RTD vs Thermocouple
RTDs and thermocouples are the two most common industrial temperature sensors. Compare accuracy, range, durability, and cost to select the right sensor for your application.
The Two Temperature Sensing Technologies
Resistance Temperature Detectors (RTDs) and thermocouples (TCs) are the two most widely used temperature sensors in industry. Both measure temperature, but they use completely different physical principles and have very different accuracy, range, and durability characteristics.
Choosing the wrong sensor type leads to inaccurate readings, premature failure, or unnecessary cost. The decision is usually straightforward once you understand the trade-offs.
How They Work
| Aspect | RTD | Thermocouple |
|---|---|---|
| Principle | Electrical resistance of a metal (usually platinum) changes with temperature | Two dissimilar metals joined at a junction generate a small voltage (Seebeck effect) proportional to temperature |
| Sensor element | Platinum wire wound or thin-film (Pt100 most common) | Two dissimilar metal wires (e.g., Type K: chromel-alumel) joined at the tip |
| Output signal | Resistance (100Ω at 0°C for Pt100) | Millivolt DC voltage (41 μV/°C for Type K) |
| Reference junction | Not required | Required — cold junction compensation (CJC) at the transmitter |
| Signal strength | Stronger — resistance change is measurable | Weak — microvolt signal requires amplification |
Why platinum in RTDs
Platinum is used because its resistance changes linearly and predictably with temperature. A Pt100 RTD has exactly 100.0 ohms at 0°C and increases by ~0.385 ohms per °C. This linearity and stability is unmatched by any other temperature sensing technology.
Accuracy and Precision
| Accuracy factor | RTD (Pt100) | Thermocouple (Type K) |
|---|---|---|
| Typical accuracy | ±0.1 to ±0.5°C | ±1 to ±2°C |
| Repeatability | Excellent — returns to same reading at same temperature | Good — but can drift over time |
| Long-term stability | Excellent — minimal drift over years | Moderate — drifts due to junction ageing |
| Resolution | High — 0.01°C achievable | Lower — limited by signal noise |
| Hysteresis | Negligible | Small but present |
When accuracy matters
If your process requires tight temperature control (±0.5°C or better) — pharmaceutical reactors, food pasteurisation, precision calibration — RTD is the clear choice. If ±2°C is acceptable, a thermocouple is simpler and cheaper.
Temperature Range
| Range | RTD (Pt100) | Thermocouple (Type K) | Thermocouple (Type N) |
|---|---|---|---|
| Lowest | -200°C | -200°C (Type T better at low temps) | -200°C |
| Typical use range | -200 to 500°C | 0 to 1100°C | 0 to 1200°C |
| Maximum | ~600°C (thin-film) to 850°C (wire-wound) | 1260°C (Type K) | 1300°C (Type N) |
| High-temperature types | Not practical above 850°C | Type K, N, S, R, B up to 1700°C+ | Up to 1300°C |
RTD range limits
Above 600°C, platinum RTDs become unstable — the platinum migrates and the resistance drifts. For temperatures above 600°C, thermocouples are the only practical choice. Type S, R, and B thermocouples (platinum-rhodium) can measure up to 1700°C.
Durability and Environment
| Durability factor | RTD | Thermocouple |
|---|---|---|
| Vibration resistance | Poor — thin platinum wire can break | Excellent — rugged junction of two wires |
| Shock resistance | Poor — wire-wound elements are fragile | Excellent |
| Response time | Slower — element is enclosed in a sheath | Faster — junction can be exposed or grounded |
| Size | Larger — typically 6mm diameter minimum | Smaller — can be as fine as 1mm |
| Moisture resistance | Good — sealed in sheath | Good — but insulation must be intact |
| Chemical resistance | Same (depends on sheath material) | Same (depends on sheath material) |
Vibration is the RTD killer
The platinum wire in an RTD is thin and fragile. In high-vibration environments (pump discharges, compressors, engines), the wire can work-harden and break. Thermocouple junctions are simply two wires welded together — virtually indestructible by vibration. For vibrating equipment, use thermocouples.
