Abstract: A leak detection system for detecting a fuel leak from a storage tank comsing a vertically positioned reference tube that spans the full useful length of the fuel tank which is generally located underground. The reference tube is filled and drained with the tank. The system also has a normally open pneumatic by-pass valve which when closed isolates the reference tube from the tank. A differential pressure gauge between the tank and reference tube reads zero pressure immediately after closure of the by-pass valve. A differential pressure reading between the tank and reference tube indicates the presence of fuel leakage from the tank. The inside of the reference tube is tapered at the top portion and bottom portion of the tube to match the cross sectional area of the tank upper dome and lower tank dome ends, while the outside diameter of the tube remains constant through out the length of the tube.