Cecil,
If I am not mistaken, using oxygen at night at low altitude is
mainly to help with vision. We rarely fly at night except for night
currency at local airport and maybe at twilight or shortly after sunset
on long cross-country trips. The oximeter just tells you whether you
need oxygen or not. By itself it does not help with high altitude or
night flying except telling you that you'd better go lower or land ;-)
I'd think that putting the 200+dollars towards the purchase of oxygen
on board is a better idea. One person complete system sells for
something like $450. Emergency hand-held system selling for something
like $320.

Hai Longworth