Cost and Complexity
| Cost factor | RTD | Thermocouple |
|---|---|---|
| Sensor cost | Higher — platinum element | Lower — base metal wires |
| Transmitter cost | Lower — simpler electronics | Higher — needs CJC and amplification |
| Wiring cost (3-wire vs extension) | 3-wire or 4-wire copper cable | Thermocouple extension wire (same type metals) |
| Wiring distance | Limited — lead resistance affects reading | Longer — but extension wire must match thermocouple type |
| Calibration | Less frequent — stable | More frequent — drifts over time |
| Replacement cost | Higher | Lower |
3-wire RTD wiring
RTDs need 3-wire (or 4-wire) copper cabling to compensate for lead resistance. Thermocouples need extension wire made of the same metals as the thermocouple itself — using copper wire introduces a second junction and errors. The extension wire is often more expensive than standard copper.
When to Choose RTD
- High accuracy required: ±0.5°C or better — process control, laboratory, calibration.
- Temperature range -200 to 500°C: The sweet spot for RTDs — stable, accurate, and reliable.
- Long-term stability needed: RTDs drift very little over years; ideal for sensors that are hard to access.
- Low vibration environment: If the sensor is not subject to significant vibration, RTD's accuracy advantage is worth the fragility.
- Repeatability critical: Batch processes, pharmaceutical, and food where the same temperature reading must be reproduced exactly.
When to Choose Thermocouple
- High temperature (above 600°C): Furnaces, kilns, exhaust gas — RTDs cannot survive.
- High vibration environment: Pump discharges, compressors, engines, gas turbines — thermocouple junctions are indestructible.
- Fast response needed: Exposed or grounded junction thermocouples respond in milliseconds — faster than sheathed RTDs.
- Small sensor required: Thermocouples can be as fine as 0.5mm — useful in confined spaces and surface measurement.
- Low cost for many points: When you need many temperature points and ±2°C is acceptable, thermocouples are cheaper.
- Harsh mechanical environment: Shock, impact, and physical abuse — thermocouples survive where RTDs fail.
Tip: A common hybrid approach: use RTDs for process temperature measurement (where accuracy matters) and thermocouples for equipment bearing and motor winding temperatures (where vibration and harshness dominate).
Thermocouple Types Quick Reference
| Type | Metals | Range | Best for |
|---|---|---|---|
| J | Iron-Constantan | 0–750°C | Reducing atmospheres, general purpose |
| K | Chromel-Alumel | -200 to 1260°C | Most common — general purpose, oxidising atmospheres |
| T | Copper-Constantan | -200 to 350°C | Low temperature, cryogenics, food |
| N | Nicrosil-Nisil | 0–1300°C | High temperature, more stable than K |
| S | Platinum-Rhodium (10%) | 0–1450°C | High temperature, laboratory, furnace |
| R | Platinum-Rhodium (13%) | 0–1450°C | High temperature, semiconductor |
| B | Platinum-Rhodium (30%) | 0–1700°C | Very high temperature |
Type K is the default
If in doubt, Type K covers most industrial applications from 0 to 1100°C. It's the most widely available and cost-effective thermocouple type. Use Type T for low temperatures and Type N or S for high temperatures.
Summary
| If you need... | Choose... |
|---|---|
| Highest accuracy (±0.5°C or better) | RTD |
| Temperature above 600°C | Thermocouple |
| Vibration or shock resistance | Thermocouple |
| Long-term stability (minimal drift) | RTD |
| Lowest cost for many points | Thermocouple |
| Fast response time | Thermocouple (exposed junction) |
| Repeatability for batch processes | RTD |
| Small sensor for tight space | Thermocouple |
| Temperature -200 to 500°C | RTD (if vibration is low) |