Industrial temperature measurement using thermocouples: Which characteristic is considered undesirable for thermocouples used in process industries?

Introduction / Context:
Thermocouples convert a temperature difference into an electromotive force (emf) via the Seebeck effect. For accurate and simple instrumentation, a near-linear emf–temperature relationship, coupled with good environmental durability, is preferred. Understanding which traits are desirable or not guides sensor selection and signal conditioning design.

Given Data / Assumptions:

• Standard base-metal and noble-metal thermocouple types (J, K, T, E, S, R, etc.).
• Industrial conditions may include oxidation, corrosion, and thermal cycling.
• Linearization can be done in software, but linear behavior reduces effort and error.

Concept / Approach:

Desirable attributes include stability, repeatability, corrosion/oxidation resistance, and a predictable emf–temperature function. Non-linearity complicates transmitter design, increases interpolation error, and demands more sophisticated compensation, especially with analog instruments. Therefore, strong non-linearity across the intended range is an undesirable characteristic in practical applications.

Step-by-Step Solution:

Verification / Alternative check:

Thermocouple polynomial tables exist precisely because intrinsic behavior is non-linear; however, types are chosen to be as linear as practical in the operating span (e.g., Type T for cryogenic ranges). The less linear the response, the more demanding the compensation.

Why Other Options Are Wrong:

Linearity: Clearly desirable. Corrosion/Oxidation resistance: Essential for service life and accuracy retention at temperature.

Common Pitfalls:

Assuming microprocessor linearization removes all issues; in reality, sensor variability and reference junction errors still benefit from intrinsic linearity.

Final Answer:

Non-linear relation of emf with temperature over the working range