Side-by-side comparison
| Parameter | Thermocouple | Thermistor |
|---|---|---|
| Operating Principle | Seebeck effect — two dissimilar metal junctions generate EMF proportional to temperature difference | NTC or PTC semiconductor resistance changes with temperature |
| Temperature Range | Type K: –200°C to +1260°C; Type B: +600°C to +1820°C | Typically –50°C to +150°C; limited by semiconductor material |
| Sensitivity | Low — 41 µV/°C for Type K; requires amplification | High — 3–6% change in resistance per °C (NTC at 25°C) |
| Linearity | Nonlinear but well-characterized by IEC 60584 polynomial tables | Highly nonlinear — Steinhart-Hart equation used for linearization |
| Output | Millivolt EMF — needs cold junction compensation | Resistance (kΩ range); needs Wheatstone bridge or resistance measurement |
| Cold Junction Compensation | Required — reference junction must be at known temperature | Not applicable — single junction sensor |
| Self-Heating | Negligible — passive sensor, no current needed | Significant — measurement current causes I²R heating error |
| Typical Component | Type K TC with MAX31855 cold junction compensator IC | NTC 10kΩ thermistor (B = 3950 K) with voltage divider |
Key differences
Type K thermocouples generate 41 µV/°C — measuring 0.1°C change gives only 4.1 µV, requiring a precision amplifier and cold junction compensator like the MAX31855 IC. An NTC thermistor at 25°C with 10 kΩ nominal resistance changes by 400–600 Ω per °C — three orders of magnitude more sensitive. But the thermistor is useless above 150°C while a Type B thermocouple measures platinum-rhodium junction EMF reliably up to 1820°C. Cold junction compensation is the most common source of thermocouple error: the MAX31855 performs this digitally, but any ambient temperature gradient at the connector introduces offset.
When to use Thermocouple
Use a thermocouple when measuring temperatures above 150°C or in harsh industrial environments — for example, monitoring kiln temperature at 900°C in a ceramic manufacturing plant using a Type K thermocouple with a MAX31855 cold junction compensator.
When to use Thermistor
Use a thermistor when high sensitivity and accuracy are required in the 0–100°C range — for example, measuring battery cell temperature to 0.05°C accuracy in a Li-ion battery management system using a 10 kΩ NTC thermistor in a voltage divider.
Recommendation
The simplest exam rule: above 150°C means thermocouple; below 150°C with precision means thermistor. Remember cold junction compensation is always required for thermocouples — failing to mention it in an exam answer costs marks in measurement uncertainty questions.
Exam tip: Examiners test cold junction compensation — know that the reference junction of a thermocouple is at the measurement instrument terminals (typically 25°C), and any error in knowing this reference temperature directly adds to the measured temperature error.
Interview tip: Interviewers at instrumentation companies like Yokogawa or Endress+Hauser ask about self-heating error in thermistors — explain that the measurement excitation current causes I²R dissipation in the sensor, raising its temperature above ambient; minimizing current (< 100 µA) and using pulsed excitation reduces this error.