Flow sensor
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. (, okudagram)

In the mid-2270s, an assistant engineer was performing computer simulations on the flow sensors of the . Before the ship went to warp factor one, the simulation's results were borderline and Montgomery Scott couldn't guarantee that the ship would work. ()

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. (, okudagram)
Sensor technology
Warp