Short answer
An RTD changes resistance with temperature. A thermocouple produces a small voltage based on a junction of dissimilar metals. That difference determines the wiring, the input hardware, and the error modes.
Quick reference: RTD (Pt100/Pt1000) vs. Thermocouple (Type J/K/T) Field Comparison Matrix
| Technical Characteristic | RTD (e.g. Pt100 / Pt1000) | Thermocouple (e.g. Type K / Type J) |
|---|---|---|
| Operating Principle | Predictable positive temperature coefficient of platinum resistance (IEC 60751) | Seebeck effect: thermoelectric voltage generated at dissimilar metal junction (IEC 60584) |
| Useful Temperature Span | -200 °C to +600 °C (industrial limit) | -200 °C to +1,250 °C (Type K up to 1,370 °C) |
| Measurement Accuracy & Drift | High accuracy (±0.1 °C to ±0.3 °C); virtually zero long-term drift | Moderate accuracy (±1.5 °C to ±2.5 °C); drifts over time due to wire aging & oxidation |
| Extension Cabling Requirements | Standard copper instrument wire (2, 3, or 4 conductors; matched resistance) | Specialized thermocouple extension/compensating cable matched to exact alloy (e.g. KX for K) |
| Terminal Block Requirements | Standard copper terminal blocks | Isothermal terminal block with Cold Junction Compensation (CJC) or thermocouple alloy terminals |
| Lead Resistance Compensation | 3-wire bridges cancel lead resistance; 4-wire four-terminal Kelvin sensing eliminates it entirely | Lead resistance adds minor series impedance but does not alter thermoelectric millivolt slope |
What to look for
Two-wire RTD wiring includes the lead resistance in the measurement. Three-wire arrangements compensate for much of that lead effect when the leads are matched; four-wire measurement separates current and voltage paths for higher accuracy.
Thermocouple systems require the correct thermocouple type and cold-junction compensation. A stable but implausible temperature can be a configuration mismatch rather than a failed probe.
When the symptom appears in the plant
An industrial oven was consistently overbaking products. The control system used a Type J thermocouple, but a technician had inadvertently used Type K extension wire when repairing a broken connection near the heating zone. The junction of the dissimilar metals created a secondary cold junction, introducing a significant offset error in the temperature reading. Replacing the extension wire with correct Type J wire fixed the issue.
The Standard Copper Terminal Splice Trap on Thermocouples
Never splice or terminate thermocouple wires using standard brass or nickel-plated copper terminal blocks without an isothermal junction. Every transition between thermocouple alloy (e.g. Alumel/Chromel for Type K, or Iron/Constantan for Type J) and copper forms a new thermoelectric junction that generates its own millivoltage proportional to the local temperature. In a plant environment, if a junction box sits near a hot furnace or drafts, these parasitic junctions can inject 5 °C to 20 °C of phantom error into the reading. Always run continuous, un-spliced thermocouple extension wire directly to the temperature transmitter or PLC card, or use certified thermocouple terminal blocks composed of the identical matching alloys.
Five checks before replacement
- Identify the sensor type: RTDs typically have 2, 3, or 4 identical looking wires (often red/white), while thermocouples use specific color-coded pairs (e.g., red/white for J, red/yellow for K in US standards).
- Measure the sensor resistance: A Pt100 RTD should read near 100 ohms at 0°C, while a thermocouple should show a near short circuit (a few ohms).
- Verify the input card configuration in the PLC or temperature controller matches the connected sensor type.
- Check the entire cable run for thermocouples to ensure the correct extension wire type was used and polarity is maintained.
- For 3-wire RTDs, ensure all three wires have identical resistance and the compensation lead is wired to the correct terminal.
Use RTDs for high accuracy at lower temperatures (<600°C); use thermocouples for high temperatures, vibration, or fast response times.
Never splice thermocouple wire with standard copper wire or standard terminal blocks; it creates unwanted parasitic junctions.
RTD extension wires must be equal length and identical gauge to prevent lead wire resistance errors.
Bench checklist
- Identify sensor technology: two/three/four copper/platinum leads indicate RTD; dissimilar alloy pair indicates thermocouple.
- Check lead resistance on RTD installations: verify that all 3 lead wires share identical length and gauge.
- Confirm thermocouple extension wire type matches the probe calibration (Type K yellow/red, Type J white/red).
- Verify cold-junction compensation (CJC) sensor temperature at the input card terminal strip.
- Inspect for accidental galvanic contact between thermocouple tip and grounded thermowell or motor casing.
Reference families
- ASTM E230 (Thermocouple Standards)
- IEC 60751 (RTD Standards)
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- What's the Difference Between Thermocouples and RTDs? Omega
Manufacturer comparison of principles, wiring, accuracy, range, drift, and response.
- Measuring Temperature with Thermocouples, RTDs, and Thermistors NI
Official sensor-fundamentals documentation explaining Seebeck voltage and RTD resistance.
- IEC 60751:2022 Platinum RTD sensors IEC
Official RTD standard landing page; the standard text may require purchase.
- IEC 60584-1:2013 Thermocouples IEC
Official thermocouple reference-function and tolerance standard landing page.