Abstract: Process and apparatus for measuring liquid level changes in horizontally disposed cylindrical storage tanks compensates for thermal expansion and contraction of the liquid in the tank during the measurements. A vertically disposed substantially cylindrical container in the storage tank has a valve which controls fluid communication between the container and the storage tank. A float and associated sensing mechanism measure the level of liquid in the container. A measurement of the liquid level is made in the beginning of the test with fluid communication open between the container and the tank. A second measurement is taken after a fixed time period during which fluid communication between the container and the tank is closed. From the second measurement a theoretical liquid level is calculated which would exist in the tank due to temperature effects, and without leakage.