tube structure 15 includes a cylindrical sense tube 47 having an open end 48 that is positioned such that the open end 48 is close to the bottom 50 of the tank 12 the sense tube 47 is constructed and arranged such that the liquid gasoline 46 can enter from the bottom open end 48 only the tank filler tube 51 is also shown fig3 shows an example embodiment of the sample tube structure 15 the sample tube structure 15 includes a housing 52 coupled to the tank 12 so as to extend outside of the tank 12 the sense tube 47 is connected to one side 54 of the differential pressure sensor 17 which can be provided in the housing 52 or in the vapor cavity 28 the other side 56 of the pressure sensor 17 is connected to the vapor cavity 28 so that the pressure sensor 17 measures the difference in pressure between the volume of the vapor cavity 28 and the volume of the sense tube 47 containing the liquid gasoline 46 the temperature sensor 26 is mounted so as to measure the temperature in the fuel tank vapor space 28 the sample tube structure 15 also includes an optional equalization valve 58 disposed in the housing 52 the equalization valve 58 can be used to equalize the pressure between the sense tube 47 and the tank vapor cavity 28 at the start of the diagnostic test in the embodiment of fig2 the processor 30 is shown to be disposed in the housing 52 of.