In article .uk,
Ed Sirett writes:

AIUI Oxygen _can_ diffuse through a warm plastic pipe non barrier pipe.

What I'm unsure about is the relative diffusion rate through the plastic
pipe as against, say, the open surface in the F&E tank.

I've no idea either, but it would be easy to test.
In a earlier posting, I worked out that a heating system when
cold looks like a 50% vacuum to an oxygen (or nitrogen) molecule
in the air outside the heating system. So take a length of
non barrier pipe, capped at one end, and with a pressure test
unit on the other end. Pump up to 1 bar above atmospheric with
air (giving same air pressure differential as you have when
heating system is cold), and watch how quickly the pressure
drops. The initial rate will be same rate air leaks in through
non barrier pipe when heating system is cold to attempt to
equalise the partial pressures of oxygen and nitrogen in the
water.

I don't actually have any non barrier pipe, so I leave this
to someone else to test. A comparison with barrier pipe and
copper tube would be ideal too.

--
Andrew Gabriel