My current glider came without an oxygen bottle. Before the new bottle arrived with the Canadian regs allowing half an hour between 10,000 and 12,500. I spent a considerable amount of that legal (if not smart) half hour trying to figure out which hand on the altimeter was what.

Before the MH, I took an Aerox up to 18,000 and believe I was conscientious in adjusting the flow. But I grounded myself for two days afterwards as I was wiped out after landing.

Higher up I treat the pulse oxymeter as a primary instrument, followed by the VNE placard.