There are circumstances in which the cylinder flows and mixtures may present a situation of hypoxia for the diver. The worst situation is using a lean cylinder by a plane at low speed. In the example given in Section 6.3, if the Ducks were VO 2 to 1.9, so the loop is fO2 0.20. Even in a VO2 of 1.35 would fO2 of 0.30, which would be 30% that diving was planned.