Thanks for advice. Your Inogen rep honest. Mayo rehab person said the same thing - don't let perfect get in the way of good - and Inogen pulse dose not the same as continuous flow.
I had 3 oximeters, one stopped working. just ordered another. I tried out Inogen with oximeter test on the treadmill. I have prescription for 2L/min. If I just slow walking Inogen 2 fine, but if I want to get up to 3 METS (2.8 miles/min I think) then I put it on 3L and that seems to work. Uses up more battery but who cares.
I will blue tooth my new oximeter so I have report for Doc.