What's a good SAC

[dmaziuk]

1) 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's when you get out of the water when the dive is over, and you have a few hundred psi left in your tank.

2. 0.5-ish but it's hard to be accurate without a WAI computer.

3. Trying to get a good angle on that $coolthing while holding it in focus. Trying to catch up with the buddy/group after I'm done. Rinse, lather, repeat.

 

[JohnnyC]

Your SAC is what it is. It will get better the more relaxed you are in the water. Generally speaking, consumption is related to workload. Don’t forcefully try to improve it, that leads to things like CO2 retention which inevitably causes your consumption to go up.

You need as much gas as your body needs. If that means you need a bigger cylinder, get a bigger cylinder.

 

[markmud]

Hi MargaritaMike,

A good RMV is .5 cu/ft./min. It works for me. I was tested for my RMV twice at Monterey. Both RMV runs were in the middle of winter with lots of rubber on (the RMV was actually less than .5 cf/minute).

A few years ago I went back and checked my log book (when I was keeping track of gas consumption). On tropical dives, I use 1 cubic foot per minute average. That is top to bottom, safety stop and all. This is multi-level diving with an average max depth of about 70'.

On a good day, I will use 55 cf on a 60 minute dive (850 psi remaining with an AL80). On a bad day, after 60 minutes I am down to 5-600 psi with an AL 80 (65 cf used).

I am 6'0" and 180 pounds.

I attribute my RMV rate to cross training (running) and a hatred for using my BCD inflator. At the Surge in Curacao, I used my BCD inflator once on the surface while we were waiting for the boat (drift dive).

If I were 6' 6" and 230 pounds, a AL 80 would never last me an hour. Size matters.

dive on,
markm

 

[RayfromTX]

First dive day of the trip in warm clear water is about rmv .54
By the third day of the trip it is down to .46
In cold dark water it's going to range between .7 and .5

 

[Graeme Fraser]

Even though I try to track RMV, I convert it in SAC for the tank I use because I can then track my usage underwater, you still have to adjust by the pressure.

(I wish my computer displayed the RMV over the last x minutes)

Check to see if your dive computer has an average depth (Av Dep) field in the dive log. If not on the DC screen it may be shown once you download onto your PC or tablet.

If so, as long as you note your start and finish pressure it's very easy to work out an accurate SAC rate for various dive profiles. Simply multiply used pressure by cylinder volume, divide by dive time, then divide by ATA (Av Dep /10 + 1). Example:

Start Pressure = 210b
End Pressure = 90b
Used Pressure = 120b
Cylinder Volume = 12L
Dive Time = 40min
ATA = 2.5 (Av Dep 15m / 10 + 1)

UP 120b × CV 12L = 1440L
1440L ÷ 40min = 36L per minute
36L ÷ 2.5 ATA = 14.4 L SAC rate

 

[MargaritaMike]

First dive day of the trip in warm clear water is about rmv .54
By the third day of the trip it is down to .46
In cold dark water it's going to range between .7 and .5

After several years of diving (about 32) and based on my PERSONAL preferences, I just can't understand why ANYONE would dive in cold dark water.

Cheers -

 

[admikar]

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?

Some interesting thinking. Keep in mind that OP was about good SAC. I don't know what tank you're diving, lets assume it's AL80. Now take a double AL80 and try to maintain 16 psi/min breathing rate, then you'll see why it is tank size dependent

 

[tmassey]

You guys are all making me feel better about my air consumption!

I mentioned mine was 0.68 cu ft / min. That is in coldish darkish water (a quarry), doubles and a drysuit, which seems in line with many other experienced divers. I feel a little better about that now! Plus I’m 6’2”/235.

And yes, all of this is RMV. Pure SAC is fairly useless unless you only dive the exact same tank configuration every single time. Easier to measure, but pretty useless in general. I personally don’t have AI, so I have to calculate based on PSI used, depth and time. So I will periodically swim squares at 33 feet at the quarry for 10 minutes, noting the start and stop pressure. Then, using the tank volume, percent used, time and pressure (33 feet is 2 ATA), I have my RMV.

 

[markmud]

And yes, all of this is RMV. Pure SAC is fairly useless unless you only dive the exact same tank configuration every single time.

Hi tmassey,

Your point is well taken.

My instructor of record, uses SAC and RMV interchangeably. The expanded acronym SAC is: "Surface Air Consumption." It is not "PSI consumption averaged over various depths with any tank".

I am a literal person. Raw SAC data is useless based on psi, other than a quick reference while u/w. It is useless for a myriad of reasons that are elementary to experienced and trained divers.

We tested my gas consumption u/w while swimming at a moderate clip while holding depth (Monterey; winter; wetsuit). This was an instructor led and supervised test.

We then used the well known formula for converting the test data to my surface air consumption. My rounded up SAC/RMV is 1/2 cf a minute.

During my Tec 40 class, I asked him about SAC/RMV. I was confused. He rolled his eyes and explained the issue. I paraphrased what he said above.

@Bob DBF earlier wrote:
"I guess it would depend on the tank you are using."

"RMV works better as it is not tank dependent, so you can leave off the tank size without confusion. The terms are used interchangeably by many, so stating SAC in cuft/min is not abnormal."

IMHO, Bob knows his sheet as usual.

Dive on,
markm

 

[Graeme Fraser]

SAC is purely RMV adjusted to ambient pressure at sea level. It's a simple formula to apply to various depths and cylinder capacities. Yes it's of limited value 'in water' but still a useful tool for gas planning.