Content oxygen cylinders: Oxygen cylinders come in different sizes and have various pressure capacities. Cylinders are identified by letter according to their size. Manufacturers stats vary, some say D cylinders (the most common size) hold 300 to 350 litres, others 415. E = 625/682 M = 3,000/3,450, 1723 G = 5,300, H = 6,900. At 12 LPM a 415 litre tank will run for an average of 35 minutes. A D tank, with 300-350 litres will run for 50 minutes at six litres per minute, or 35 minutes at 10 LPM, or 30 minutes at 10 LPM. An E tank, with 600 litres would run for 1 and 3/4 hours at 6 litres per minute, one hour ten minutes at 10 LPM or one hour at 10 LPM.