Rudy, I would offer the following possibility.  In an empty tank if you get some condensation in the tank it can also evaporate out and leave the tank when conditions are right.  In a tank with fuel in it the fuel forms a seal since the water sinks to the bottom and any water that gets into the tank by whatever means cannot evaporate. bticbw. All the best, Gary Hagstrom Crackerjack-in a heated bed for winter Iron RIver, WI