SAC rate too low?

[lolskim2008]

I've been working on my trim and breathing techniques in the water to lower my SAC rate and I've got it down to .497 cu ft/ min, my question is, when is your SAC rate considered too low?

 

[mikemill]

Um, 0?

 

[ScubaInChicago]

Um, 0?

not a good day.....

 

[Sas]

I've been working on my trim and breathing techniques in the water to lower my SAC rate and I've got it down to .497 cu ft/ min, my question is, when is your SAC rate considered too low?

If it is not providing you with adequate air to live? Not sure what you are getting at. 0.497 is fine (about the same as most of my buddies), so don't worry about yours. The lowest SAC I've seen is 0.24. Mine is about 0.39 but can drop to 0.3 sometimes. It can also increase to 1.16 one time when I was testing to see what it was like swimming very hard.

 

[Bubbletrubble]

I've been working on my trim and breathing techniques in the water to lower my SAC rate and I've got it down to .497 cu ft/ min, my question is, when is your SAC rate considered too low?

You're looking for a "number" answer. Well, SAC rates are going to vary from diver to diver. Some experienced divers while stationary (such as during a safety or deco stop) have SAC rates that can get down in the 0.3 cuft/min range. Divers with larger lung volumes will typically have higher SAC rates, e.g., all other things be equal, a large man might have a higher SAC rate than a small woman.

In the vast majority of cases (healthy individuals), the SAC rate is too low when one shows physiological signs of inadequate ventilation. One of the most common signs of this is the post-dive headache caused by skip breathing (CO2 retention). Know that some individuals have adapted to higher levels of CO2 in the blood which alters blood pH and can induce physiological changes in respiration and/or metabolism. These changes can initiate/potentiate narcosis. These individuals should take precautions against physical over-exertion during a dive.

As a newer diver, I wouldn't get too hung up on SAC rate. The danger is that you'll be more prone to skip breathe. Instead, just try to relax more. Streamline your gear. Achieve horizontal trim. Take your time. Hover whenever you can. Your consumption is...whatever it turns out to be. Take this into account in your gas planning. If you need more gas, get a bigger tank.

 

[DA Aquamaster]

As BT is saying, too much emphasis on SAC can quickly get you into trouble. There is another thread currently running discussing the same issue where you can get details.

Also, a SAC is never a fixed number, but is usually fairly consistent based on the dives you do. For example in doubles and a stage in a high flow cave where a fair amount of swimming effort is needed, I can plan on my SAC being right around .65 cu ft per minute, and this is consistent from dive to dive per the average depth function of my computer and the gas used. In a low flow cave, it will be closer .60. On my deco stops I plan on a resting SAC of about .4, although on longer deco where I really relax it will drop close to .3.

It is important to note here that I am also not trying to stretch my gas but rather I am ensuring I stay efficient yet well ventilated to keep CO2 levels down.

When you say your SAC is .497, it really means nothing without also providing the general context. Some people will sit in front of the television and breathe of a reg, then claim a SAC somehwere around .2 and imply it is somethig special. Not really. It is also almost impossible to scale up to a real dive.

Similarly, some divers may do a resting SAC in the water, but again it tells you very little, except maybe what you might expect on a deco stop.

A swimming SAC works better, but how you set it up is important. Ideally, you will swim at a steady pace and a steady depth for 5 minutes to get into a steady balanced state regarding your metabolism, CO2 production etc, then note your SPG pressure a depth and the time to the second, and then maintain that depth for 5 to 10 minutes while continuing to swim at the same steady pace. The longer the swim the better, and the deeper the depth the better as both help to give you a more accurate calculation.

Then note the end time and the end pressure and crunch the numbers for the gas used perminute that that depth.

You can also run trials at different workloads to see how that impacts your SAC. That works much better than taking a resting SAC and trying to scale it up with wild ass guess constants like 1.5, 2.0 etc.

Ideally, you will also have a computer or bottom timer with an average depth function. That along wqith careful recording of start and end pressures and notes on the dive will let you develop a very accurate picture of your SAC under different real world conditions.

 

[Rhone Man]

It is too low when you have a bloody great headache after each dive. How low can you really go is the great philosophical question of limbo dancers; but for both limbo dancers and divers, it really is a feature of how big you are. No matter how fit you get, some skinny girl who weighs 98 lbs dripping wet is going to have a lower SAC than you.

Also remember that your SAC will vary wildy on conditions. My "optimum" SAC is down about 0.38, but I rarely hit that on real dives, and some dives I am nearly at 0.7 (ie. double).

In comparitive terms, here was a poll from a while back where people posted their "average" SAC.

http://www.scubaboard.com/forums/advanced-scuba-discussions/264147-how-small-your-sac.html

 

[GrumpyOldGuy]

I would offer one tidbit, it is not so much your SAC, but how you are achieving it. It varies to much to work with a hard number. There is a difference between reducing your needs for 02/C02 exchange and suppressing your bodies mechanisms that insure adequate exchange is taking place. The first is low/no risk, the second can be dangerous.
- Reducing SAC by trimming, moving slowly, not using your hands, etc... is completely safe.
- Slow, full breaths are generally considered safe (relative to normal breathing).
- Slowing down your breathing (skip breathing) and holding your breath (airway open) for a few seconds is starting down the slippery slope that can lead to elevated CO2 levels, headaches and potential blackouts. This techniques work to a degree, but the point where this becomes dangerous is a fuzzy, moving target, different for every person and dive condition.

 

[Mike Boswell]

I've been working on my trim and breathing techniques in the water to lower my SAC rate

Why? What is your goal?

I don't ask this to be facetious, but if your goal is to make your air last as long as most people, you are probably already there.

If your goal is to stay down as long as more experienced buddies, you will probably get there just by diving more and learning to relax.

If/when you do get your SAC down below average, then, unless you are a solo diver, your bottom time will be limited by other divers.

For most of us, a low SAC just means you will end most dives with lots of unused air left in your tank.

 

[Slamfire]

Similarly, some divers may do a resting SAC in the water, but again it tells you very little, except maybe what you might expect on a deco stop.

I monitor my SAC rate for virtually every dive I do. Instead of measuring it for only a small portion of the dive I take the entire dive into account. Yes, the rates will vary from day to day but when you accumulate 100+ sac rates measured this way, then you can start seeing patterns and getting more realistic standards. For instance, I have a pretty good idea of how much my sac rate will increase if I jump into the water with motion sickness. The same goes for diving against currents of different intensities or when you've had to deal with problems under water.

Recently I discovered a small leak coming out of necklace 2nd stage because my SAC rate for the last dives had increased slightly. At first I'd chucked it to having an off day. I would ask buddies to do a bubble check, and everything would come out fine because they would focus more on the first stage area and hoses. After a while of having SAC rate slightly higher than usual I came to realize that it is not very likely to have 4 "off days" in a row over a period of 2 weeks. So I looked closely and found the leak. Basically I had adjusted the regulator to breath slightly too easy.

I echo what others mentioned before about focusing too much of reducing the SAC and going into skip breathing and CO₂ retention. Keep on breathing normally. Instead of focusing on the breathing itself start focusing on the things that'll affect air consumption:

• Bouyancy
• Trim
• Efficient movement
• Being in the right head space - control panic/fear when facing uw problems
• relaxation and enjoyment
• Breathing with your "stomach" not your chest - opera singer style
• I've heard doing yoga greatly helps. I haven't tried it though.