The system is composed by the PD meter type SBM 150 with strainer, a balance pipe, a vent pipe for air eliminator, mounted on a 4 wheels trolley according. The meter will be connected via the two quick couplings, on one side to the tank truck via a flexible hose, and on the other side to the underground tank via another flexible hose. To avoid air measurement during transfer is foreseen the balance pipe. It has a length roughly 1 meter higher than the highest level of the liquid inside the tank truck. Two sight glasses installed on inlet strainer and on output collector, permit to verify liquid presence in the meter after and before the discharge procedure. Once the tank truck discharge valve is open, the liquid will flow by gravity through the meter, which shall start measuring, and both the vent pipe and the balance pipe will be filled of liquid till the level of the liquid inside the tank truck. The more the tank truck is emptying, the lower the level will be, but the meter manifold design is such that the meter will always remain full of liquid at all time, even at the end of the unloading. At delivery end, liquid inside the hose downstream the meter will be empty thanks to air flowing from the balance pipe.