European cylinders have the cylinder details stamped on the neck.
Volume in litres, Working pressure in bar, test pressure in bar, Serial number, date of manufacture and manufacturing specification.
1. AVAILABLE GAS - TURN PRESURE - RESERVE
A 10litre cylinder filled to 225bar.
10x225 = 2250 litres of available gas.
Crude rule of thirds 2250/3 = 750 litres of gas as a reserve
Or 225(bar)/3 = 75litres as the reserve pressure (in bar).
Turn point for the dive = 75(bar)x2 = 150bar. so turn for home at 150 bar, on the surface at 75 bar.
2. HOW LONG?
If your breathing rate is 25litres a minute (surface breathing rate). Planned depth is 20m.
25x3 (absolute pressure at 20m = 3 bar) = 75 litres a minute
150bar available for dive (crude rule of thirds) = 20 minute dive time, (10 minutes out, turn at 150bar, 10 minutes back, on surface with 75 bar (10 minute reserve).
3. WHAT IS MY BREATING RATE?
Enter water with 10 litre cylinder filled to 200bar, to a depth of 20m. for 10 minutes. (Square profile dive)
Gas on exiting 140bar.
200-140 bar = 60 bar used.
60 bar x 10 litres = 600 litres of gas used.
Depth = 20m, (3 bar absolute), time in 10 minutes.
600/10 = 60 litres per minute at 20 meters (3 bar absolute)
60/3 = 20 litres a minute surface breathing rate.
Hope that helps.
Gareth
(Someone will check the math and find a mistake

)