Sorry. Are y'all using different calculations from these? Based on your definitions, there must be some difference. According to the NOAA Air Requirement Formula, SAC is not cylinder specific. Actually RMV is calculated based on SAC for a specific cylinder constant. Also, RMV is a volume measurement of consumption at the surface, not at depth. So if you have other equations/definitions, I'd love to see them.
If their calculation gives an output in PSI per minute, then it is tank dependent. You don't have to go any farther than the unit analysis to see it, regardless of what they might say about it.
If I use 50 psi per minute from an AL 80, I am using 1.28 cu ft per minute:
77 cu ft / 3000 psi = .0257 cu ft per psi, .0257 cu ft per psi x 50 psi = 1.28 cu ft.
If I am using 1.28 cu ft at a depth of 30 feet:
1) I am at 1.9 ATA;
30/33+1 = 1.9
2) and my SAC/RMV/SCR or what ever else you want to call my rate of gas use at depth corrected to 1 ATA at the surface will be .67 cu ft per minute.
However, if I was diving a steel 72, my SAC is now
71 cu ft / 2475 psi = .0286 cu ft per psi, .0286 cu ft per psi x 50 psi = 1.43 cu ft., 1.43 / 1.9 = .75 cu ft per minute.
It's not a huge shift, but it's a change. However, if I have a set of double 100s and I use 50 psi per minute at 30 ft, I'm an incredible gas hog:
200 cu ft /3442 psi = .0581 cu ft per psi, .0581 cu ft per psi x50 psi = 2.90 cu ft., 2.90 cu ft / 1.9 = 1.53 cu ft per minute.
Thus any unit of gas consumption, if defined by psi per minute, is extremely tank dependent.
Actually RMV is calculated based on SAC for a specific cylinder constant.
Taking that back to cave diving practice, if I have a SAC rate (whether you call it SAC, RMV, SCR or even just plain Bob) of .6 cu ft per minute, and I am doing a cave dive with a diver using an pair of AL80s, another diver using a pair of steel 72s while I am still using a pair of LP 95s I'll know that:
The pair of steel 72s has a tank factor of 5.8 (0.286 x 100 = 2.86, rounded to 2.9), the pair of AL 80s have a tank factor of 5.2, and the LP 95s have a tank factor of 7.2 (95/2640x2=.0719, .0719x100 = 7.2). This means that 100 psi is equal to 5.8 cu ft, 5.2 cu ft and 7.2 cu ft resepctively.
If we are diving into a cave with strong outflow where I'm comfortable using a fill third of gas, then I'd plan on using 1200 psi of my 3600 psi filled LP 95 tanks, and that would be 86.4 cu ft (7.2x12=86.4). However, if I lose all my gas I'll need to exit on another diver's reserve gas, so the smallest volume tanks come into play here. in this case, 1/3rd of the double 72s needs to be considered since they hold 154 cu feet at 2475 psi, providing a smaller third of only 47 cu ft. (The AL80s hold 154 cu ft at 3000 psi and have a 51 cu ft "third", however if the steel 72s were overfilled to 3000 psi, then they'd hold 174 cu ft (5.8 x 30=174) with a 58 cu ft "third" rand the AL 80s would now be the smaller set.)
So I'd want to limit my larger tanks to a smaller third of only 47 cu ft to match the steel 72s. 47 / 7.2 = 6.53, or 650 psi, meaning that if I started with 3600 psi, I'll now turn the dive at 2950 psi rather than 2400 psi. to ensure I use no more than 1/3rd the volume of the smallest set of tanks on the team.
That's where a correction based on tank volume comes into play, but whether you do it all in psi, work with cubic feet or use a tank factor that creates a volume of gas per 100 psi unit. all the tanks involved have to be normalized so that each diver knows how much gas in psi can be used on the dive.
In planning terms, if the average depth of the cave is 90 ft and I am swimming 50 feet per minute, with a SAC of .60 with 47 cu ft available for the penetration I know that I will be at 3.7 ATA (90/33+1=3.7) using 2.24 cu ft per minute (3.7x.60=2.24), and can swim for about 21 minutes (47/2.24=20.98) and 1050 ft (21 minutes x 50 feet per minute) before I have to turn the dive.
Of course, I could work it in psi as well:
0.60 cu ft per minute in double LP 95s is 8.3 psi/minute (7.2/100 = .60/X, [.60x100]/7.2 = 8.3),
8.3 x 3.7ATA = 30.71 (30 psi/minute)
650 psi "third" / 30 psi per minute = 21 minutes, x 50 feet per minute = 1050 ft.
So it really does not matter what you use or what you call it, as long as you understand the basic concepts involved, and I am pretty sure that if I dive another 28 years, the argument about SAC versus RMV will still be raging and will still be just as meaningless - and psi or BAR will still be tank dependent if divers are trying to equate them with a specific volume.