Question Definitions for SAC and RMV?

[Lorenzoid]

I really wish GUE did not invent these new terms. It wasn't necessary.

I agree it wasn't "necessary." Nevertheless, this thread shows how much confusion surrounds the older terminology, so I can understand their motivation. Start afresh, clearly define the terms, and don't worry about what terms others may use (and confuse). There's something to be said for that approach.

 

[scubadada]

I'm glad I posted this thread. I was falsely under the impression that most divers use the same definitions for SAC and RMV that I do. I'm surprised at the diversity in the use of these terms.

As long as you know your surface gas volume consumption/time, it doesn't matter if you call it SAC, RMV, SCR, or something else, you have the ability to calculate your gas use on a dive. It's the units, not the name.

I'll continue to call it RMV Average Gas Consumption

 

[Divin'Papaw]

I'm glad I posted this thread. I was falsely under the impression that most divers use the same definitions for SAC and RMV that I do. I'm surprised at the diversity in the use of these terms.

As long as you know your surface gas volume consumption/time, it doesn't matter if you call it SAC, RMV, SCR, or something else, you have the ability to calculate your gas use on a dive. It's the units, not the name.

I'll continue to call it RMV Average Gas Consumption

I too find it interesting. When I learned the terms, it was driven home by my instructors that SAC is tank dependent, and you must apply the tank factor to get the tank independent RMV. That is how I use the terms to this day.

 

[Manatee Diver]

SAC is your consumption adjusted to surface pressure. It can be pressure or it can be a volume unit; like in science classes you need to give your units (cuft/min or psi/min or L/min or bar/min).

RMV is a medical term that has a similar meaning and units that some divers are attempting to co-opt as their own instead of trying to get divers to agree to a new term because divers are too lazy to give their units. If we need a new term to separate pressure and volume SAC rates, make up a new term don't create confusion by using an established medical term.

 

[Cheizz]

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

 

[scubadada]

SAC is your consumption adjusted to surface pressure. It can be pressure or it can be a volume unit; like in science classes you need to give your units (cuft/min or psi/min or L/min or bar/min).

RMV is a medical term that has a similar meaning and units that some divers are attempting to co-opt as their own instead of trying to get divers to agree to a new term because divers are too lazy to give their units. If we need a new term to separate pressure and volume SAC rates, make up a new term don't create confusion by using an established medical term.

Hi @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?

 

[L13]

I find it gratifying to discover that almost everyone is using a term that is consumption/time/ATA. The use of a non-ATA compensated number seems to have disappeared! I tend to advocate for understanding others perspectives, but plan to never mention the non-ATA compensated versions in the future now that it has virtually disappeared. I don't want to be the one keeping it alive!

The pressure vs. volume issue is easily dealt with by stating units, and I am fine with RMV being redundant with one of the SAC definitions.

 

[Lorenzoid]

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.

 

[drrich2]

RMV is a medical term that has a similar meaning and units that some divers are attempting to co-opt as their own instead of trying to get divers to agree to a new term because divers are too lazy to give their units. If we need a new term to separate pressure and volume SAC rates, make up a new term don't create confusion by using an established medical term.

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.

Just now I opened MacDive on my desktop computer to see what it said for SAC, and was a bit surprised to see RMV instead showing up on the opening dive listing display. Went to upper left screen corner, MacDive, down to Settings, went in and found out both are offered, enabled SAC also, and I see for example on one dive SAC 19.78 and RMV 0.60. Units are PSI/Minute and CF/Minute, respectively.

So that's how a popular Macintosh and iOS-based dive logging software program is handling this.

From other's posts:

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.

Yes, Oceanic has used SAC as pressure/min and RMV as volume/min for a long time...long predating Shearwater's use of AI and their definitions....which are the same. it seems to be the Europeans who are the holdouts....

Does anyone know of any currently marketed dive computers or dive log software that is using the terms SAC and/or RMV in a different way?

Put another way, does the diversity in terms use Scubadada describes here:

I was falsely under the impression that most divers use the same definitions for SAC and RMV that I do. I'm surprised at the diversity in the use of these terms.

exist amongst dive computers and dive log software?

 

[Cheizz]

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.

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).