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• For high pressure cylinders of nitrogen (Max Dog or equivalent) or helium, regulation is essential. Flow regulators are provided with the Max Dog system or can be purchased from gas providers. The Max Dog regulator shown in Fig 5.9.2 gives the pressure of nitrogen in the cylinder, and allows the output flow rate to be set at 15 litres/ min. Using this regulator, flow rate is constant throughout the hypoxic death.
• For disposable ‘Balloon Time’ helium cylinders at pressures of 1.7MPa, Exit has designed gas flow control fittings expressly for this purpose (Fig 5.11). These fittings give an initial flow rate of ~20 litres/min when the tap is opened fully. Exit helium flow fittings are available at: http://bit. ly/9swOxk
For the low-pressure nitrogen system described (Fig 5.3) with filling pressures of 2.8MPa (400psi), a standard LPG regulator restricting the outlet pressure to ~ 2.8kPa with a jet (size ~2mm) will provide the appropriate flow rate.
Fig 5.9.2
High pressure Max Dog nitrogen regulator with click flow rate output
Fig 5.9.1
LPG regulator and 2mm jet used to control nitrogen flow