Which device is typically immersed in a process liquid to measure flow?

• A. Turbines and paddlewheels
• B. Ultrasonic transducers
• C. Resistance temperature detectors
• D. pH probes

Answer: (A)

Flow is often measured by putting a moving part directly in the liquid so the liquid’s motion drives that part. Turbine meters and paddlewheel meters do exactly that: a small rotor sits in the flow, and as the liquid moves it spins. The rotation rate is directly related to the volumetric flow, so these devices are immersed in the process liquid to sense flow accurately. Ultrasonic transducers measure flow by sound waves and can be mounted on the outside of a pipe or inserted without the rotor in contact with the liquid, so they aren’t typically described as being immersed in the liquid in the same way. Temperature sensors (resistance temperature detectors) and pH probes monitor temperature or chemical properties, not flow.