The flow sensor was an integral component of the warp propulsion system on Federation starships. The sensors monitored the balance of fuel in a ship's engines.
In the 23rd century, Montgomery Scott wrote the technical manual Operating Protocol - Flow Sensors. This manual was the standard reference work into the mid-24th century. (TNG: "Booby Trap", okudagram)
In the mid-2270s, an assistant engineer was performing computer simulations on the flow sensors of the USS Enterprise. Before the ship went to warp factor one, the simulation's results were borderline and Commander Montgomery Scott couldn't guarantee to Captain James T. Kirk that the ship would hold up. (Star Trek: The Motion Picture)
On the Galaxy-class starships, there were port and starboard flow sensors. An engineer could monitor and control these sensors from the Engineering station on the main bridge of the ship. (TNG: "The Defector", okudagram)