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There are two sizes of cylinder available, a larger cylinder which contains 420 litres (14.9 cubic feet) of compressed helium at a pressure of ~ 1.5MPa or a smaller cylinder that contains 250 litres (8.8 cubic feet) of compressed helium at a similar pressure. (Fig 5.5).
While there is enough helium in a small cylinder for a peaceful death, close control of the gas flow rate in the small cylinders is essential.
The larger 420 litre cylinders give greater margin for error and are preferred for this reason. This cylinder will produce a usable stream of gas that will run for approximately 20 minutes; more than enough time for a peaceful death to occur. Useful flow rates are obtained from either cylinder (>5 litres/min for 15 minutes with the small cylinder, and 25 minutes with the larger - see Fig 5.5a).
Fig 5.5a: Flow Rate, Pressure and Volume of Helium with Time for 420 & 250 litre Cylin- ders using the Exit Gas Control Jet Assembly