SAC rate calculation...

[scottfiji]

I don't have an air-integrated computer.

If I want to know my SAC rate for a given multi-level dive, and my buddy's computer calculates their SAC rate, and we both swim near each other (similar depths), then I can do the following, right?

my SAC = (my buddy's SAC * my air usage (in cubic feet) ) / my buddy's air usage

thanks,
Scott

 

[ShakaZulu]

Gee Scott, put down the crackpipe my friend..........

 

[scottfiji]

Gee Scott, put down the crackpipe my friend..........

I've tried, but I just can't seem to do it...

 

[hermosadive]

I think it will be just as easy to learn the following calculations.

http://home.flash.net/~table/gasses/sac.htm

You'll have to adjust the formula to allow for your DCPC or Daily CrackPipe Consumption.

 

[scottfiji]

You guys are both really good at not answering questions... I hope the MOD's boot both of you off...

people, please feel free to ignore these goofballs and answer my question....they are both kranky because they are not diving today.

 

[The Kraken]

Just take your buddy's computer generated sac rate and multiply it by the percentage you get when you divide your volume consumption by his.

IE: His pooter said his SAC was .45.
He used 60 cf of air and you used 50 cf. 50/60 = .83. So, following along, .83x.45 = your SAC of .37.

 

[spectrum]

You guys are both really good at not answering questions... I hope the MOD's boot both of you off...

people, please feel free to ignore these goofballs and answer my question....they are both kranky because they are not diving today.

OKay, I'll bite....

Algebra says your formula will work. You must as you say convert both consumptions to cubic feet. Pressure drop would also work if the tanks are the same size.

Note my credentials but I think I've learned a lot here and I'm about to learn more.

Pete

 

[fmw625]

IE: His pooter said his SAC was .45.
He used 60 cf of air and you used 50 cf. 50/60 = .83. So, following along, .83x.45 = your SAC of .37.

Kraken, The SAC in the artical is in psi per minute and is in whole numbers. this confused me because I use to seeing SAC in decimal numbers like you posted. Your numbers are in cubic feet per minute. So a .43 SAC on a AL80 on the surface equals about 162 min. At 33ft 162/2 or 81min and at 66ft 162/3 or 54 min, Am I doing this right?

 

[kalvyn]

I don't have an air-integrated computer.

It makes no nevermind of your computer is air integrated or not. Does your dive computer or gauge give you average depth for a dive? If it does, that's what you need to know to compute your SAC rate.

For example, I just got back from a couple dives with OE2X, Uncle Pug and NWGratefulDiver. UP and I dove as a team on the second dive, and my average depth was 51 fsw.

So, with an average depth of 51 fsw, a dive time of 56 minutes and I used 2200 PSI on the dive (using an E8 119 with a short fill), my SAC rate was .53. I used this link to do the math for me:

http://www.spearfishing.org/bruces_tips/java/sac.html

Hope this helps!

Jimmie

 

[DA Aquamaster]

Kraken, The SAC in the article is in psi per minute and is in whole numbers. this confused me because I use to seeing SAC in decimal numbers like you posted. Your numbers are in cubic feet per minute.

Technically speaking "SAC" is by definition PSI per minute and is consequently tank dependent. For example 1 cu ft in an AL 80 is 39 psi (3000 psi / 77 cu ft) but 1 cu ft in a Faber steel 95 is only 27 psi (2640 / 95) and in a PST 120 is 22 psi. (2640 / 120).

So if you figure your SAC with one tank and then use another, you need to convert the SAC found with one tank to cu ft and then convert that cu ft /min figure to psi /min in the new tank. When you convert a SAC to cu ft/min, you have an RMV (Respritory Minute Volume).

Similarly, if your buddy's computer figures a SAC, you need to know the volume at a given service pressure for his tank so that you can convert his SAC to an RMV. You can then compare the total volume of air he used with the volume of air you used and use that same ratio to figure your RMV.

But lately it is common to do everything in cu ft and call it a SAC even though it is really an RMV. It can get confusing id the diver does not recognize the difference.

So a .43 SAC on a AL80 on the surface equals about 162 min. At 33ft 162/2 or 81min and at 66ft 162/3 or 54 min, Am I doing this right?

That works but it is confusing and less usable. It makes more sense to adjust the Surface Air Consumption, (in either cubic feet or psi) for the depth as that is purpose of figuring a SAC in the first place.

Plus you can then figure your consumption at various depths and do gas planning for a multilevel dive, deco stops etc, which otherwise would become very cumbersome and confusing to do dividing how long a tank lasts at the surface by the atmospheres of pressure at each depth.

Gas consumption at depth is figured with this equation: [1 + (depth/33)](SAC).

So with a SAC of .43 cu ft., your air consumption at 66 ft will be [1 + (66/33)](.43) = 1.29 cu ft/ min. So at 66 ft. your 80 will last 59 minutes with no reserve (77 cu ft/1.29 cu ft/min=59 min), or 49 minutes with a 500 psi reserve (2500 psi / 39 psi/cu ft = 64 cu ft, and then 64 cu ft /1.29 cu ft/min = 49 min)