What is a good SAC?

[MichaelBaranows]

I am new to diving and I was wanting to know where my SAC compared as a new diver. I dive using a standard 80cf tank and have been diving around 25' and my SAC a minute seems to be about 30. I was thinkng that I read about 45 reef dives at 60' and there is no way that I can make a dive that long.

Right now I am not worried cause I have been doing lake dives and my buddy has a SAC than me. So I know to watch my air to make sure he has enough. But once I start making deeper dives I will need to improve my SAC.

 

[ScubaSixString]

I am new to diving and I was wanting to know where my SAC compared as a new diver. I dive using a standard 80cf tank and have been diving around 25' and my SAC a minute seems to be about 30. I was thinkng that I read about 45 reef dives at 60' and there is no way that I can make a dive that long.

Right now I am not worried cause I have been doing lake dives and my buddy has a SAC than me. So I know to watch my air to make sure he has enough. But once I start making deeper dives I will need to improve my SAC.

I read somewhere the average is 25. I calculated mine (sitting on my couch with the reg in my mouth (probably useless as i had no other gear on)) and it came in at 25. I dove the next day to 25 ft and was at 500 psi in 30 minutes.

A "good" SAC is one that gets the job done. with a SAC of 30 for a given size tank ( i would reccomend converting to Respiratory Minute Volume (take the volume of the tank used (3000 psi for example) divide by size (80cf) to give you psi per CF. (37.5) then divide your SAC by that number (30/37.5 )to get RMV=0.8.

The RMV will allow you to plan using different size tanks. At any rate. you mentioned diving to 45-60 ft. Lets run the numbers.

For 45 ft= Depth consumption rate (DCR) = SAC (30) x (45+33)/33 =70.9 (round to 71) psi. a minute.

3000/71= 42 minutes at that depth.

For 60 ft= DCR= 30 x (60+33)/33=84.54 (round to 85) psi

3000/85= 35.3 minutes.

NOTE: These figures assume that you start with 3000 lbs. the DCR's won't change but your dive time will if you start with less.

YOU SHOULD KNOCK AT LEAST 4-5 minutes off these times to account for leaving air in your tanks/ emergencies/etc. AT LEAST.

Other than that, fitness, comfort, and low workload will improve air consumption. If you find a trick, let this air hog know

 

[mikkilj]

The RMV will allow you to plan using different size tanks.

Is RMV = SCR?

Sorry for such a newbie question... We have all of our numbers converted to cu. ft. per minute so that it translates to different tank sizes...

 

[MichaelBaranows]

That is alot of information. I think I might have to read it a few times to realy understand everything you wrote.

 

[ScubaSixString]

Is RMV = SCR?

Sorry for such a newbie question... We have all of our numbers converted to cu. ft. per minute so that it translates to different tank sizes...

No need to apologize. I have way too much free time on my hands...thats the only reason i was able to answer

RMV (Respiratory Minute Volume) is based on the Volume of gas you breathe (IE cubic feet.) So you could use that number to determine how long a 120CF tank would last based on a RMV determined using an 80CF tank. If your numbers are converted to CF then you are using RMV and not SAC/SCR (although the difference is purely semantics. Consuming .66 CF per minute at the surface IS a surface consumption rate)

 

[lamont]

by 30 do you mean 30 psi / min / ata on an Al80?

if so, that's around 0.75 cu ft / min / ata which is a pretty standard newbie SAC rate and a good rate to use for planning. around 0.60 cu ft / min / ata can usually be obtained by any experienced diver, 0.50 is pretty good (i'd be happy with that), and 0.30 is right around the best you'll typically see.

 

[ScubaSixString]

That is alot of information. I think I might have to read it a few times to realy understand everything you wrote.

I'd be glad to answer any questions you have (with the caveat that i'm rather new to this too.... like i said, i burned an 80cf tank down to 500 psi in 30 minutes at only 25 ft)

 

[ScubaSixString]

That is alot of information. I think I might have to read it a few times to realy understand everything you wrote.

The math is a matter of accepting that it works (i tried to figure out where the numbers came from and got lost)

Consumption rate at depth = SAC x (Depth + 33) /33

 

[Rick Murchison]

RMV (Respiratory Minute Volume) is a term borrowed from medicine. There it is usually expressed in liters per minute, and as it's a medical term it applies to the surface. As adapted to diving, it is usually expressed in standard cubic feet (1 cubic foot at standard temperature and pressure) per minute, corrected to surface pressure. SCR (Surface Consumption Rate) is usually expressed the same way - CF/M or more often abbreviated CFM. If you use CFM you can use that with any size tank easily.
(Some folks will say that RMV is "actual volume breathed per minute" regardless of depth - it's all the same thing)
SAC rate is usually expressed in psi per minute and is only good for one size tank.
There are several threads (search on SCR or RMV) here with detailed explanations and formulae.
Rick

 

[The Kraken]

Anything greater than zero . . .

the K