fmw625:
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.
fmw625:
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)