How do I determine how many pounds of propane will be left in an "empty" tank at a given temperature?

My grill runs on propane and I understand that when the tank is "empty" there is still some propane in it, there is just not enough pressure to allow it to escape. In a standard 20 lb tank what is this amount of propane left in an empty tank when the tank is at room temperature. When the tank is in a cold outdoor environment, there will be more propane left in the tank when it is "empty" than in a warm environment. How do i determine the amount of propane left in the tank at different temperatures? I know this is thermodynamic related and I just need some one to get me started moving in the right direction with an equation or something...