Question Definitions for SAC and RMV?

[Wibble]

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.

Interesting. I was told it was YOUR 'air' consumption in one minute on the surface expressed in litres.

 

[scubadada]

Just to give you my 2c: I know and use only SAC (in lt/min) and with it I can do all the necessary gas planning calculations for my dives, if needed (not very often to be honest). On top of that, it is very convenient that Subsurface uses the exact same definition hence I can use its' gas planning feature out of the box...

It is not only divers who use the term SAC and RMV variably. Dive software also reveals the same variation. As @stepfen wrote, Subsurface uses SAC in volume/time/atm.

MultiDeco uses RMV in volume/time/atm, cu ft or liters

The Shearwater Cloud (and the Teric onboard log) reports only SAC in pressure/time/atm. You must calculate your own RMV in volume/time/atm by knowing the characteristics of the tank(s) you used

In this example with an AL80 13.23 psi/min x 0.258 cu ft/psi = 0.34 cu ft/min.

Oceanic OceanLog reports SAC in presuure/time/atm and RMV in volume/time/atm. You are able to enter the tank used from a dropdown list, including a default

It is unlikely that the terms SAC and RMV will ever be standardized. Use units when communicating with others.

 

[scubadada]

Good video. Rather that specifically do RMV measurements for parts of several dives. I measure RMV for entire dives over the range of conditions I dive in. I have been looking at RMV since I bought a hoseless AI computer in 2010. This makes it very easy as the start and end pressures/pressure used, average depth, and dive time are all captured (enough to calculate SAC). Some download software also allows you to enter the tank(s) used and will automatically calculate your RMV (see post #82, above)

I have my RMV for most all of the dives I have done since 2010, 1792 dives. My average RMV is 0.36 cu ft/min with a standard deviation of 0.04 cu ft/min. So, I know that about 95% of my dives have had an RMV of between 0.28 and 0.44 cu ft/min. The actual range is 0.24-0.63 cu ft/min. The 0.24 was essentially standing or kneeling in the sand watching Great Hammerhead sharks in Bimini, Bahamas, almost no swimming involved. The 0.63 was the opposite end of the spectrum, a dive in cold (47 degrees in a wetsuit), dark water with high current on the stern section of the Chester Poling in Cape Ann, Massachusetts.

I already had about 450 dives when I started to record RMV. My RMV has not changed much over those 12+ years, only a slight decrease with a little less variability. Of course, exertion raises my RMV. Nearly equally important is being cold, which also raises my RMV significantly. This information is very useful in gas planning. I double my avg RMV to calculate my pony gas volume.

 

[lowwall]

The actual range is 0.24-0.63 cu ft/min. The 0.24 was essentially standing or kneeling in the sand watching Great Hammerhead sharks in Bimini, Bahamas, almost no swimming involved.

You are one cool customer. I imagine most folks have an elevated respiration rate in that situation regardless of the minimal exertion.

 

[wetb4igetinthewater]

It is not only divers who use the term SAC and RMV variably. Dive software also reveals the same variation.

Who do you think writes this software? Divers or people consulting divers. As these terms have branched into different definitions....

It is unlikely that the terms SAC and RMV will ever be standardized. Use units when communicating with others.

Yes. What is interesting about this conversation is a datapoint on how over time the percentages of people who adhere to one definition or the other changes over time.

 

[drrich2]

My average RMV is 0.36 cu ft/min with a standard deviation of 0.04 cu ft/min. So, I know that about 95% of my dives have had an RMV of between 0.28 and 0.44 cu ft/min.

Of course, exertion raises my RMV. Nearly equally important is being cold, which also raises my RMV significantly.

It's a bit of a tangent, but that RMV is excellent and has me wondering how much of that is inherent to the diver's individual physiology (i.e.: do some people naturally breathe less than others? What is the impact of being a small woman over a large man, and how then do we explain big guys with low RMV?), and how much is determined by acquired factors (e.g.: mental calm, advanced dive skill with little wasted motion and efficient fin kicks)? And then there's cardiovascular/aerobic fitness, how large a cross-sectional profile you need to push through the water, and breathing technique manipulation (whether the derided 'skip breathing' or using yoga techniques)?

Inadequate thermal protection and exertion are known factors.

In a nutshell, what should a given diver's RMV be? This comes up when people post asking about bigger-than-usual tanks, others tell them dive more and their RMV will come down, etc...

 

[scubadada]

Hi my friend @drrich2

I have asked myself these same questions many times, too bad I have not really been able to answer them.

