Dynamic behavior of instruments: Which of the following is considered an undesirable dynamic characteristic of a measuring instrument?

Introduction / Context:
Instrument performance is judged by static and dynamic characteristics. In dynamic operation, the speed and fidelity with which an instrument tracks changes in the measurand are vital for control and safety. Undesirable dynamic traits degrade responsiveness and can destabilize closed-loop systems.

Given Data / Assumptions:

• Dynamic characteristics include time constant, time lag, hysteresis, overshoot, and dead time.
• Static characteristics include accuracy, bias (static error), precision, resolution, and reproducibility.
• We seek the dynamic trait that is undesirable.

Concept / Approach:

Time lag (or dead time) is a delay between the input change and the beginning of output response. It reduces phase margin, complicates controller tuning, and can cause oscillations or poor disturbance rejection. While some dynamic smoothing is inevitable (first-order time constant), a pure lag is especially troublesome because it provides no new information to the controller during the delay interval.

Step-by-Step Solution:

Verification / Alternative check:

Control theory shows that additional dead time lowers achievable closed-loop bandwidth for a given robustness, raising integral absolute error for setpoint changes and disturbances.

Why Other Options Are Wrong:

Reproducibility: Desirable static property. Dead zone: Typically categorized as a static nonlinearity; undesirable but not primarily a dynamic attribute. Static error: A static bias; again, not a dynamic characteristic.

Common Pitfalls:

Conflating dead zone (static insensitivity) with dead time (dynamic delay); both are harmful, but the question asks specifically for a dynamic characteristic.

Final Answer:

Time lag (delay between input change and output response)