Resistance temperature detectors (RTDs): which material listed is never used as a sensing element in industrial resistance thermometers?

Introduction / Context:
Resistance temperature detectors use the predictable change of electrical resistance with temperature in certain metals and alloys. Material selection dictates accuracy, stability, and usable temperature range. This question checks familiarity with common RTD elements and highlights one metal that is unsuitable for this purpose.

Given Data / Assumptions:

• Industrial RTDs emphasize linearity, stability, and repeatability.
• Common RTD materials: platinum (standard), nickel and copper (legacy/industrial), and certain nickel–iron alloys.
• We seek the material that is not used as an RTD element.

Concept / Approach:
Platinum (Pt100, Pt1000) is the international standard due to stability and near-linear response. Nickel (Ni120) and copper RTDs exist for specific ranges and cost considerations. Alloys such as 70% nickel + 30% iron (Balco-type) are used in instrumentation circuits. Lead, however, is soft, has poor mechanical properties, low melting point, and an unsuitable temperature coefficient for precision RTD service, making it inappropriate as a sensing element.

Step-by-Step Solution:
List typical RTD materials: platinum, nickel, copper, and certain Ni–Fe alloys.Evaluate lead’s properties: low melting point, creep, instability.Conclude: lead is never used as an RTD sensing element.

Verification / Alternative check:
Standards (IEC 60751) specify platinum characteristics; vendor catalogs also offer nickel/copper variants, but none use lead for sensing elements.

Why Other Options Are Wrong:
Nickel: Historically used, good sensitivity; limited high-temp stability vs Pt.Copper: Good linearity near ambient; used in some HVAC/instrumentation.30% iron + 70% nickel: Employed in measurement circuits and specialized sensors.Platinum: The standard industrial RTD material.

Common Pitfalls:
Confusing lead as “wiring material” with “sensing element”; lead may appear in solder, not as an RTD element.

Final Answer:
Lead