Instrumentation check: which device is commonly used to measure the rotational speed (rpm) of a stirrer shaft in laboratory and industrial bioreactors?

Introduction / Context:
Accurate measurement of impeller speed is fundamental for controlling mixing intensity, oxygen transfer, and shear in stirred tanks. The standard instrument for rotational speed is the tachometer, available in contact and non-contact (optical or laser) forms.

Given Data / Assumptions:

• We need to measure shaft rpm directly or via motor feedback.
• Laboratory or industrial bioreactor context.
• Device recognition rather than calibration details.

Concept / Approach:
A tachometer measures angular speed, typically returning rpm or rps. In bioreactors, speed readouts may come from motor encoders (a form of tachometer) or external handheld devices used for verification. Other instruments target different physical quantities: anemometers measure air velocity, manometers measure pressure differences, and barometers measure atmospheric pressure.

Step-by-Step Solution:
Identify required variable: rotational speed (rpm).Select instrument class: tachometer/encoder for angular velocity.Confirm alternatives do not measure rotation directly (anemometer, manometer, barometer).Conclude tachometer is the appropriate device.

Verification / Alternative check:
Most bioreactor controllers display rpm using internal motor tach feedback; periodic verification with an external optical tachometer ensures calibration accuracy.

Why Other Options Are Wrong:
Anemometer: measures linear air speed, not shaft rpm.Manometer: measures pressure head, not rotation.Barometer: measures atmospheric pressure, not rotation.

Common Pitfalls:
Confusing motor nameplate frequency (Hz) with actual shaft rpm after gearing; neglecting to account for slip in induction motors without feedback control.

Final Answer:
Tachometer