What's a good SAC

[Bob DBF]

I have found the shorter the dive and when shore diving - my SAC goes high. For me several reasons - if I have to inflate my wing and find others that may have been lost - as well as fighting the waves to get out and under the waves is energy that is using air. For me the shallower the dive and the longer it goes the lower the SAC / RMV evens out. If I am not hitting the surface much and I can relax - usually on a solo dive - my SAC is much lower because I am not following / catching up / or concerned about others - only myself.

Yeah, when filling lift bags my SAC gets really bad. Mine is a bit over .7, I can get it to almost half that if I cherry pick the dive or portion thereof, but I'd rather have a realistic idea of the gas I use for planning my normal dives. I've found over time you use the air you use, if I can't stay under long enough, I just bring more gas. Even with my SAC rate, I've never had a problem completing dives with others.



Bob

 

[rhwestfall]

for me: 0.5 (deco or drift diving) to 0.7 (normal cold water diving wet). Throw on the Dry Suit for really cold and a set of doubles, and it is 1.0...

 

[scubadada]

Minor nitpicking: True volume on al80 is 77.4 cu. ft. not 80.

Could Shearwater possibly have gotten something wrong?

I have always used 77.4 cf in my calculations

Dive Gear Express uses 77.6 cf

 

[scubadada]

I would think that if my Perdix AI says that I am breathing 16 psi/min, it doesn't care what size tank I have. It sees what the tank pressure is at time zero and then at time 1 and subtracts one from the other. Why would it be tank size dependent?...

An AL80 has 2.58 cf/100 psi, a HP steel 120 has 3.49 cf/100 psi

Somebody on SB used to have a great status message, 4 out of 3 people have trouble with fractions

 

[2airishuman]

Now that I have a dive computer that talks to my laptop computer, I have been looking at different data on my dive history. One of the pieces that I have been looking at is my SAC. I know what mine is, but I'm curious as to what other people have for a SAC. Is 8 psi/min good? Is 12 psi/min good? Is 16 psi/min good? Obviously, 8 is better than 16, but how does that compare to what other divers have for their SAC?

The question is threefold: 1) What is a good SAC?, 2) What is your average SAC?, and 3) When does your SAC increase or decrease during a dive?

Well, the tongue-in-cheek answer is that it's like MPG, R-values, and crop yields -- exaggeration is part of the culture. I'm sticking to PG-rated examples.

What is a good SAC. Well it depends mainly on muscle mass, because that determines your resting metabolism. Muscle mass in turn is affected by gender, body size, and how much you go to the gym. So if you are a 5'2" woman with relatively little muscle development you should have a SAC that's less than half of mine, because I'm tall, male, and lift weights.

I tracked my RMV on every dive for a while and found that it generally runs between 0.6-0.8 cfm with some exceptions on either side of that. So that's like 23-31 PSI with an AL80, which I hardly ever use. It isn't really a goal for me to cultivate a low SAC, because I like to move around and see things, and be active. I find that I have a lower SAC on tropical dives where the viz is fantastic and I'm more inclined to stay in one place.

If you can't control your trim then you'll flail around and your SAC will go up, but there's more to the story than that.

Though you did not ask, I am a big fan of the idea that people who breathe more should dive larger cylinders. While I usually dive HP120s, I now find that on most dive boats my own air will not limit dive duration if I have a 100 cf cylinder that is actually full at the start of the dive.

 

[Storker]

) What is a good SAC?, 2) What is your average SAC?, and 3) When does your SAC increase or decrease during a dive?

1) It all depends on the dive and on the diver. Some people seem to have gills, others just blow through a lot of gas.
2) Somewhere between 15 and 20 SLM (surface liters per minute), but I've logged everything from about 12 to more than 25.
3) Short dives increase my apparent RMV, since I use 300 bar tanks. The compressibility effects at pressures above some 200-230 bar affect the measured RMV. Then there's physical exertion (my lowest RMVs have been logged on lazy drift dives), stress level and exposure protection. I routinely log some 10-15% lower RMV numbers when I'm diving wet abroad than when I'm diving dry back home.

I'm asking SAC. I guess RMV would be a good statistic to track also.

IMNSHO surface volume per time is the only meaningful parameter, since it removes any dependency on tank volume.

 

[arew+4]

I generally average .55 to .6, have had a few down into .4Xs and few in the .6Xs. So i not super concerned about consumption, could be better but I'm a fairly large guy. Kind of jealous of my wife and daughter with their consistent .35 to .45 but we give them 63's when possible so we all end up needing to get out of the pool at about the same time.

Does using nitrox have much of an effect on consumption? How about other gases like trimix?

 

[northernone]

Depending who I'm diving with I am between .36 cubic feet per minute and 2.0 cf/m RMV.

Workload, health, mindset and environment all have a significant part.

Practicing efficient breathing patterns also radical changed my consumption.

 

[Degenerate]

I would think that if my Perdix AI says that I am breathing 16 psi/min, it doesn't care what size tank I have. It sees what the tank pressure is at time zero and then at time 1 and subtracts one from the other. Why would it be tank size dependent?
Cheers -

When talking about scuba tanks the pressure means nothing unless you add the tank volume to the equation as well.
16psi in a 10L tank is about 1,1bar, or 11L.
16psi in a 15L tank is also about 1,1bar, or 16.5L.

I have 2 tanks, one 8L 300 bar and one 2x7L 300 bar.
Just looking at the pressure would give me really strange values as my doubles hold almost twice as much as even though the max pressure is the same.
I can end a dive with my doubles with 150 bar (half tank) left no sweat, the same dive profile with my single 8L would mean an almost empty tank when I surface.
Don't know if I misunderstood you, but watching the pressure is absolutely useless unless you take the volume into account as well.

 

[Saniflush]

Somebody on SB used to have a great status message, 4 out of 3 people have trouble with fractions

The one that I always liked that is similar is, there are three kinds of people in the world. Those that are good at math and those that are not.