An emergency gas planning & reserve example for a dive to the Basic Open Water Limit of 18m/60':
Emergency "Stressed" Breathing Rate of 30 litres/min SCR (Surface Consumption Rate), with 18 meters (2.8 ATA) max depth NDL, and with 1 minute stops every 3 meters to surface (can be performed as 30 seconds hold at depth and then 30 seconds to ascend 3 meters to the next shallower depth and hold again for 30 seconds; then repeat these ascend 30sec & hold 30sec intervals every 3 meters --all the way to the surface):
2.8 x 30 x 1 = 84
2.5 x 30 x 1 = 75
2.2 x 30 x 1 = 66
1.9 x 30 x 1 = 57
1.6 x 30 x 1 = 48
1.3 x 30 x 1 = 39
1.0 x 30 x 1 = 30
Sum Total: 399 litres gas needed to ascend to surface for an emergency contingency.
Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank for each member of the buddy team, which has a metric rating of 11 litres/bar. So 399/11 = approx 36 bar.
That's 36 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 36 x 2 = 72 bar. Therefore your Air Reserve is 72 bar showing on your SPG --if there is no emergency air sharing contingency at that exact instant at 18m depth with 72 bar remaining, just continue the dive as normal but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5m safety stop with your buddy with no less than 50 bar showing on the SPG.
Note: an arbitrary value of 3 meters/min emergency ascent rate as used above is a very conservative, slow and easily controllable ascent rate for the novice diver buddy team performing an emergency air share. Even though the max recommended ascent rate is 9m/min, I would not calculate the above air reserve plan with an ascent rate any faster than 6m/min.
