There have been tests of portable units and the major issue with
them going up in altitude is not that they don't work, but they
get a lot because the compressor works harder.

That is really an interesting point. Prior to this thread--I had never
given any thought to the possibility of anyone using a portable concentrator
in the cabin of an aircraft, as my interest had been to eliminate the use of
refillable bottles for normal use. I had simply presumed that the
compressor(s) would be driven directly by the engine(s) and that any
necessary intercooling would be provided prior to the concentrator unit; or
alternatively that a purpose build self contained unit would be installed.
Therefore, this thread has been something of a revelation.

Peter