i should think when refilled the amount of uncompressed contaminate in the bottle would be very small as its only atmospheric pressure and the refill gas would be hundreds times greater so u would not even notice
Leaving it empty and open would allow atmosphere in.
Its not that the next batch of gas would be tainted because it should be vacuumed prior to filling. The problems start when moisture/condensation/ice could form on the of the cylinder wall and begin to corrode it from the inside. This has been proved in the past to cause cylinders to rupture and with fatal consequences.
Seems to be quite common that a valve on a cylinder is left open to vent any remaining contents when it is 'empty'.
We tend to close the valve once its finished and mark the cylinder 'MT'
We recently cut a couple of old W size oxy cylinders in half so they could be scrapped. They were really rusty on the inside surfaces.
Are the inside faces of the cylinders checked at any time during their life Gasman?
Cylinders should be periodically tested and refurbished.
I have never had direct involvement but they have to be checked by an approved company; Inspected inside and out, refinished, new valve, pressure tested & re-stamped. You should see a plastic test ring beneath the valve with a date on. Plus a load of other information stamped into the shoulder.
i would think checking for internal corrosion might be done in same way as Air Receivers (air compressor tanks like you typically get in car repiar workshops). If i remember correctly those have to be certified every 13 months under health and safety legislation. I think (but i might be wrong) that the inspectors who come to the premises to do this use an ultrasonic thing which tells them the thickness of the metal from the outside.
These inspections are usually done through your main insurance co who sub it out. eg. if you have a motor trade policy with Norwich Union/Aviva and have this optional 'cover', then they send someone from Bureau Veritas to do it. AXA sub it out to HSB Haughton. etc etc