Side-by-side comparison
| Parameter | RTD | Thermocouple |
|---|---|---|
| Sensing Element | Platinum wire or thin-film resistor (Pt100: 100 Ω at 0°C) | Two dissimilar metal wires (Type K: NiCr/NiAl; Type J: Fe/CuNi) |
| Temperature Range | –200°C to +850°C for Pt100 (IEC 60751) | Type K: –200°C to +1260°C; Type R/S/B to +1820°C |
| Accuracy | Class A Pt100: ±(0.15 + 0.002|T|) °C — very high | Type K IEC Class 1: ±1.5°C or 0.4% of reading, whichever is greater |
| Linearity | Nearly linear — resistance increases ≈ 0.385 Ω/°C for Pt100 | Nonlinear — requires polynomial correction or lookup table |
| Stability | Excellent long-term stability — drift < 0.1°C/year | Drifts faster, especially at high temperatures (grain growth in wire) |
| Signal Conditioning | 3-wire or 4-wire Wheatstone bridge; small resistance change needs precision amplifier | Millivolt EMF with cold junction compensation IC (MAX31855, AD8495) |
| Self-Heating | Significant if excitation current > 1 mA — use < 1 mA excitation | Negligible — passive device, no excitation required |
| Cost | Higher — Pt100 element and 4-wire transmitter | Lower — simple construction, low cost for Types K and J |
Key differences
A Class A Pt100 at 100°C has accuracy ±0.35°C; a Type K thermocouple at 100°C has accuracy ±1.5°C — RTDs are four times more accurate in this range. Pt100 resistance varies linearly at 0.385 Ω/°C; at 100°C it reads 138.5 Ω, easily measured with a 4-wire bridge. The 4-wire configuration eliminates lead resistance error — critical when lead resistance is comparable to the 0.385 Ω/°C sensitivity. Thermocouples need no excitation but need cold junction compensation; every 1°C error in knowing the cold junction temperature adds 1°C to the final reading.
When to use RTD
Use a Pt100 RTD when accuracy better than ±0.5°C and long-term stability are required — for example, a 4-wire Pt100 in a climate-controlled pharmaceutical storage room monitoring 20°C ± 0.2°C for drug stability compliance.
When to use Thermocouple
Use a thermocouple when temperature exceeds 600°C or when fast response and low cost are more important than accuracy — for example, a Type K thermocouple inserted into a 950°C furnace lining for process monitoring in a heat treatment plant.
Recommendation
For exam problems, choose RTD (Pt100) when the question mentions accuracy, stability, pharmaceutical, food, or laboratory environments. Choose thermocouple when temperature is above 500°C or when a replaceable, low-cost sensor is described. That split covers 95% of sensor selection exam questions.
Exam tip: Examiners ask why 4-wire RTD connection is preferred over 2-wire — explain that 4-wire eliminates lead resistance by using separate current-carrying and voltage-sensing leads, so the measured resistance is purely that of the Pt100 element regardless of cable length.
Interview tip: Interviewers at process control companies ask the practical difference in commissioning — say that RTDs need 4-wire cabling and a precision current source (typically 1 mA), while thermocouples need thermocouple-grade extension wire matched to the TC type; using copper extension wire for a Type K thermocouple introduces a second uncompensated junction.