Why do you crack an oxygen tank?

You "crack" an oxygen tank by opening the cylinder's main valve briefly to blow out contaminants from the main valve before attaching the regulator. This is done because contaminants inside the valve or at the valve outlet (dust, dirt, organic material, bugs, fragments of plastic from valve seats, etc.) act to initiate regulator fires and explosions ignited by heat from adiabatic compression of oxygen into the regulator when the oxygen main valve is first opened. These fires ignite the brass regulator itself and are very dangerous. They are avoided by keeping oxygen equipment clean, by cracking a new oxygen cylinder prior to use, and by opening the main valve slowly when a regulator is attached.