Question Definitions for SAC and RMV?

[scubadada]

This topic comes up over and over. Let's see what you think. Please vote once for SAC and once for RMV or vote for #5 and elaborate in your post or #6 if you don't know what this is all about.

 

[tursiops]

I only voted for 4, because the V in RMV means Volume, just as the S in SAC means Surface. All else is arguable...although if there were agreement on RMV then that leaves SAC for pressure/time.

 

[inquis]

Am I answering what I feel *should* be the case or what I think the prevailing understanding is in the community? I'd rather SAC be whatever units I tack on the end (pressure or volume based). It's simply two ways to view consumption. However, I voted that SAC is pressure-based and RMV is volume based because I believe that distinction is made by the majority of divers.

 

[scubadada]

Am I answering what I feel *should* be the case or what I think the prevailing understanding is in the community?

What you think and use is what I meant, thanks

 

[tbone1004]

I firmly believe they should be used interchangeably. Anyone who votes for pressure though on RMV is 100% incorrect, so if you're going to vote for that just understand that the definition of RMV is not up for debate and is strictly a volumetric measurement, though since it is a medical term the use of cubic ft would never be used and IMO it should only be LPM, but that's probably the engineer and stickler for units in me.

The conundrum with my opinion is that Shearwater has essentially cemented the use of SAC to dictate psi/min/atm and the term RMV for cfm/lpm then there is little point in continuing to argue over semantics.

 

[drrich2]

My understanding is that SAC (surface air consumption) represents the volume gas (cf3) used per unit time (minute) adjusted for its surface equivalent (which you achieved by dividing by atmospheres).

So, a few things come from that:

1.) It's cylinder independent, particularly to U.S. divers accustomed to following their gas volume using PSI. I can see where someone accustomed to using the tank's physical volume in liters might think about it a bit differently.

2.) The SAC is a constant in that it is independent of depth; a diver going through the same tank volume in 60 minutes at the surface (1 atm pressure) or 30 minutes at 33 feet deep (2 atm pressure) has the same SAC rate. This makes SAC depth-independent as a value, so we can compare SAC rates across different dives for a diver, or the same dive between divers.

This is important, because if you and I were to do a demanding dive together, it'd be useful to know who has the higher SAC (it'd be me), because in terms of gas supply (if we had the same size tanks), I'd be the 'rate limiting diver' for duration.

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Without going to look it up, and with the stipulation that I think SAC and RMV are sometimes used interchangeably, here is what I think RMV 'should' be:

RMV, Respiratory Minute Volume, is the amount of gas (cf3) consumed per unit time (minute), with no adjustment made for depth. So, all other things being equal, your RMV at 33-feet deep is double your RMV at the surface.

So SAC is RMV adjusted for ambient pressure (as due to depth), and a more useful metric in making comparisons topside. RMV is good to have in mind when you're at 100-feet with an 80-cf tank on your back, and need to be mindful it won't last long at that depth.

 

[scubadada]

I firmly believe they should be used interchangeably. Anyone who votes for pressure though on RMV is 100% incorrect, so if you're going to vote for that just understand that the definition of RMV is not up for debate and is strictly a volumetric measurement, though since it is a medical term the use of cubic ft would never be used and IMO it should only be LPM, but that's probably the engineer and stickler for units in me.

The conundrum with my opinion is that Shearwater has essentially cemented the use of SAC to dictate psi/min/atm and the term RMV for cfm/lpm then there is little point in continuing to argue over semantics.

American Underwater Products/Oceanic... has used the same definitions for a long time, before 2010, when I started downloading my VT3. Shearwater did not make it up.

 

[tbone1004]

@drrich2 DAC is more appropriate for depth consumption and is a commonly used term.

RMV is an actual medical definition and the device used to measure it uses vanes and actually directly measures the volumetric flow of gas past the vane. Since RMV is directly measuring or calculating volume as a function of respiratory volume and respiratory rate it would not be appropriate to use RMV as depth air consumption since we are not measuring volumetric consumption but are measuring pressure consumption

@scubadada I'm not saying they made it up, just said they cemented it when they started using it in their manuals. This is not discounting AUP's use but the prevalence of transmitters in recreational diving is considerably higher now than it ever has been and IMO Shearwaters use of the terms when compounded with TDI's use of the terms have certainly cemented them.

 

[scubadada]

Hi @tbone1004

AUP/PPS/Oceanic... has marketed the MH8A transmitter since 1997, there are many more out there than Shearwater is responsible for. Now, current brand importance/recognition, you got it.

 

[Lorenzoid]

RMV is not a term I learned from any diving course or reading materials; I do see SB posts using the term. I use the terms Surface Consumption Rate (SCR) and Depth Consumption Rate (DCR), which are in units of volume per minute. I believe SCR and SAC have the same meaning.