Platinum resistance thermometry behavior: As temperature increases, how does the electrical conductivity of platinum (used in RTDs) change?

Introduction / Context:
Resistance temperature detectors (RTDs) exploit the predictable change in resistance of metals with temperature. Platinum is the industry standard because of its linearity, chemical stability, and reproducibility. Understanding the direction of change of conductivity (the inverse of resistivity) is fundamental.

Given Data / Assumptions:

• Material: high-purity platinum wire or thin film.
• Normal RTD operating range (e.g., −200 to 600 °C).
• Ohmic behavior with negligible self-heating effects at proper excitation.

Concept / Approach:
For metals, resistivity increases with temperature due to increased lattice vibrations scattering conduction electrons. Therefore, resistance increases and conductivity (σ = 1/ρ) decreases as temperature rises. This monotonic trend underpins the positive temperature coefficient (PTC) response of platinum RTDs.

Step-by-Step Solution:

Verification / Alternative check:
Standard RTD equations (e.g., Callendar–Van Dusen) show resistance increases with temperature, confirming decreasing conductivity.

Why Other Options Are Wrong:

• Increases / increases exponentially: Opposite to metallic behavior in normal ranges.
• Remains constant: Not for metals; semiconductors may show opposite trends.
• Random changes: Platinum is prized because it is predictable, not random.

Common Pitfalls:
Confusing metals with semiconductors; semiconductors often show conductivity increasing with temperature, unlike metals.

Final Answer:
Decreases