| Characteristic | Thermocouple (TC) | RTD |
|---|---|---|
| Principle | Seebeck effect (temperature difference → voltage) | Resistance-temperature relationship (metal resistance changes with temperature) |
| Output Signal | Voltage signal (μV level) | Resistance change (Ω level) |
| Self-Powered | Yes (no external excitation needed) | No (requires constant current/voltage excitation) |
| Characteristic | Thermocouple | RTD |
|---|---|---|
| Typical Range | -200°C to +2300°C | -200°C to +850°C |
| Common Types | Type K (-200 to 1260°C) | PT100 (-200 to 850°C) |
| Accuracy | ±1-2°C (requires cold junction compensation) | ±0.1-0.5°C (higher accuracy) |
| Linearity | Poor (requires polynomial compensation) | Excellent (near-linear) |
| Characteristic | Thermocouple | RTD |
|---|---|---|
| Sensing Element | Junction of two dissimilar metals | Platinum/nickel wire wound or thin film |
| Mechanical Strength | High (but junction is fragile) | Low (brittle, sensitive to vibration) |
| Chemical Stability | Depends on metal type (e.g., Type K is oxidation-resistant) | Platinum is corrosion-resistant (but sensitive to contamination) |
| Response Time | Fast (0.1-10 seconds) | Slower (1-30 seconds) |
| Characteristic | Thermocouple | RTD |
|---|---|---|
| Signal Conditioning | Requires high-gain amplification + cold junction compensation | Requires precision current source + Wheatstone bridge |
| Wiring Requirements | Requires compensation cables (same material) | Standard copper wires (3-wire eliminates lead resistance errors) |
| Typical Cost | Low (100) | High (500, platinum is expensive) |
| Long-Term Stability | Moderate (prone to aging) | Excellent (platinum is highly stable) |
| Scenario | Thermocouple Advantages | RTD Advantages |
|---|---|---|
| High-Temperature | Boilers, metallurgy (>600°C) | Laboratory baths (-50 to 300°C) |
| Industrial Processes | Fast-response temperature control | High-precision monitoring (pharmaceuticals, food) |
| Harsh Environments | Engine exhaust temperatures | Cleanroom monitoring |
| Cost-Sensitive | Household appliance overheat protection | Metrology calibration standards |
graph TD A[Temperature >600°C?] -->|Yes| B[Choose Thermocouple] A -->|No| C{Need accuracy >0.5°C?} C -->|Yes| D[Choose RTD] C -->|No| E{Need fast response?} E -->|Yes| B E -->|No| F{Budget limited?} F -->|Yes| B F -->|No| D
Prefer Thermocouples when:
Ultra-high (>600°C) or ultra-low (<-100°C) temperatures
Fast response required
Budget constraints with moderate accuracy needs
Prefer RTDs when:
Medium-low temperature range (-200 to 850°C)
High accuracy needed (±0.1°C level)
Long-term stability critical (e.g., calibration standards)
Key Fundamental Difference:
TCs are "active" temperature-to-voltage converters, ideal for dynamic measurements
RTDs are "passive" resistance-based, ideal for steady-state precision measurements
Note: Modern smart transmitters can automatically identify TC/RTD types and provide cold junction compensation and linearization, simplifying system design.
