Flow sensor identification.

• A. Thermocouple
• B. Float switch
• C. Pressure transducer
• D. Orifice plate

Answer: (D)

Measuring flow in a pipe typically relies on a primary element that creates a pressure difference which can be translated into a flow rate. An orifice plate is exactly this: a flat plate with a precise hole that creates a restriction in the flow. As liquid passes through the hole, its velocity increases and pressure drops on the upstream side while a lower pressure is felt downstream. A differential pressure sensor then reads this pressure difference, and with calibration it translates to a flow rate. The device has no moving parts, making it rugged and inexpensive for many applications.

The other devices listed don’t measure flow directly. A thermocouple detects temperature, not flow. A float switch senses liquid level, not how fast the liquid is moving. A pressure transducer measures pressure, which is not a flow measurement by itself (though it can be used with a restriction like an orifice plate to infer flow).