Let's do some gas planning practice starting with a 0.75 cuft/min SAC rate (also known as RMV -Respiratory Minute Volume, or also referred to as Surface Consumption Rate "SCR", a term I like better), a common conservative reference planning figure that even most novice divers can achieve. 0.75 cuft/min in Imperial is approximately 22 litres/min in Metric.
With a 22 litres/min volume SCR and a variety of different tank sizes, your pressure SCR in bar/min obviously varies inversely, depending on the size tank in use :
22 litres per minute -divided-by- 11L/bar tank (AL80): 2 bar/min
22 litres per minute -divided-by- 13L/bar tank (AL100): 1.7 bar/min
22 litres per minute -divided-by- 22L/bar twinset (double AL80's): 1 bar/min
22 litres per minute -divided-by- 32L/bar twinset (double Pressed Steel 104's): 0.7 bar/min
[note: 1bar/min is equivalent to 14.5 psi/min -working with metric system pressure units in bar is so much easiest than US/imperial PSI units]
Your volume SCR, 22 litres/min in the example above, is the "arbitrary" constant across all metric tank ratings ("Arbitrary" in this instance because consumption rate depends on environmental conditions, physical fitness, workload etc). In the example above, the goal IMO is to utilize a pressure SCR that is commonly understood to be predicated on the metric tank rating in use, because your SPG is is in bar pressure units --NOT volumetric litre units.
So for example, going to 30m (100 feet) depth using a single 207 bar full 11L/bar cylinder with a 22 litre/bar volume SCR, which is the same as a pressure SCR of 2 bar/min: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on a 11L/bar tank in nominal conditions, you would expect to consume 80bar of gas (8 bar/min multiplied-by 10min equals 80 bar) and your SPG reading to be down or show a delta of 80bar. . . another 10 min period at 30m would consume another 80 bar for a total consumption of 160 bar of your gas supply --way too close to possibly running out of air on your ascent & safety stop for a 11L tank --so you would in this instance stay no more than 5min or 40 bar consumed maximum to give yourself (and also your buddy in an emergency out-of-gas air sharing contingency) some extra safety margin. . .
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Elementary Gas Planning and The 18m Beginner/Novice OW Limit. . .
A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Open Water. . . again much easier & understood better with Pressure Bar Units in the metric system.
For a single 11 litre tank (AL80), a total of 11 litres/bar metric tank rating and a volume Surface Consumption Rate (SCR) of 22 litres/min*ATA -same as a pressure SCR of 2 bar/min*ATA (divide 22 litres/min*ATA by 11 litres/bar equals 2 bar/min*ATA)- using an example NDL air dive to 30m (4 ATA) depth in Open Water.
Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with one minute stops every 3 meters to the surface,
-->Just "tally the depth ATA's":
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3
Sum Total: 26.5
Multiplied by 2 bar/min*ATA equals 53 bar Rock Bottom absolute reading remaining on your SPG. --this also happens to be the pressure in bar needed for one person in an emergency contingency to reach the surface with the above minimum decompression ascent profile.
So ideally for a two person buddy team, multiply 53 by 2 which is 106 bar for both to reach the surface (sharing in a buddy Out-ot-Gas contingency).
But realistically, for two experienced divers stressed: 106 bar plus 30% of 106 bar equals 138 bar Rock Bottom SPG reading.
For two novice divers stressed: 106 bar plus 100% of 106 bar equals 212 bar (!!!) A single full 11L/bar (AL80) Cylinder is 200 bar!
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .