I use an oxygen generator in my airplane when oxygen is required by the FAA (above 12500 ft for 30+ minutes, above 14000 ft at all). This has a visual flow-mete

I use an oxygen generator in my airplane when oxygen is required by the FAA (above 12500 ft for 30+ minutes, above 14000 ft at all). This has a visual flow-meter on it, but oxygen generators have an additional failure mode where gas is flowing, but is not oxygen enriched, so regular blood oxygen monitoring makes sense. I also monitor when above 12000 ft but before oxygen is required, to decide if I want to get on it early. I actually use a fingertip reader as my primary, but upgraded my Apple watch to an oxygen-capable one to have a backup instrument.