Why is my butane lighter not working?

As the fuel in a butane lighter is used up, why doesn't the liquid fuel under pressure slowly and completely become more gaseous as the pressure inside decreases?

• Why does it still appear to be full, half full, quarter full, etc. with continued use?

• Answer:

  The pressure inside doesn't decrease, that's why they use butane. See, at room temperature, liquid butane has a vapor pressure of about 2 atmospheres. That means that a liquid pool of butane will evaporate until it has 2 atmospheres of butane above it, at which point it will stop. Lighters are built to contain that much pressure, so they maintain 2 atmospheres of butane vapor over the liquid. When the lighter is used, it releases butane vapor, and the liquid butane evaporates until the pressure is restored. The amount of liquid consumed is comparatively tiny, compared to the gas released, but over time the liquid is used up. As long as there is both liquid and vapor in the lighter, the pressure is the same (assuming a constant temperature). A lighter that's almost completely full and just has a tiny pocket of vapor will have the same pressure as a lighter with only a drop of liquid left. Once the last bit of liquid is used up, all you have is gas, which will decompress until the pressure is gone. But since it's only at 2 atmospheres, there's not much pressurized gas to be used.