Originally Posted by whydowebother
Thanks, I'll have another look in the AFM but I'm pretty sure it's a factory fitted system.
You might want to take a look at the aircraft manufacturer's website and see if there is an AFM supplement listed for the oxygen system.
Most manufacturers do not make the full supplements available on their website, but they typically will provide a document that gives you the dates of most recent revision to both the AFM and the AFM supplements. If you see that there is a supplement published, try giving the manufacturer a call (call their customer support department) and ask them if they would be so kind as to send you the latest issue of the AFM supplement for the oxygen system. Most manufacturers will do this at no charge if you (i.e. the aircraft operator) have a subscription to the AFM revision service for the aircraft.
Be aware that during the production history of the aircraft, the manufacturer may have fitted more than one type (or size) of oxygen system. So, before you call the manufacturer, get together with your maintenance technician and find out exactly what the specification of the system fitted to your aircraft is.