On Saturday, October 12, 2019 at 6:00:05 AM UTC-5, Maxx Ruff wrote:
Bought an 'FDA Approved' Finger Pulse Oximeter Blood Oxygen
Saturation SPO2 Heart Rate O2 Patient Monitor from fleabay to try out
($25-ish).

Although it was a bit clunky sitting on the end of my finger, it appeared
to work reliably well on the ground (slight overcast?) or airborne under
cloud.

At altitude it repeatedly failed in very bright sunlight. (Measures light
absorption of the blood in your finger)
Whenever I uncovered my hand in bright sunlight to read the display it
immediately lost the plot. Wonderful!

As an experiment I covered the device and part of my finger in the foil
from a snack bar leaving the display (mostly) uncovered, it worked
much better but was even more inconvenient.

Don't buy cheap??! (YMMV)

MR

Yes. SoaringRx on pulse ox accuracy
https://www.danlj.org/~danlj/Soaring...-p18-20-22.pdf
Geez! Hard to believe that I wrote that 7 years ago...
and on high-altitude breathing in general
https://www.danlj.org/~danlj/Soaring...ygen-risks.pdf
Enjoy!
DrDan