Thermocouples – order of magnitude of generated EMF The electromotive force (EMF) produced by standard thermocouple junctions for practical temperature differences is typically of what order of magnitude?

Introduction / Context:
Thermocouples generate a voltage due to the Seebeck effect when two dissimilar metals form junctions at different temperatures. Understanding the scale of this EMF is crucial for selecting appropriate signal conditioning (amplifiers, cold-junction compensation) and for appreciating noise susceptibility and lead resistance effects.

Given Data / Assumptions:

• Standard industrial thermocouples (Types K, J, T, etc.).
• Temperature spans from tens to several hundred degrees Celsius.
• Reference (cold junction) compensation is applied in practice.

Concept / Approach:
Typical Seebeck coefficients for common thermocouples lie around 20–60 microvolts per degree Celsius. Over a 100 °C difference, this yields roughly 2–6 millivolts, placing the EMF squarely in the millivolt range for ordinary temperature differences. Although the per-degree sensitivity is in microvolts per degree, the total EMF across practical spans sums to millivolts, not full volts or kilovolts. Hence thermocouple signals require low-noise amplification and careful wiring to avoid parasitic thermoelectric effects at connectors.

Step-by-Step Solution:

Verification / Alternative check:
Thermocouple tables (e.g., NIST) list EMFs on the order of millivolts over common ranges (for Type K: ~4.095 mV at 100 °C relative to 0 °C).

Why Other Options Are Wrong:

• Microvolts: Correct scale per degree but not for typical aggregate EMF across practical spans.
• Volts / Kilovolts: Far too large for passive thermocouple junctions.

Common Pitfalls:
Confusing per-degree sensitivity units with the total EMF across the span; neglecting cold-junction compensation in instrumentation design.

Final Answer:
Millivolts