How is the ascent time from 50 fsw to 40 fsw factored into the oxygen period?

The ascent time from 50 feet of seawater (fsw) to 40 fsw is included in the calculation of the oxygen period because any time spent at a given depth, regardless of whether it involves a descent or ascent, contributes to the overall experience of partial pressures of oxygen. In this context, as the diver ascends, they are still considered to be under the effects of the dive until they reach a shallower depth where the partial pressure of oxygen is different.

Including the ascent time ensures that all phases of the dive are accounted for when determining how long a diver is exposed to elevated levels of oxygen. This is important for understanding potential oxygen toxicity and for planning safe dive profiles. The consideration of ascent times reflects a comprehensive approach to physiology and safety in dive training, ensuring divers maintain a clear awareness of their exposure to varying levels of oxygen throughout the entirety of their diving activities.