hi raider
before I can answer any of your questions, it would be helpful to know what size the O2 tank is and what settings does the regulator have on it for flow control? The reason I'm asking is that if it's just a small 240 litre tank and the regulator does not go below 1 lpm (litre per minute), then you would have only 240 minutes of oxygen.
oxygen tanks can be refilled/exchanged at most welding supply companies.