How's your SAC?

[Diver0001]

During AOW, one of the things we were taught was how to measure our SAC. We went through the calculations and logged it after each dive, but none of the instructors gave us any idea of what an average or good number is. Is it one of those things where guys just don't like to compare SACs? What's a good number?

An average adult at rest will use about 12 litres a minute. In diving anywhere between 12 and 14 is average to good. The 14 to 18 range is common for beginning divers and when diving in current. In stressful situations you can count on your SAC being well over 20 litres per min.

There are extremes on both the high and low side of what I just mentioned. Many divers can reach a SAC of 10lpm or a bit lower over time. I can't do that unless I'm diving in the tropics.

And remember, knowing your SAC has a function for dive planning. it's not a competition.

R..

 

[jonnythan]

That's pretty good!

My SAC rate goes up almost 50% when switching from singles to doubles ... then again, so does everything else ...

... Bob (Grateful Diver)

The cool thing about going from single 130s to double 130s is that the increase in SAC does not affect your bottom time at all. Even if you breathe the single tanks down to rock botom, on the second dive you have to leave 20cf+ on shore in the "empty" tank while you dive the second. With doubles you get to take that 20cf back down with you on the second dive.

Then, when you'd normally have two tanks with 40cf in each of them, neither of which are good enough for a meaningful dive, you have 70-80cf still in your doubles!

I can't believe I bought a pony bottle just so I could avoid putting my tanks together. Geez.

 

[Diver0001]

That's pretty good!

My SAC rate goes up almost 50% when switching from singles to doubles ... then again, so does everything else ...

... Bob (Grateful Diver)

You've probably got a trim issue, Bob. My SAC is the same when I'm on twins. Even rigged for wreck diving (twin 12's, 7 litre stage, reels, balloons, lights, drysuit) I'm a very consistent 12-14 litres per min (.42-.45). That's only a shade higher than I have in my puddlestomping gear.

R..

 

[Charlie99]

In warm water single tank diving I run pretty consistently:

just under 0.4cfm -- very lazy drift dive, slow wandering around the reef
0.45cfm normal swim around the reef speed, perhaps 1/2 kt
0.5 cfm -- 1 kt transit speed. going somewhere, but not fighting current
0.6cfm to ??? -- swimming hard, fighting a current.

These numbers, as are most so far posted, are lower than what you see on a typical resort boat. Rarely do I end a group boat dive with less than 1000psi.

A diver with a 1.0cfm SAC will stand out as an airhog.
At 0.75cfm, you may or may not be the first one up out of a group of 6.
At 0.6cfm you are at the average or better than a typical diver in a resort place like Cozumel, Maui, or the Florida Keys.

SAC is best treated as a number that you use for planning, and a number that gives you some feedback on how efficiently you are swimming.

Charlie

BTW, to convert Diver0001's numbers use 28.3 liter/min = 1 cu ft/min.
When using an AL80, figure 38.75psi/cubic ft. (SAC numbers are not very repeatable, so 40psi/cu ft is an easier approximation)

 

[-hh]

An average adult at rest will use about 12 litres a minute. In diving anywhere between 12 and 14 is average to good. The 14 to 18 range is common for beginning divers and when diving in current. In stressful situations you can count on your SAC being well over 20 litres per min.

There's 28.3 liters in a cubic foot, so 12lpm --> 0.42 ft^3/min (cfm)

14, 18 and 20 lpm would be .50, .64, and .71 cfm respetively.

IMO, its going to depend quite a bit on how much active swimming you're doing on the dive...its not uncommon for 'zoomers' to be up at 2.0 cfm.

As per the USN Dive manual, their average breathing consumption rates are listed as 1.4 for Normal and 2.5 for Heavy work. Of course, this is with a Mk21, so there's some efficiency losses present, plus this is a planning document of which they expect "all" divers to be no worse than. IIRC, in older versions, they've also published a "Very Heavy Work" value of 4.0 cfm


-hh

 

[teknitroxdiver]

I calculated mine the other day, it was .5. However it fluctuates from .4 to .7 depending on conditions. I've found that if I'm leading the dive, or doing a solo dive it tends to be much lower than in a group trailing a DM.

 

[Daryl Morse]

Calculating SAC is also a good way to track your progress as a diver ... as you get more experienced and learn how to dive more efficiently you should see your SAC rate dropping ... sometimes significantly.

I started tracking my SAC when I started diving with a computer, which was after the first 20 or so dives. Without knowing the exact value, I could tell my SAC had already dropped quite a bit from the early dives just by noting the longer bottom time.

Since I started tracking the numbers, it gradually decreased to 0.6-0.65 cfm and recently I've been hitting 0.5 cfm on some dives. It makes a big difference for me if I concentrate on my breathing, particularly at the start of the dive. I tend to hoover quite a bit while initially decending unless I consciously breathe slowly.

 

[Hyper-limits]

IMO the best way to check your SAC rate is to swim a little faster than normal, anybody can have a really good SAC while diving in perfect condition (no currents).
My average is .6 when diving doubles and stage and usually around .5 for deco and single tank diving.

 

[Doc Harry]

My SAC:

0.50 at rest at 70 degrees F
1.00 during heavy work at 70 degrees F
0.75 for travel at 70 degrees F

I have confirmed my SAC and planning over multiple dives:

I plan my dive using estimated SAC rates (e.g., estimate 0.6 for descent, 0.75 for travel, 0.5 for safety stop, etc.). Then I record my PSI and elapsed times during the dive. Of course, I follow my dive plan.

Finally I use my actual dive data and compare to my planned numbers. I notice the differences and make adjustments to my estimates during the next dive plan.

I have become pretty good at estimating my gas usage to an accuracy of 200-300 pounds at the end of the dive.

Of course, if you don't dive your plan then the numbers don't really apply.

 

[teknitroxdiver]

Well, I just whipped out a little Excel spreadsheet and calculated my SAC over 21 dives, and came up with .495. Lowest was .429, highest was .604 on a dive involving filming some very fast eagle rays.

I did the same with my sister's data: She had a SAC rate of .392. Her lowest was .355, her highest was just .411. Wow.