http://www.craggyaero.com/Oxygen.htm

Looks smaller at flight level 240 it was minus 24 C and I had gloves on it
was bloody cold I do not think I would take the gloves or glove off to see
my saturation.

The best thing when wave flying is to breath oxygen for a hour before the
flight or going above 8000 ft thus saturating your body with oxygen.

It worked for me http://www.mals.net/bunyan04/pages/Picture%20001.htm