Abstract: Apparatus for measuring changes in volumetric content of a storage tank having a predetermined relationship between changes in liquid level and the change in volumetric content. The apparatus includes a tape windable onto and off a reel co-acting with a counterweight. The tape has means at spaced intervals along a selected portion of the length thereof consisting of performations or, alternatively, at least one notched edge driving a digital counter through a drive pulley. The means at spaced intervals on the tape positively drives the counter with a controlled amount of slip between the tape and the counter driving mechanism corresponding to predetermined non-linear relationships that may exist between the float movement and volume change whereby there is a direct reading of changes in volumetric content of the storage tank throughout the entire extent of travel of the tape.