Suggest a sensor that could be used as part of the control system for a furnace to monitor the rate at which heating oil flows along a pipe. The output from the measurement system is to be an electrical signal which can be used to adjust the speed of the oil pump. The system must be capable of operating continuously and automatically, without adjustment, for long periods of time.   source:  Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering 6th Edition by W. Bolton  question 2.16

Suggest a sensor that could be used as part of the control system for a furnace to monitor the rate at which heating oil flows along a pipe. The output from the measurement system is to be an electrical signal which can be used to adjust the speed of the oil pump. The system must be capable of operating continuously and automatically, without adjustment, for long periods of time.

source:  Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering 6th Edition by W. Bolton 

question 2.16