Identify the device that is not a speed-measuring instrument: compare psychrometer, stroboscope, and tachometer based on their primary measurement function.

Introduction / Context:
Industrial and laboratory instruments are often identified by the measurand they target. Distinguishing among devices that measure rotational or periodic speed versus those measuring fluid properties is basic instrumentation literacy.

Given Data / Assumptions:

• Psychrometer: a humidity instrument (dry-bulb and wet-bulb thermometers).
• Stroboscope: visualizes periodic motion and can infer rotational speed.
• Tachometer: directly measures rotational speed (rpm).

Concept / Approach:
Only one of the listed devices does not measure speed by design. A psychrometer measures relative humidity via temperature depression; a stroboscope and a tachometer both relate to speed—one optically by matching flash frequency, the other via mechanical/electrical pickup.

Step-by-Step Solution:

Verification / Alternative check:
Operation manuals and standards categorize psychrometers under hygrometry, whereas stroboscopes/tachometers fall under speed/rotational measurement.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing a stroboscope’s optical method with purely visual effects; it is a legitimate speed measurement tool when calibrated.

Final Answer:
Psychrometer