Industrial sensing: which of the following devices could form the sensing element in a sensor assembly for measuring a physical quantity?

Introduction / Context:
Sensors convert physical phenomena—temperature, pressure, position, flow—into signals that control systems can interpret. The sensing element is the component that directly experiences the measurand and initiates the transduction chain.

Given Data / Assumptions:

• A thermometer directly measures temperature and thus can be a sensing element.
• Brakes and clutches are actuators, not sensors.
• Relays are switching/logic elements; they route power or signals but do not measure a physical quantity by themselves.

Concept / Approach:
Differentiate components by role: sensing (detects), actuation (applies force/torque), switching (routes). A thermometer provides a quantitative reading of temperature, often forming the front end of control loops (e.g., PID temperature control).

Step-by-Step Solution:
1) Identify which option directly couples to a measurand.2) Recognize thermometer as a primary transducer (e.g., bimetallic, thermistor).3) Exclude actuator and switching components (brake, clutch, relay) and non-sensing hydraulics.4) Select “Thermometer.”

Verification / Alternative check:
In instrumentation diagrams, thermometers/RTDs/thermocouples occupy the “sensor” position; relays are found in control panels as outputs or interlocks.

Why Other Options Are Wrong:
Brake/clutch: apply or transmit torque. Relay: electromechanical switch. Hydraulic pump: energy source; none of these detect a measurand.

Common Pitfalls:
Assuming any electrical component can sense; most are for control or power, not measurement.

Final Answer:
Thermometer.