Industrial instrumentation – thermocouple sensitivity comparison Which of the following thermocouple pairs will produce the highest electromotive force (emf) output for the same hot-junction and cold-junction temperatures?

Introduction / Context:
Thermocouples generate a measurable electromotive force (emf) because of the Seebeck effect. Different metal combinations have different Seebeck coefficients, so for the same temperature difference between the hot and cold junctions, some thermocouples produce a larger voltage than others. Selecting the most sensitive type improves signal-to-noise ratio and measurement resolution in industrial temperature measurement.

Given Data / Assumptions:

• The hot-junction and cold-junction temperatures are identical across options.
• Output comparison is based solely on inherent Seebeck coefficient of each pair.
• Standard, widely used types are implied by the metal pairs listed.

Concept / Approach:
The generated emf is approximately proportional to the temperature difference: E ≈ S * ΔT, where S is the effective Seebeck coefficient of the thermocouple pair. Among common thermocouples, the Chromel–Constantan pair (Type E) has one of the highest Seebeck coefficients, typically larger than Iron–Constantan (Type J) and much larger than the noble-metal Platinum–Platinum+Rhodium types (Types S/R), which trade sensitivity for stability at high temperature.

Step-by-Step Solution:

Verification / Alternative check:
Thermocouple reference tables show that for the same span (for example 0 to 600°C), Type E develops a higher millivolt output than Type J and much more than Type S/R. This corroborates the selection.

Why Other Options Are Wrong:

• Platinum–platinum + rhodium: Excellent stability at high temperature but low sensitivity; produces smaller emf per degree.
• Iron–constantan: Moderately sensitive, but less than Chromel–Constantan.
• All same: Incorrect because Seebeck coefficients vary significantly by metal pair.

Common Pitfalls:
Confusing high-temperature capability with sensitivity. Noble-metal thermocouples withstand higher temperatures but do not provide the largest voltage per degree at moderate ranges.

Final Answer:
Chromel–constantan