Question Definitions for SAC and RMV?

[scubadada]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

 

[L13]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

I suspect most don't use it at all. Especially for those who dive metric, it adds very little if anything.

 

[tursiops]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

I dive Shearwater, who defines SAC as pressure/min.

I suspect most don't use it at all. Especially for those who dive metric, it adds very little if anything.

Pressure per minute is extremely useful to tell you how you are doing during a dive. No calculations needed. The only measurement being made is tank pressure; everything else is derived. If all I know is my personal volume per minute usage, that does not help me when I look at my SPG and see 70 bars, or 1200 psi. I also have to know my tank size and make a calculation to utilize what my SPG is telling me.
The point is, volume/minute is very nice for general dive planning, but pressure/minute is more useful during a dive.

 

[drrich2]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

I don't use it at all. Too much to keep up with. I've got a 130-cf HP steel tank for diving at the local quarry, which I haven't done in years, I use 100-cf AL tanks when I can get them (e.g.: VIP Diving in Bonaire, CocoView Resort in Roatan, some liveaboards like the Indo-Siren), and I've used steel 120's (don't recall HP or LP offhand, probably LP) when available (Jupiter, Florida and North Carolina).

I use...well, RMV apparently...after the dive to get a sense how my gas consumption rate on that dive compares to my usual. I don't use it during the dive to try to regulate my breathing by it, and my dive computer offers estimated gas time remaining (at current depth and usage rate), so I don't have a practical use for it.

 

[tursiops]

my dive computer offers estimated gas time remaining (at current depth and usage rate)

It does this based on pressure/minute; it does not care what your RMV is.

 

[Lorenzoid]

For all of you who define SAC as volume/time/atm, what do you call gas consumption in pressure (psi or bar)/time/atm, or do you not use it at all?

As tursiops said, it is more useful for tracking one's own consumption during a dive, and therefore we don't need a term for it to discuss it with others before or after the dive. Occasionally, we might mention it in the dive briefing, and it would go something like: "When I reach the bottom I expect I will be using X psi per minute" (though more commonly, we talk about "X psi per five minute segment").

 

[scubadada]

...Pressure per minute is extremely useful to tell you how you are doing during a dive. No calculations needed. The only measurement being made is tank pressure; everything else is derived. If all I know is my personal volume per minute usage, that does not help me when I look at my SPG and see 70 bars, or 1200 psi. I also have to know my tank size and make a calculation to utilize what my SPG is telling me.
The point is, volume/minute is very nice for general dive planning, but pressure/minute is more useful during a dive.

I have dived an AI Oceanic VT3 since 2010, it gives a very accurate air time remaining, including safety stops or deco stops. The computer does not give SAC (pressure/time) or RMV (volume/time). Neither SAC nor RMV is in the on board log. Both SAC and RMV are given in the computer download to OceanLog.

I have been diving a Shearwater Teric since 2019. It can display gas time remaining, not including stops, disabled in deco. It can display SAC during the dive. SAC is included in the on board log and is in the download to Shearwater Cloud. RMV must be calculated using tank characteristics. My Teric home screen is configured to display both GTR and SAC.

When diving, like most, I pay most attention to my depth, dive time, and tank pressure. I will occasionally glance at my VT3 ATR. I have learned that the Teric GTR tends to underestimate the dive time remaining near the beginning of the dive but is quite close nearer to the end of the dive. I have not figured out why there is this discrepancy. I rarely look at the SAC, as it does not give me valuable additional information. I know when I am relaxed and breathing at a normal rate and I know when I am breathing hard. I will occasionally look at SAC to see just how much I am breathing with exertion. I almost always dive with an AL80 so I know that my normal rate is about 14 psi/min or 0.36 cu ft/min

I recently posted information on the new Mares Sirius AI computer Mares Sirius, another computer running Buhlmann. It was interesting to see that it allows you to enter the tank characteristics for each of its 5 transmitters so that the computer could display RMV. This information would be no more useful to me than SAC.

 

[scubadada]

As tursiops said, it is more useful for tracking one's own consumption during a dive, and therefore we don't need a term for it to discuss it with others before or after the dive. Occasionally, we might mention it in the dive briefing, and it would go something like: "When I reach the bottom I expect I will be using X psi per minute" (though more commonly, we talk about "X psi per five minute segment").

I believe @Lorenzoid is the one respondent to the poll who voted for different definitions.

He uses surface consumption rate (SCR) in volume/time, that would be the same as SAC or RMV, depending on which you use to designate volume/time.

He also uses depth consumption rate (DCR) which appears to be his SCR compensated for depth, neither SAC nor RMV. This is the value that we all use to calculate our gas consumption at depth during a dive.

 

[Lorenzoid]

Yes, I suppose so. Thanks to that training agency that blazes their own path--existing definitions be damned.

 

[wetb4igetinthewater]

Nearly 2/3 of respondents who voted on the definition of SAC in the thread poll use the same definition.

Well Galileo was condemned for his theory that the sun revolved around the earth, so.....