An RTD (Resistance Temperature Detector) measures temperature through the predictable change in electrical resistance of a metal as it heats up.
A transmitter converts this resistance change into a standard 4-20 mA signal for the control system.
This guide covers the working principle, construction types, wiring configurations, key parameters, and how to select the right RTD for your application.
The sensor works on a simple principle: the electrical resistance of a pure metal increases linearly with temperature. Platinum is the preferred material because it is chemically stable, highly pure, and produces a well-defined, reproducible resistance-temperature relationship defined by IEC 60751.
RTD Working Principle: How Resistance Changes with Temperature
Every metal contains a lattice of positively charged atoms. When current flows, electrons move through this lattice. At higher temperatures, the atoms vibrate more intensely and collide more frequently with passing electrons, increasing the opposition to current flow. This opposition is called resistance.
Three properties make this measurement accurate: a predictable temperature coefficient of resistance (TCR), a stable linear response, and low self-heating error. Click any term to expand.
RTD Construction: Wire-Wound vs Thin-Film
The sensing element is made by forming a length of pure platinum wire or film into a defined geometry on a substrate. The two main construction types are wire-wound and thin-film. Each has distinct advantages for different applications.
| Feature | Wire-Wound RTD | Thin-Film RTD |
|---|---|---|
| Construction | Fine platinum wire coiled around a ceramic or glass mandrel and potted in protective powder | Thin platinum layer deposited on a ceramic substrate by sputtering, then laser-trimmed to exact resistance |
| Temperature range | Minus 200°C to 850°C | Minus 50°C to 500°C (standard); specialist types to 600°C |
| Accuracy | Highest: Class AA and Class A tolerances achievable | Good: Class B standard; Class A available in premium types |
| Vibration resistance | Poor: wire can deform under vibration, changing resistance | Excellent: the thin film is bonded directly to the substrate |
| Response time | Slower: larger thermal mass | Faster: smaller element with lower thermal mass |
| Cost | Higher | Lower: suitable for high-volume applications |
| Typical use | Laboratory, calibration standards, high-accuracy process measurements above 300°C | Industrial process control, HVAC, food processing, pharmaceutical |
RTD Resistance Formula
The Callendar-Van Dusen equation gives the exact resistance at any temperature. For most industrial purposes above 0°C, the linear approximation is accurate to within 0.4% up to 400°C.
R₀: nominal resistance at 0°C (100 Ω for Pt100, 1000 Ω for Pt1000)
α: temperature coefficient = 0.00385 Ω/Ω/°C (IEC 60751)
T: temperature in °C
Pt100 at 200°C: R = 100 × (1 + 0.00385 × 200) = 100 × 1.77 = 177.0 Ω
Pt1000 at 200°C: R = 1000 × (1 + 0.00385 × 200) = 1770.0 Ω
The transmitter stores the Callendar-Van Dusen coefficients and applies the non-linear correction automatically. The engineer enters only the sensor type and range.
See the Pt100 calibration guide for the resistance-to-temperature table used during calibration.
RTD Wiring Configurations: 2-Wire, 3-Wire and 4-Wire
Lead wire resistance adds to the measured resistance and causes a positive temperature error. The wiring configuration determines how much of this error is compensated. See the sensor connections guide for full wiring diagrams for each configuration.
2-Wire RTD
Simplest wiring. The transmitter measures lead wire resistance plus sensing element. Lead error cannot be separated.nnot be separated. A 1 Ω lead causes approximately 2.6°C error in a Pt100.
Use only with very short cables (under 2 m) or with a Pt1000 where the relative lead error is smaller.
3-Wire RTD
The most common industrial configuration. A third wire allows the transmitter to measure one lead and subtract it from both sides of the bridge.
Requires all three wires to be identical. Standard for industrial Pt100 installations with cable runs up to 50 m.
4-Wire RTD
Two wires carry excitation current; two separate wires measure voltage across the element only. Lead resistance is completely eliminated regardless of cable length.
Used for laboratory reference standards and measurements requiring accuracy better than 0.1°C. See the platinum RTD advantages guide.
Transmitter in Head (Head-Mount)
A compact transmitter mounted in the sensor connection head. The short element-to-transmitter distance eliminates lead resistance. The transmitter outputs 4-20 mA over the loop cable.
The preferred configuration for new installations, removing the need for special extension cable. See the temperature transmitter guide.
Resistance and Temperature Calculator
Sensor Material Types and Temperature Ranges
| Material | Nominal R at 0°C | Range | TCR (α) | Application |
|---|---|---|---|---|
| Platinum (Pt) | 100 Ω (Pt100) or 1000 Ω (Pt1000) | Minus 200°C to 850°C | 0.00385 Ω/Ω/°C | All industrial and laboratory applications. Most accurate and stable. |
| Nickel (Ni) | 100 Ω or 120 Ω | Minus 60°C to 250°C | 0.00617 Ω/Ω/°C | HVAC, refrigeration, lower cost applications where full platinum accuracy is not needed |
| Copper (Cu) | 10 Ω | Minus 200°C to 260°C | 0.00427 Ω/Ω/°C | Motor winding temperature monitoring; linear but less stable than platinum |
| Nickel-Iron (Ni-Fe) | 604 Ω | Minus 100°C to 200°C | 0.00518 Ω/Ω/°C | Bridge circuits and specific military applications |
IEC 60751 Accuracy Classes per IEC 60751
IEC 60751 defines four accuracy classes for platinum RTDs. The class applies to the sensing element only, not to the complete measurement chain including the transmitter and lead wires.
| Class | Tolerance at 0°C | Tolerance Formula | Typical Use |
|---|---|---|---|
| AA | ±0.1°C | ±(0.1 + 0.0017 × |T|) | Laboratory reference standards, primary calibration |
| A | ±0.15°C | ±(0.15 + 0.002 × |T|) | High accuracy process measurement, precision calibration |
| B | ±0.3°C | ±(0.3 + 0.005 × |T|) | Standard industrial process control (most common) |
| C | ±0.6°C | ±(0.6 + 0.01 × |T|) | General purpose, non-critical temperature indication |
Class B is the default for most industrial Pt100 and Pt1000 sensors. At 100°C, Class B tolerance = ±0.8°C. Specify Class A for critical measurements.
See the temperature transmitter calibration guide for the full verification procedure.
Watch: What Is an RTD and How Does It Work?
RTD Sensor Questions
External References
- Resistance Temperature Detector: Construction and Working Principle | Electrical4U
- What Is an RTD Temperature Sensor? | Atlas Scientific (updated 2025)
What We Learn Today
- An RTD (Resistance Temperature Detector) measures temperature through the predictable increase in electrical resistance of a metal as temperature rises
- Platinum is the standard RTD material: Pt100 (100 Ω at 0°C) and Pt1000 (1000 Ω at 0°C) follow IEC 60751 with TCR = 0.00385 Ω/Ω/°C
- Wire-wound RTDs offer higher accuracy and wider range (to 850°C); thin-film types offer faster response, better vibration resistance, and lower cost
- The 3-wire configuration compensates for lead resistance and is the industrial standard. The 4-wire configuration eliminates lead error completely for highest accuracy.
- IEC 60751 accuracy classes: AA (±0.1°C), A (±0.15°C), B (±0.3°C), C (±0.6°C), all measured at 0°C
- Platinum RTDs are more accurate and stable than thermocouples. Thermocouples cover higher temperatures and respond faster. Choose based on the application requirements.