Anyone not measure their SAC?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Yes, I understand the rescue/return part of gas planning. The important part.

But the point that I was going for was that one's gas consumption rate is variable and one can easily determine one's extremes to set a range. After a few simple trials, one can have a reasonable estimate of one's "nearly fell asleep on the couch" consumption rate, an average dive consumption rate, and a constant moderate finning consumption rate. Some find it interesting to compare a given dive to one's range. Yes, I believe that this discussion belongs in basics.

I would caution anyone trying to determine a max stressed consumption rate that they do it with help and in water shallow enough to stand up in. I did this in a PSD course. The drill was to fin as hard as possible for one minute, you get a tap on the head by your assistant, and then just go to your necklace reg. That was an eye-opener. Never, ever get air starved underwater. The slightest problem can be disastrous.
 
Please do not get hung up on the technical definitions of SAC, RMV, or any other term that are sometimes used. Different people define the terms differently. In threads like this in the past, people have gotten very huffy in declaring that the definition they were taught is the only correct one, so the definitions everyone else was taught are dead wrong. The way the terms are used by different people, SAC and RMV are virtually interchangeable terms.
 
Yes, I understand the rescue/return part of gas planning. The important part.

But the point that I was going for was that one's gas consumption rate is variable and one can easily determine one's extremes to set a range. After a few simple trials, one can have a reasonable estimate of one's "nearly fell asleep on the couch" consumption rate, an average dive consumption rate, and a constant moderate finning consumption rate. Some find it interesting to compare a given dive to one's range. Yes, I believe that this discussion belongs in basics.

I would caution anyone trying to determine a max stressed consumption rate that they do it with help and in water shallow enough to stand up in. I did this in a PSD course. The drill was to fin as hard as possible for one minute, you get a tap on the head by your assistant, and then just go to your necklace reg. That was an eye-opener. Never, ever get air starved underwater. The slightest problem can be disastrous.
As the video pointed out:

Nominal Working SAC: 0.75 cf/min or 20 l/min;
Relaxed (Safety Stop/Deco): 0.5 cf/min or 15 l/min;
Emergency/Stressed: 1.0 cf/min or 30 l/min.

I would also add "Drift Diving with the Current in Tropical Waters": 0.4 cf/min or 11 l/min.

Because of immersion barometric effects, any physical exertion with an elevated RMV above 1 cf/min and approaching 2 cf/min (30 l/min to 60 l/min) cannot be sustained for more than a couple of minutes at depth; while forcing RMV below 0.3 cf/min or 9 l/min at depth -usually indicative of skip breathing- should be seriously discouraged even as a method to conserve air consumption: Either case can lead into spiraling metabolic CO2 poisoning, Hypercapnia and unconsciousness.

For reference, at the surface, a normal resting RMV is around 0.2 to 0.3 cf/min (5 to 8 l/min).
 
Last edited:
@lowviz makes a really good point. I've heard some folks bragging about their SAC being well below 0.5. With perhaps a few exceptions, the only way most of us are going to get there is to measure it while at rest. If you consistently shoot for measuring your SAC while at rest, you have no idea how much you will be consuming if you suddenly find yourself needing to swim. I have a good idea what my SAC is for: at rest, an average "working" dive, and while working hard. For gas planning I tend to err on the higher side as I don't like surprises.
 
I try not to aim high or low when planning. If I use more air than expected, then I will turn on pressure. If I use less then expected then I will turn on time. I try not to limit my plan by being overly conservative because I will always turn on the first parameter that is reached. If I plan TOO conservatively, I limit my dive plan. I am new at this so I may change later. Yesterday was our one year scubaversary.
 
Hi folks. Just wondering if anyone on here doesn't record their air consumption. I don't log dives at the moment and haven't bothered to record my gas use per dive. This is not a problem as I always seem to have plenty for my dives and redundancy. If and when I get a bit more technical I will log but at the rec level I just don't see the point. Thanks.

I calculate SAC now and then just for fun, but not on the vast majority of my dives. If one is doing the same kind of dive frequently, the more useful information is simply learning over time how much gas you use on one of those dives. Most of us tend to have a certain kind of dive that we do most frequently. For example, I have a very good idea of how much gas I am going to use (in units of PSI) with an Al 80 on a 60' tropical vacation kind of dive. I don't need to calculate anything.

