Air consumption rates.

[Akimbo]

Okay, I am a vintage diver, and have not heard of the term RMV related to diving and air usa. To me, it is "root mean value," but this is from either statistics or math. So, enlighten me!

SeaRat

We used RMV, Respiratory Minute Volume, exclusively in Navy and commercial diving. I never heard of SAC rates until visiting Scubaboard. Not sure about what combat swimmers used but that was what the USN diving manual used. RMV was normally expressed in Liters for volume but I have also seen it used in Cubic Feet . In my limited experience, RMV was always a measurement at sea level and adjusted for altitude or depth.

Respiratory minute volume - Wikipedia, the free encyclopedia

 

[KenGordon]

I think that once you have become a reasonably skilled and experienced diver, your RMV is essentially fixed by individual variables. For my last 742 dives, my RMV has been 0.37 +/- 0.04 cu ft/min (mean +/- SD). That means 95% of my dives have an RMV of 0.29-0.45, the actual range is 0.28-0.63. My RMV is remarkable unaffected, though adversely, by effort and by cold. My current average RMV of 0.35 is barely lower than the 6 year average, I don't expect it to change much in the future. I'm not at all sure what personal factors fix your RMV. Having been a competitive swimmer between the ages of 4 and 18 may have been a positive. I have exercised regularly my entire adult life and am a nonsmoker. I am not a particularly small man at 5'10" and 185 lbs. My breathing pattern of a slow, moderately deep inhale, a short pause, and then a slow exhale, has not changed in more than 1000 dives.

I am aware of a few men, and several women, with average RMVs lower than mine.

Have you tried measuring your RMV sitting on the sofa watching (unexciting) tv?

You are saying that your air consumption can be as low as that of a 10 year old child sitting watching TV. Maybe I will incentivise him in a future experiment - "you can watch to until you run of gas..." sort of thing.

Low air consumption seems to be a major form of willy waving here. It would be interesting to know what average competent divers manage. My typical warm blue water dives are more like 15l/m. I know that under stress (failing to perform a shutdown for example) it can hit 35l/minute or more.

 

[Skeptic14]

My SAC was right around 1.0 cu ft/min for my first dozen dives... then as I started dialing in weighting and having a few ah ha moments about buoyancy and trim it's dipped down to in the .6s, I wouldn't mind if I could get closer to .5 but I don't feel any rush to artificially improve it, just allow natural improvement as my log book gets thicker.

My wife's SAC was pretty much immediately .35-.40 from dive 1 and she always seems to use only half the amount of air as I do, she's only 120lbs and I'm 185lbs so I'm sure that matters. That said, we've had dive guides that were my size that used no more air than my wife and perhaps a little less... so I can believe there are male divers with .25-.30 SACs... I suspect if you have 10k dives and dive 365 over time you become a virtual fish

 

[Lorenzoid]

. . . That said, we've had dive guides that were my size that used no more air than my wife and perhaps a little less... so I can believe there are male divers with .25-.30 SACs... I suspect if you have 10k dives and dive 365 over time you become a virtual fish

I've often wondered why that is. Is it really that the people who dive every single day are more relaxed than the rest of us? More physically fit? What is it exactly?

 

[KenGordon]

I've often wondered why that is. Is it really that the people who dive every single day are more relaxed than the rest of us? More physically fit? What is it exactly?

The ones with a high SAC don't get the job?

 

[dlofting]

I've often wondered why that is. Is it really that the people who dive every single day are more relaxed than the rest of us? More physically fit? What is it exactly?

I've wondered about this too and I think it's that they are more relaxed diving than the rest of us (or at least than me). I walk or hike every day, going about 40-45 km a week at a moderate pace of about 5 km per hour. I know that my respiratory rate is less when I'm walking on level ground than it is when I'm diving. There is no way that the average physical workload of diving is greater than the average physical workload of walking at 5 km per hour.....it just isn't. The only answer I have is that I'm much more relaxed while walking or hiking than I am when I'm while diving.....and it makes sense as I walk/hike 20 times as much as I dive, so while one activity is second nature to me, the other has many more "unknowns" and is therefore, more stressful.

 

[admikar]

One curious thing is that, I get a lot lower SAC during night dives, on same site, same conditions.
Somehow, I am a lot more relaxed watching at my hot spot, than from swiveling my head around to see everything there is to see.

 

[tbone1004]

normal is considered 0.4-0.6 in my experience, for experienced, relaxed divers, in normal physical condition, under normal diving circumstances. New divers with SAC rates in the 0.6+ range are usually there because they aren't relaxed, and those below 0.4 are either skip breathing, or have something else going on that allows them to use less air. DPV diving properly, decompression, drift diving, etc etc are all situations where normal divers are able to get below 0.4, but to be that low while kicking is rare. My normal sac rate if I'm relaxed and not working too hard is about 0.45cfm or roughly 13lpm, but I never use that for gas planning. If I'm "working" i.e. kicking at anything faster than about 50fpm, it jumps up to 0.6cfm or 17lpm, and that is what I typically count on for my dive planning. For emergency planning, it is always 1cfm or 28lpm as that is not an abnormal SAC rate if you are excited for whatever reason. People say that is too conservative, but no one has ever died from having too much gas in an emergency situation so I choose to put that buffer in place

 

[uncfnp]

Ken. Thanks for linking the video. Very interesting but I must be missing something. I didn't see anything about skip breathing, breath holding or divers trying to intentional reduce SAC/RMV. In fact, the variables he discussed are mostly beyond a divers control, such as individual physiology, equipment and gas density

As to your contention that divers lie about their SAC/RMV. I don't doubt that some rates may be somewhat embellished or prehaps rounded up/down with math to the diver's benefit but I have no doubt that there are divers out there with incredibly low rates and I don't think under .4 is rare. During my TDI AN course we calculated the rate for one of our dives. The instructor calculated out to .27 but no I didn't check his SPG but I did watch him dive. I have no reason to disbelieve him.

Another time I had the privilege to dive with a veteran diver in Florida with her horsecollar BCD, plastic backplate and the tiny HP tanks that I have heard called "bowling balls." Both dives she stayed to the 60 minute time limit and it was dives max to about 80 feet. Don't know her rate, I doubt she cared but it had to be ridiculously low.

But in the immortal words from dumpsterDiver's sig line...

"SCUBA Diving: The only sport where grown men will brag about how low their sac is."

 

[John C. Ratliff]

When I was an instructor in the 1970s and 1980s, I put together this sheet about calculating the Surface Air Consumption Rate (SAC). I used this technique for a period, but have not been using it recently.

What I now use is my Suuno Cobra dive computer. I've included a graph of my May 10, 2015 dive in the Clackamas River, which shows both the dive profile graph and the SAC. In this dive, the river was quite high, and I worked pretty hard during the dive. I normally do swim in the river, cross-current and into the current on the bottom. This, obviously, will influence the SAC. 0.58 cubic feet per minute is equal to 15.9 liters per minute. (Note: I'll have to check this out, as I need to make sure I entered the correct tank configuration into this calculation.)

SeaRat

PS, I just checked, and I'm now using my Suunto Dive Manager on my Mac. In this computer, the program is not as comprehensive as in the PC world (the screen on my PC laptop went gunny, so I haven't been using it recently). When I changed the tank size to 69 cubic feet (it was a single 72), I came up with a 1.45 cfm SAC rate. That's 42 liters per minute. I told you I was working!