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 rate that 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 :
For the most common tank used at Dive-Ops overseas & worldwide (AL80):
22 litres per minute -divided-by- 11L/bar tank (AL80's metric tank rating): 2 bar/min;
22 litres per minute -divided-by- 13L/bar tank (an AL100 tank): 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- 34L/bar twinset (double Pressed Steel 104's with a cave fill): 0.7 bar/min
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18m Beginner/Novice OW Limit
A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Open Water. . .
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 -same as a pressure SCR of 2 bar/min*ATA (divide 22 litres/min by 11 litres/bar)- using an example NDL air dive to 30m (4 ATA) depth in Open Water.
Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with slow conservative one minute ascent stops every 3 meters to the surface,
-->Just "tally the 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 (!!!) --Note a full AL80 is 207 bar pressure. . .
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .