Turbines and paddlewheels immersed directly in a process liquid are used to measure which quantity?

• A. Flow
• B. Temperature
• C. Pressure
• D. Density

Answer: (A)

The key idea is that these devices convert the motion of the liquid into a measurable signal. A turbine or paddlewheel sits directly in the flowing liquid, so it spins as the fluid moves. The faster the liquid flows, the faster the turbine or paddlewheel rotates. Since the cross-sectional area is fixed, the rotation rate (or the pulse rate from a pickup) is proportional to the volumetric flow rate. That makes them tools for measuring flow, not temperature, pressure, or density. Temperature sensors respond to heat, pressure sensors respond to force per area, and density measurements require different methods; they don’t rely on the rotating motion produced by the liquid’s flow.