SAC is useful for extrapolating to other kinds of dives. If you know your average SAC (averaged over a variety of kinds of dives), you can use it to predict air consumption reasonably accurately for any dive.
 
sac rate is measured in volume/time/ata.

SAC = Surface Air Consumption. RMV = Respiratory Minute Volume. Both are volumetric calculations of gas consumption.

As has already been observed, both terms are commonly used pretty much interchangeably. Often, one can infer what the person really means by the context. Or, it doesn't matter.

However, it sure would be nice if everyone got on the same page. For that, it only makes sense (to ME) that RMV is a volumetric measurement. I mean, it has "Volume" right in the name. And, if RMV is going to be volume over time, then it makes sense to me to agree that the ambiguous term, SAC, be used for pressure over time. There are reasons why we use both, so I think we, as a community, need a term for volume over time AND a term for pressure over time.

Thus, when I say RMV, I always mean volume per unit of time. If I mean pressure per unit of time, I will say SAC. But, if I'm just talking about generic consumption - as in, for comparison purposes, I will generally say SAC. Obviously, in almost all contexts, if you are comparing consumption you would be assuming the same size cylinder, so SAC or RMV would always give the same answer. If SAC is higher, then RMV is higher. If SAC is lower, then RMV is lower.

I try not to aim high or low when planning. If I use more air than expected, then I will turn on pressure. If I use less then expected then I will turn on time. I try not to limit my plan by being overly conservative because I will always turn on the first parameter that is reached. If I plan TOO conservatively, I limit my dive plan. I am new at this so I may change later. Yesterday was our one year scubaversary.

For planning purposes, I think what is most important is planning your reserve. Meaning, conducting a planning exercise to determine how much gas you will use is cool. But, it is not as important as planning how much gas you will NOT use (short of an emergency) if you see what I mean. For planning gas reserves, I would always aim for the high side. Yes, it can result in a turn pressure that means you feel like you are cutting your dives a lot shorter than is really necessary. But, that is the price you pay for being sure that if something really bad happens, you will have enough gas to get you and your buddy home safely. If you're doing recreational dives and having to cut a lot of dives short, it might be time to start using bigger tanks. I love my HP120s for single tank NDL diving...
 
Hi folks. Just wondering if anyone on here doesn't record their air consumption. I don't log dives at the moment and haven't bothered to record my gas use per dive. This is not a problem as I always seem to have plenty for my dives and redundancy. If and when I get a bit more technical I will log but at the rec level I just don't see the point. Thanks.

I have dived with AI since Day 1, so I have my gas consumption recorded for every dive I have done. But, I surveyed a group of divers at our local scuba happy hour a few months ago. I think there were about 10 divers that I asked. NONE of them knew their SAC/RMV or even how to figure it out.

My opinion is that, if you know what size tanks you have been using and what depths you were diving to, and you know that you are consistently able to do your dive and get back to the surface with a reserve amount that you are happy with, that is the most rudimentary form of logging your consumption. I mean, it does give you a basis for planning your future dives.

However, the biggest problem with it is the phrase "a reserve amount you are happy with." Is that contentment born of real knowledge? Or do you have undeserved confidence in your reserve based simply on what a dive leader told you or some other thing? These are questions that each diver has to answer for themselves.

Made up example: You always get back to the surface with 500 psi (or 40 bar) left in your tank. You are satisfied with that and feel like it's safe because a divemaster on the boat told you to come with that much. If you haven't done anything to verify empirically that that is an adequate reserve, then I would call it false confidence. I mean, how do you KNOW that, if your buddy goes out of air on you right when you were about to turn the dive anyway (so you're already getting low on gas), you have enough gas for both of you to breathe (at what would probably be an elevated rate) all the way through an ascent at a safe speed, with a safety stop? Do you even know, at ALL, what your buddy's consumption rate is, in order to figure out if you have enough reserve for the both of you? If you dive with an insta-buddy, do you ask? Obviously, if your buddy asks you the same question, you don't have an answer, so your buddy can't really plan his/her reserves with any confidence.
 
Planning wise my dives are now all come back up when you have to. At first I would compute what my air consumption was but now in my home waters on an hp100 with nitrox I am usually NDL limited and not air limited so it does not matter.
 
Al my planning is done on a SAC of 15L/min.

When measured, it goes from 10 to 18 with a, average @ 13.

Since I carry 95% of the time a 15L tank, the calculations are easy: 1bar/minute at the surface.
 
https://www.shearwater.com/products/teric/

Back
Top Bottom