Water Finding Paste [1]

A paste that reacts with water and which can be applied to the lower end of a tank gauge stick, before the stick is inserted into a liquid fuel storage tank [2]. If water is present on the bottom of the tank, the paste changes color to the depth of the water. The operator is thus able to detect not only the presence of water in the tank, but the depth of the water layer as well. 

See also Gauge stick [3].