rotometer
/rəʊˈtɒmɪtə/noun
1A hand-held measuring device incorporating a small wheel that can be rolled over a map or plan, the revolutions of the wheel giving the notional distance travelled on the ground.
2A device with a transparent wall that can be fitted into a pipe or tube to indicate the rate of flow of fluid passing along it.
Origin
Early 20th century. From roto- + -meter. Perhaps originally an alteration of rotameter.