Easiest, if not cheapest, is to get one of the big lab gas suppliers to
make you up a cylinder and then very gently bleed it into your
incubation container. There is nothing unpleasant in your mix and it can
vent to atmosphere, the bleed rate can be low because your G's are not
going to make much impcat on it.
Peter Harris, Reading, UK.
AKA <P.J.Harris at reading.ac.uk>