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One bottle system (20 cm in diameter) with sterile water filled to the 0 mark to provide a water seal and a low resistance one-way valve. The distal end of the drainage tube is immersed 2 cm below water. This depth determines the hydrostatic pressure needed to overcome expiration. The collection chamber should ideally be placed at least 100 cm below the chest to prevent the chamber fluid getting sucked back during inspiration.
