Three minutes of tidal breathing are required. The shallower breaths take longer to raise the O₂ concentration in the alveoli but is normally more pleasant for the patient (Fig 1).

With a circle system the oxygen flow should be at least equal to the patient's minute volume (6 L/min or above) in order to keep the inspired oxygen concentration at 100%.

For a Bain system the oxygen flow must be greater (9 L/min or higher) to minimize rebreathing of both nitrogen and carbon dioxide.

Fig 2 shows the oxygen flow needed for tidal breathing method using a circle system.