Oxygen consentration
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I hope you're having fun up there, haha. Coincidentally, the reason that aircraft is limited to 25,000ft, when it clearly has the power to get much higher, is due to the lack of diluter-demand oxygen masks, which you might have seen in some of my other aircraft. The face masks that are required for flight above 18,000 feet are insufficient to keep the body properly oxygenated above 25,000, hence the declining SpO2.
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I think that behavior might actually be expected. The continuous-flow masks are only effective up to about FL250. As you approach that altitude the oxygen partial pressure becomes too low for the mask to maintain normal blood oxygen saturation, so the SPO2 reading starts to drop. The FL250 ceiling is basically the altitude where the system can still keep the pilot functional, not necessarily at 90% saturation. For extended flight, most guidance recommends keeping SPO2 around 90%, and increasing oxygen flow or descending if it starts dropping into the high-80s.
