 99.3 cf, and a Catalina AL100 has 100 cf.

 

[Miyaru]

I'm amused....and convinced that it makes more sense to define a tank by it's (pressure independant) water volume

 

[inquis]

NOAA and Navy would agree with you, but our scuba training agencies maybe not.

I wonder if that's because of difficulty teaching it to the masses without different identifiers? Seeing SAC on both sides of an equation will send a lot of people into a tailspin.

Obviously, a subscript of (p or v) fixes that in educational materials -- and retains the "pronouncibility" of SAC -- but I don't see it taking off. Would be nice if it did, IMO. Units still need to be given to sort out Imperial or metric.

 

[Colliam7]

I'm amused....and convinced that it makes more sense to define a tank by it's (pressure independent) water volume

And, I am (probably, equally) convinced that the parameter of interest is the gas capacity at the working pressure. The capacity is the basis of my dive plans, and cylinder selection. And, as with so many things in diving, we should do what works best for us.

 

[Miyaru]

I wonder if that's because of difficulty teaching it to the masses ...

Very good point. Let's say you have a brand new diver, who comes up with the inevitable question How long does that tank last?
The instructor says: That depends on how much air you breathe per minute.

The brand new diver, still belonging to the masses and unaware of the endless SAC/RMV discussion on the internet, will ponder about a certain volume of air (s)he's breathing per minute. For most divers, that will be in liters per minute, and for a few divers from one of those other 3 countries, it will be something imperial.
Not a single new diver will think in terms of pressure difference per minute. They'll only start doing that after reading topics such as these.

 

[Storker]

And, I am (probably, equally) convinced that the parameter of interest is the gas capacity at the working pressure. The capacity is the basis of my dive plans, and cylinder selection. And, as with so many things in diving, we should do what works best for us.

Problem is, for us simple metric brains those tank factors of yours are unnecessarily confusing.

If my tank is, say, 15L 232 bar, I have *pauses for two seconds* about 3500 liters of gas at surface pressure. Let's say I'm a hoover in a drysuit, so I breathe about 20 L (actual liters, not recalculated liters) per minute. At the surface, that gas will last me some 175 minutes. At 10 m/2 ATA, half of that. At 20m/3 ATA, one third of that. At every time during my dive, I can easily calculate my remaining gas ("SPG shows 100 bar. That's uh, 1500 L of gas at surface pressure, 500L down here at 20m. If I stay here, I have just about 25 minutes before I'm empty").

No tank factors, just pressure and water volume. Oh, and Boyle of course.

 

[tursiops]

Not a single new diver will think in terms of pressure difference per minute.

Not sure about that. The only gauge they have on their tank reads in pressure.

 

[inquis]

They'll only start doing that after reading topics such as these.

Yep, I think the relevance to the masses ends there. Considering a pressure rate is only necessary *while* diving to predict remaining time, in a no-stop situation (again, masses) that's less of a concern.

Not to mention an increasing number of them will be using AI computers which jumps right over pressure and shows them remaining time.