Marc,

Your experiment seemed to confirm our thinkng that we would need
oxygen if flying over 10,000ft. Last July on our trip from NY to MI,
when ATC asked us to climb from 8K to 10K, we were a bit concerned but
was glad to see that our performance did not seem to suffer. In the
next 3 long x-country trips, we filed at 10K and 9K and taking turns
flying 3-4hrs stretches. Again, we never noticed any problems. Of
course, we tried to have plenty of water and food. Few years ago, in
our trip to Ecuador, we stayed at Quito, 10K elevation for several
days. The first night, we both had headaches from dehydration and had
to drink quite a few of the hotel $3.5 Evian bottles!

The levels which you monitored also in the range indicated in this
article
http://www.mountainflying.com/oxygen.htm

"The atmospheric pressure decrease at 10,000-foot altitude causes 523mm
Hg ambient air pressure resulting in 87 percent hemoglobin saturation
and 61mm Hg arterial oxygen."

"The body requires hemoglobin saturations of 87-97 percent and arterial
oxygen at 60-100mm Hg (millimeters of mercury) in order to function
normally. Below this level the body is hypoxic"

I'd expect that smokers or people with emphysema etc. may have lower
hemoglobin saturation at lower altitudes.

My thinking was that as long as we take precaution to use oxygen at
above 10K, we would not need to monitor our body oxygen level if we
remain healthy. Your experience and others convinced me that the
oximeter is a very useful device and may be worth getting. I still
plan to get the Oxygen supply first and will rely on yawning to monitor
our fatigue until there is a good sale on the oximeter ;-)

Hai Longworth