Abstract: A tube vertically disposed in a liquid storage tank, in which the tube has a cross-sectional area which changes with height in proportion to the variation in the tank's cross-sectional area. The tube is filled through an open valve near the bottom of the tube and the valve is closed for the remainder of the test. Other valves above the liquid level are kept open to maintain equal vapor pressures in the tank and the tube. A first sensor measures the fluid level in the tank, while a second sensor measures the differences in fluid levels between tank and tube over a period of time. Ambient height changes are identical for liquid in both tank and tube, whereas leak related height changes occur only in the tank. A second identical tube may be provided when levels are affected by the tube or sensor, or when the tube is required to hold the sensor and is open to the tank.