The P-40 has one of two types of oxygen systems-demand or constant flow. The demand type is the newer, improved system and it has been installed in as many airplanes as	possible. Some of the older types, however, still use the constant flow system. Your Pilots' Information File contains thorough discussions
A few facts about the two systems follow: DEMAND TYPE
1. Use it when or before you reach 10,000 feet indicated altitude. (On flights of 4 hours or more, begin using oxygen between 8000 and 10,000 feet.) Never wait until you are above 10,000 feet. 2. The rubber mask hose has a metal connector by which you attach it to the oxygen regulator. Make	sure it is connected tightly. It slips out easily, so you have to keep checking the connection. 3. A low-pressure oxygen cylinder of 1000 cubic-inch capacity is in the fuselage baggage compartment. Refill the cylinder by attaching a master cylinder to the filler valve just inside the fuselage access door. Do not fill to pressures greater than 450 psi.