Question Definitions for SAC and RMV?

[drrich2]

If you're in a group in which people dive different tanks, then you'd need to mention the tank size as well.

It's also less useful for a diver who dives different size tanks. If you use 80-cf AL 3,000 PSI rated fill pressure tanks when you have to, 100-cf AL 3,300 PSI rated fill pressure tanks when you can, and even better prefer 120-cf steel tanks (which could be LP or HP) when available at some destinations (e.g.: Cozumel, MX, Jupiter, FL and Morehead City, NC), a term independent of a given tank volume/rated fill pressure is more useful.

 

[Lorenzoid]

That's why I would use SAC rate as a volume of air per minute, corrected for average depth. That way, tank size does not have to be mentioned (it is there in the calculation of the volume of air).

Right. Terminology should be the product of a need to communicate precisely and efficiently. A term that requires one to elaborate on the tank size doesn't seem efficient to me.

I don't have a term for my consumption rate in units of pressure/minute. If my teammates and I need to discuss that, we just say something like "I estimate I will consume X psi per minute when I reach the planned depth." I can't think of any context in which we would discuss how many psi per minute we consume on the surface.

 

[Manatee Diver]

That's interesting, you use SAC to describe both pressure/time and volume/time by designating the units. When you communicate with others, do you say SAC in X units to make sure you are understood?

I would almost never give it in psi/min, but yes I use SAC for both. Regardless of what you call it you need to give your units, how do I know if you are using imperial or metric?

I understand your thinking, but as they say, the tail does not wag the dog. Even if all here agreed with your assertion, I don't think we collectively have the leadership influence in the recreational dive industry to significantly change how they do this.

I'm not an instructor I can give anyone the finger if I want. And this is a hill I will die upon.

But I think it is stupid to use not just a medical term, but a medical term that use the same units (when in metric) and has a close to similar meaning.

 

[Divin'Papaw]

What would be the context of a conversation in which a diver mentions their SAC in units of pressure/minute? It would only be useful if the tank size is implicit. If you're in a group in which people dive different tanks, then you'd need to mention the tank size as well.

I see users of shearwater AI computers who are not initiated or self-educated in what it means regularly say things like, "my SAC is 15.1" without any idea what that means. I also see the same divers comparing the SAC reported by their shearwaters without realizing that, because they have used various different tanks, they are comparing apples to oranges. I see it all the time on dive boats in South FL.

 

[inquis]

I was falsely under the impression that most divers use the same definitions for SAC and RMV that I do

Other posters have indicated various agencies teach the same definitions you use (SAC for pressure-based, RMV for volume-based). (I think PADI, IANTD and TDI/SDI were all mentioned. Plus the various AI computer manufacturers.) The number of poll respondents using SAC for volume-based is indeed interesting, and many supporting comments seem to be from the UK. I wonder if BSAC teaches the volume-based SAC and everyone intuitively knows to divide by the tank volume if pressure is desired.

 

[Cheizz]

I see users of shearwater AI computers who are not initiated or self-educated in what it means regularly say things like, "my SAC is 15.1" without any idea what that means. I also see the same divers comparing the SAC reported by their shearwaters without realizing that, because they have used various different tanks, they are comparing apples to oranges. I see it all the time on dive boats in South FL.

I assume Shearwater need a tank size as well in order to calculate SAC rate? all the other information they get from your dive profile and transmitter info.

 

[drrich2]

That's why I would use SAC rate as a volume of air per minute, corrected for average depth. That way, tank size does not have to be mentioned (it is there in the calculation of the volume of air).

But SAC rate as it's being used is expected to vary with tank, and RMV is not, from what I understand.

If SAC is PSI/minute corrected for depth, for example, then 500 PSI breathed off a 120-cf 3,442 PSI rated fill pressure HP steel tank and 500 PSI breathed off an 80-cf 3,000 PSI rated AL tank won't be the same amount of air (mass, volume, whatever).

So SAC needs to be understood in terms of the specific type of tank used, and RMV does not. If I understand correctly. By my use is with Imperial units, as common in the U.S.

You said:

This is how I learned to calculate SAC rate in metric...

SAC (l/min) = (BAR used * Tank size in L) / dive duration in minutes / ((average depth in meters / 10) + 1 )

Example:

(150 bar used * 12 L tank) / 60 minute dive / ((15 m avg depth / 10) + 1 )

(150*12)/60/((15/10)+1) = SAC rate of 12 L/min

So when you calculate SAC, you apply pressure and tank volume (so in effect, you calculate a volume/amount of air).

When we (typical U.S. use) use SAC, if I understand correctly, we only use pressure, don't factor in tank volume, and so your SAC and our SAC are different things?

And dive culture is somewhat international. Wow. RMV is sounding better all the time...

 

[Manatee Diver]

I assume Shearwater need a tank size as well in order to calculate SAC rate? all the other information they get from your dive profile and transmitter info.

It doesn't care. Shearwater's SAC rate is only used inside the computer for GTR (gas time remaining) and it limited to just a single transmitter, the tank size isn't going to change during the dive, so a pressure based SAC works fine.

They could add tank size to Shearwater Cloud, but Shearwater isn't trying to compete with Subsurface or other dive logging apps.

 

[L13]

What would be the context of a conversation in which a diver mentions their SAC in units of pressure/minute? It would only be useful if the tank size is implicit. If you're in a group in which people dive different tanks, then you'd need to mention the tank size as well.

SAC in pressure/min is mostly useful to the diver during the dive. For conversations with others, volume/min would be more useful.

But, for some divers, SAC = pressure/min always, because ...tradition...

 

[inquis]

I assume Shearwater need a tank size as well in order to calculate SAC rate?

It's measuring pressure directly, so a pressure-based consumption is easy. They also state it sidesteps potential errors & complexity if tank size were introduced. Plus, a volume-based metric isn't terribly useful during a dive (their focus, of course).