The ambiguity of an SAC expressed in units of pressure over time is the absence of a measure of volume. If a diver says that he breathes at the rate of 2.5 psi/min he has not told us anything useful until we know what size tanks he is breathing from. The math is simple - no more complicated than freshman high school algebra. It is all doable in your head (or at least in mine).
I find it simpler to reckon my gas volume in fractions of my total tank volume. For example if I am diving my LP95's, then I have 250 CF to start (at 3500 psi). At my breathing rate of .4 SCFM that is a bit over 600 minutes at 1ATA or 75 minutes at 230 feet. A one third rule gives me 25 minutes at depth. All this is planned beforehand.
Checking my spg on the dive should show my gas being depleted at a rate such as to bring me to my 2/3 mark at my ascent time. I do not really need to do any complicated calculations on the dive, I can tell by spot checks if I am on track.
For example, 12 minutes (half way) into the dive I should be at about 5/6 of my starting pressure or about 2900 psi. Note that these numbers are all approximations. You can only read an spg to +/- 100 psi and you can only measure your breathing rate to +/- 0.1 SCFM (if that).
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A Better & Clear Gas Planning Methodology Utilizing Metric System:
If you understand your pressure per time rate is based on your tank's rating, then there is no ambiguity. And it's easier to do it all pre-planning & on-the-fly using the Metric System. . .
For your example of LP95's at 230 feet/69 meters:
Your SCR (Surface Consumption Rate) of 0.4 SCFM converts to 11 litres/min*ATA in metric.
Your total metric tank rating for double LP95's is 30 litres/bar.
Therefore your pressure SCR rated for your double LP95's, is 11
litres/min*ATA divided-by 30
litres/bar which equals approximately 0.4 bar/min*ATA.
So at a depth of 230' or 69 meters (same as 7.9 ATA):
Your DCR (Depth Consumption Rate) will be 0.4 multiplied-by 7.9 equals 3.2 bar/min. Hence for every minute of elapsed time, I expect the SPG to decrease by 3.2 bar when at a depth of 230'/69m.
Thus in 10 minutes of nominal swimming at 230'/69m, I expect the SPG to be down 32 bar from the previous reading. In 10 more minutes, I expect the SPG to be down another 32 bar; in 5 more minutes after that, I expect the SPG to be down 16 bar from the previous reading --and now you're at your third's-turn pressure of 80 bar total delta consumed, with 160 bar remaining pressure (out of 241 bar full fill) as indicated by the SPG.
Re-Check Arithmetic Thirds Turn Pressure & Time: Your fill of 3500 psi is equivalent to 241 bar in metric. One-third of 241 bar is approx 80 bar delta.
Summing the above expected SPG time check readings at respectively 10min, 20min and 25min elapsed time at 230'/69m: 32 bar plus 32 bar plus 16 bar equals 80 bar total delta consumed. Check -->As you stated above and is confirmed, your planned third's turn pressure in this example would be at the 25 minute mark with 80 bar consumed. Your expected SPG reading at thirds turn pressure should indicate 160 bar remaining (241 bar full fill minus 80 is approx 160 bar remaining).
This illustrates my point using your own example that there is no ambiguity in the use of pressure units per time rate during the dive, provided that you understand that it is mathematically dependent upon your total tank rating of the cylinder(s) in use. . . (
And again, since your SPG reads in bar units so it makes better sense to work with pressure units instead of volume units).