I am 5' 10" tall and weigh about 185 pounds, hardly svelte. I was first certified in 1970 and dived steel 72s with J-valves in Southern California. I almost always had more gas than my buddies and rarely pulled my J-valve. Skipping to modern times, for me, starting in 1997, my gas consumption has always been on the low end. By the time i started recording my RMV at about 450 dives, my RMV was already fixed and has not changed significantly. I see that as the limit of my gas consumption and don't see that it is at all likely to improve. Is this my physiologic set point?

I was a competitive swimmer from the age of 4 through high school. The breathing for much of swimming has some similarity to that for diving. In swimming, there is a quick inhale, a brief pause and a slow exhalation during exertion and repeat with immediate inhalation. In diving, the inhalation is slow, a brief pause, a slow exhalation followed nearly immediately by inhalation. During normal breathing on land the inhalation is followed by nearly immediate exhalation and any pause is before the next inhalation, somewhat the opposite. I have often wondered if all my swimming somehow trained me and my body for scuba diving. My only buddy during my early diving who had a similar air consumption to me, was another diver from my high school swim team.

I have never smoked tobacco and generally have exercised 6+ days per week for my entire adult life. I think that my diving is pretty good (buoyancy, trim, propulsion, expended energy...) but not out of the ordinary for an experienced diver. Exertion and being cold are the only 2 easily identifiable factors that reliably increase my RMV.

Richard, you are an experienced diver and have been in a wide variety of environments. I would imagine that you are also at your physiologic set point, whatever actually determines that. Have you followed your RMV and seen that it is relatively constant?

I have been interested enough in this topic to have posted my thread on gas consumption 6 1/2 years ago. It is still going strong and has had 634 votes in the poll. There is a good discussion of factors influencing RMV in that thread.

Average Gas Consumption

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is. 1 cu ft = 28.32 liters 1 liter = 0.035 cu ft 28.3 l/min

scubaboard.com

I find the average and the wide distribution to be very interesting. From the last summary:

Good diving, take care,

Craig

 

[tursiops]

My RMV has been pretty stable too for many years, but increases 10-15% if I carry my big camera, and changes up or down a bit if my dive configuration changes (wetsuit, BCD, AL vs steel tank and size, fins).

 

[Joneill]

My RMV has been pretty stable too for many years, but increases 10-15% if I carry my big camera, and changes up or down a bit if my dive configuration changes (wetsuit, BCD, AL vs AL tank and size, fins).

Curious - with the high dive count/experience you have, what is your RMV on a relaxed, warm water dive?

 

[drrich2]

Richard, you are an experienced diver and have been in a wide variety of environments. I would imagine that you are also at your physiologic set point, whatever actually determines that. Have you followed your RMV and seen that it is relatively constant?

At near 6'1", around 265 lbs, non-smoker, fairly sedentary so not in great shape, carrying a compact camera, my RMV on benign tropical dives tends to run around 0.65 cf/min/atmosphere. If I'm really relaxed and low effort, 0.60 is good. If I mildly drift, hang on a mooring line, kneel in the sand or some such, and it's not my 1st dive of the trip, might dip a bit lower.

Some factors are fairly obvious, but their impact isn't always consistent. I can put my hand with fingers extended in a tub of water and thrust it with low resistance. If I ball my fist up and do the same thing, I expect more resistance due to greater cross-sectional area (not stream-lined). We see that in the shapes of dolphins, tuna, etc... The dolphin isn't laterally flattened the way the tuna is, but both are stream-lined to minimize resistance pushing through the water. We're taught to go slow, since the force needed to push through the water rises dramatically with speed.

But there are other large cross-sectional area folks who have better SACs. I know one SB member who's been diving at least off and on about as long as I've been alive, has been a scuba instructor and is a cave diver (so his skill and technique are dialed in to a really advanced degree). I think his cross-sectional area is a bit larger than mine, but I'd bet his SAC is lower than mine.

So I know lower SACs are achievable. How low, the best techniques for getting there and the cost, I don't know.

Per someplace called BreatheSimple.com:

"Our pulse rate when we’re in a relaxed state is a good indicator of our overall health. A normal resting heart rate for adults ranges from 60 to 100 beats a minute. Generally, a lower heart rate at rest implies more efficient heart function and better cardio-vascular fitness. For example, a well-trained athlete might have a normal resting heart rate closer to 40 beats a minute."

"The normal breathing rate for an adult at rest is 12 to 20 breaths per minute. Exhale times are about 1.5 inhale times. Just like RHR, the lower RBR, generally the fitter you are. Athletes may have an RBR around 8 per minute. RBR is higher in babies and pre-teens and tends to increase with age in adults. Training to promote periods of RBR around 6 minute can help stress reduction."

In theory, this would suggest physical fitness may be a big factor. One of the gym rats will have to get back to me on that...