SAC Rates - The Good, The Bad, The Average

[scubadada]

When we breathe we breathe in air (EAN21). When you breathe out, theres still most of the oxygen left in that air. We are simply unable to utilize the ammount of oxygen present in air and even less so the oxygen in richer mixes.
We do however fill our lungs when we breathe and as we cant absorb more oxygen from nitrox than we do for air, we have to breathe at the same rate wether we have air or EAN in the tank.
Also, what trigger us to breathe is not lack of O2, but the need to expell the CO2 that build up.

Exactly the question I asked on Marine Science and Physiology thread. Obviously, O2 delivery can be increased with increased FiO2 or mechanical ventilation would be ineffective. I still do not know what drives SAC, O2 requirement or CO2 elimination.

Best, Craig

 

[lowviz]

Control of respiration - Wikipedia, the free encyclopedia read the "Determinants of ventilatory rate" section, that may be what you are looking for.

 

[scubadada]

Control of respiration - Wikipedia, the free encyclopedia read the "Determinants of ventilatory rate" section, that may be what you are looking for.

Thanks, very useful. So, seems like SAC is controlled by CO2 production and metabolic rate. "Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate." Efficiency of diving and overall fitness may very well determine this.

Best, Craig

---------- Post added April 16th, 2012 at 07:20 PM ----------

[h=2]

Oxygen Consumption - - -[/h]

Hello Craig :

Varying Oxygen Consumption

The consumption of oxygen is a function of several factors. Naturally, physical activity is the first that comes to mind; all measurements are therefore made in resting individuals.

Nervousness is a factor and indicates a diver who is not relaxed underwater; training is usually the answer for this problem. Cold and shivering will also increase consumption of oxygen.

Being in good physical conditions is the best answer for low consumption.

Oxygen Content of Mix?

The oxygen content will not change the breathing rate. This rate is controlled by the carbon dioxide in the blood and is indicative of muscle activity. Surprisingly, oxygen is not controlling. People have walked into low-oxygen environments and died while breathing normally. This can occur in rooms where the air is replaced by something with low oxygen [for fruit storage].

Poring liquid nitrogen in a shallow area will drive out the oxygen and replace it bynitrogen. Breathing nitrous oxide for recreational purposes has resulted in death when the oxygen [in a separate cylinder] became exhausted.

A few years ago [October 1999], a Learjet with low oxygen caused all of the passengers,including pro golfer Payne Stewart, and the pilot to pass out and die. The plane crashed near Aberdeen SD.




Dr Deco :doctor:

From Marine Science and Physiology. Appears fitness level and exertion play a major role in CO2 production and SAC.

Best, Craig​

 

[Sheri SGR]

Assuming I calculated correctly mine is around .20 but I am very good on air, female, fit, etc. My sister who is female and fit but less diving than me uses about twice as much. your SAC rate will dcrease as you dive more and depending on your fitness level you might have some opportunity there as well.

 

[TSandM]

My next stop is ??? who knows ???​

This is what worries me. Some time ago, I diagnosed myself as a CO2 retainer -- it's extremely easy for me to lower my breathing rate to the point where my CO2 goes up. I tend to run about .35 or so now; it's not too difficult to drop it below that, but I try not to. I've had some ugly narcosis events that have proven to me that risking anything that increases my impairment at depth is to be avoided.

Honestly, we all breathe. We have to. We need to breathe a certain amount to keep our pCO2 normal -- where is where it SHOULD be, for very strong reasons. If your minimum breathing requirement limits your dives, buy bigger tanks.

 

[meierdierck]

what are some constructive ways to lower your sac... I know fitness, although I am a bit surprised runners and athletes are not much better than mine, I am a big guy, big lungs, I am very relaxed in the water, what can I do to lower my sac rates?

 

[lowviz]

...//... We need to breathe a certain amount to keep our pCO2 normal -- where is where it SHOULD be, for very strong reasons. ...//....

Respiratory physiology is wickedly complicated.

CO

Just breathe, your body works better that way. My coldwater SAC is about 0.7+ I have come to peace with this.


How to lower one's SAC? - good trim and lots of in-water practice.

 

[TSandM]

meierdierck, lowering SAC rate is accomplished by two things -- increasing diving efficiency, which means horizontal trim, quiet hands, and reducing velocity -- and an efficient breathing pattern, which is simply rhythmic and slightly deeper than ones on-land breathing pattern. The now-jettisoned fin pivot introduced the concept of breathing in until one starts to rise, and then exhaling until one starts to sink, and then inhaling again. This pattern will create the most efficient ventilation of the lungs you can have underwater.

Combine that with a quiet, relaxed, stable diver, and add a kick like the frog kick, which allows a rest or glide phase, and you can lower the gas consumption quite a bit. But never beyond what is necessary to maintain a normal CO2. That's counterproductive.

 

[Divedoggie]

We can all manipulate our SAC rates by changing our breathing pattern, but we really should resist the temptation. Breathing through a reg underwater is not natural, but we should try to do it as naturally as possible.

I find that the more I dive, the less I think about my breathing. When I am teaching and working underwater, I breathe normally, naturally and as required. With challenging students my SAC goes way up, with easy students or just diving for fun in easy conditions' the SAC goes way down. As long as I plan ahead and carry appropriate back gas, I really don't worry about it or try to change it.

Solid buoyancy, perfect trim, moving slowly, effective finning, a calm demeanor, and relaxed rhythmic breathing, make the SAC rate level off where it is supposed to be.
IMO experienced divers lower their SAC rates because they are more comfortable underwater, more relaxed, and their bodies are demanding less O2. At some point, no matter how relaxed and efficient, we breath what we breath and the SAC fits the individual.

As TS&M suggests, the best way to prolong a dive or to increase the safety margin with a planned minimum reserve, is to use a higher volume tank. It worries me that some suggestions on Scubaboard could encourage risky diving habits. Improper breathing and the resulting CO2 buildup can significantly increase the risk of DCS.

Those seeking advice should research the advisor's profiles, previous posts and even then, draw a bell curve to see where the concensus lies. Look to a trusted outside source. DAN has some excellent articles on their website.

Please be careful.

 

[fjpatrum]

So far mine has ranged from about .4 to 1.3, depending upon water temps, effort, etc. I think I average right around .65-.7. I don't think it's great, but so far I haven't been dissatisfied with any of my dive times.

I've been trying to track mine so that I can use it for gas management more than anything else. I figure breathe how you breathe and there's not much point in "trying" to get a better SAC.