Question Definitions for SAC and RMV?

[Joneill]

My Shearwater Teric will display real time SAC if you want to see it. I almost always dive rental AL80s and know that my average RMV of 0.36 cu ft/min translates to a SAC of 14 psi/min. I don't glance at my SAC often, most frequently when I am significantly exerting myself. I have seen SACs as high as around twice my average. It's interesting.

The Teric on board log gives you your average SAC for the entire dive, as does the downloaded log. Shearwater currently does not have a mechanism for entering your tank characteristics so that RMV can be automatically calculated. You must calculate the RMV manually by multiplying by the tank factor, 0.258 cu ft/psi for an AL80. I, and others, have suggested to Shearwater that they fix this.

On the other hand, my nearly 13 year old Oceanic VT3 does not give any real time or on board information about gas consumption. However, in the download, both SAC in pressure/time and RMV in volume/time are displayed. The tank characteristics can be chosen from a dropdown list and there is a default, AL80 for me.

I posted this in detail on another thread RMV math…

Just a clarification that the "real time" SAC displayed on AI-enabled SW computers is actually an average SAC over the last 2 minutes (still quite useful for "biofeedback", but it's a slightly lagging indicator):

 

[Lorenzoid]

Just a clarification that the "real time" SAC displayed on AI-enabled SW computers

How useful is knowing one's real-time SAC? Is the point simply to see if you are breathing harder or less hard than some baseline? (You can tell I don't use an AI computer.) I would think consumption rate at depth is all a diver really needs to see in real time.

 

[scubadada]

I have very much enjoyed the lively discussion so far, thanks to everyone who has participated.

I have used the same definitions of SAC and RMV since I started calculating them from my Oceanic VT3 in 2010. I did not get my SDI Solo certification until 2013. I decided to go back to the SDI Solo Diving Manual to see what it had to say about gas consumption. I have a manual dated 2007, 2010.

Boy, was I surprised

SAC is defined as Surface Air Consumption in volume/min (never pressure), measured at the surface, at rest
RMV is Respiratory Minute Volume, also a surface value in volume/min, but calculated from actual diving and including workload and stress factors such as temperature, visibility, current...

The manual suggests using the SAC to calculate gas requirements by multiplying it by a dive factor, something like 1.5 for an easy dive and 3 for a hard dive.

I have never used the SDI Solo Diver mechanism for calculating gas requirements. My average RMV over 1792 dives since 2010 is 0.36 cu ft/min with a std deviation of 0.04 cu ft/min. It has not significantly changed over that period, I already had about 500 dives. My RMV has had a slight decrease with a lttle less variation. So 95% of my dives are between 0.28 and 0.44 cu ft/min. I generally use 0.44 for general dive planning and twice my average RMV for emergency planning like my pony capacity.

 

[tursiops]

I would think consumption rate at depth is all a diver really needs to see in real time.

Yeah, pretty much. But if I glance down at my Teric after a few minutes of hard swimming and it says "35 psi/min" how do I interpret that? Is it because I'm at 35 ft, or because I am working hard? Best to show me SAC so I don't have to work out the effect of depth...
If my concern is gas remaining, that's what GTR is for.

 

[Joneill]

How useful is knowing one's real-time SAC? Is the point simply to see if you are breathing harder or less hard than some baseline? (You can tell I don't use n AI computer.) I would think consumption rate at depth is all a diver really needs to see in real time.

"Current" consumption rate is what is it is giving you and that can give you an indication if you are not relaxed - so feedback to adjust breathing/relax/slow down if I see it trending higher. Just a good feedback loop for me if I see it heading past 16-18!

 

[scubadada]

How useful is knowing one's real-time SAC? Is the point simply to see if you are breathing harder or less hard than some baseline? (You can tell I don't use an AI computer.) I would think consumption rate at depth is all a diver really needs to see in real time.

It's not useful, just interesting to see how much your gas use goes up when you are working hard.

My diving is very simple, mostly no stop, about 5% light deco. My Teric has the ability to display a lot of information on the home screen. I landed on this display. I replaced TTS on the second line with my transmitter pressure. I put SurfGF, GTR/SAC, and temperature on the 3rd line. During a dive, the 4th line has dive time. I use the floating north pointer as it is generally all I need while drift diving.

 

[Joneill]

Yeah, pretty much. But if I glance down at my Teric after a few minutes of hard swimming and it says "35 psi/min" how do I interpret that? Is it because I'm at 35 ft, or because I am working hard? Best to show me SAC so I don't have to work out the effect of depth...
If my concern is gas remaining, that's what GTR is for.

Agreed - I'd interpret the 35 pisi/min against what I expect to see based on past experience (my "baseline"). Unless I was really huffing it in very strong currents 35 psi/min would mean something was seriously wrong for me.

 

[Joneill]

It's not useful, just interesting to see how much your gas use goes up when you are working hard.

My diving is very simple, mostly no stop, about 5% light deco. My Teric has the ability to display a lot of information on the home screen. I landed on this display. I replaced TTS on the second line with my transmitter pressure. I put SurfGF, GTR/SAC, and temperature on the 3rd line. During a dive, the 4th line has dive time. I use the floating north pointer as it is generally all I need while drift diving.

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At least, for me, I'd say that info can be VERY useful as it gives you awareness to a possible issue and feedback to perhaps do something differently to relax/lower your breathing rate !

 

[scubadada]

Agreed - I'd interpret the 35 pisi/min against what I expect to see based on past experience (my "baseline"). Unless I was really huffing it in very strong currents 35 psi/min would mean something was seriously wrong for me.

0.90 cu ft/min with an AL80, 2.5 times my average! I'm pretty sure I would be acutely aware of breathing at that rate without having to see my SAC.

 

[lowwall]

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 believe the use of RMV predates SAC in diving. The Navy Diving Manual only uses RMV and does not use the term SAC at all. For example here is the definition of RMV from the 1973 version of the manual:

Respiratory rate is the number of breathing cycles in one minute. A normal, average rate would be between 10 and 20 cycles per minute. Respiratory minute volume (RMV;, or minute volume) is the total volume of air moved in and out of the lungs in one minute. It is computed by multiplying the tidal volume by the respiratory rate. Since both of these factors are variable, RMV; will vary (governed by the rate of activity) and can range from about 6 liters per minute to more than 100 liters per minute.

What do the early scuba manuals have to say about this?