An RTD (Resistance Temperature Detector) is a precision temperature sensor that measures heat by correlating the predictable increase in electrical resistance of a pure metal element—typically platinum—to the surrounding temperature. When you wire an RTD into a control loop, it fundamentally changes your circuit design: instead of generating its own millivolt signal like a thermocouple, an RTD is a passive resistor that requires an external constant current source or Wheatstone bridge to excite it and measure the resulting voltage drop.
The Core RTD Definition and Operating Principle
At the bench level, an RTD relies on the Positive Temperature Coefficient (PTC) of pure metals. As the metal lattice heats up, atomic vibrations increase, scattering electrons and raising electrical resistance. The industry standard for platinum RTDs is governed by IEC 60751, which mandates a specific purity level and defines the alpha coefficient ($\alpha$). For a standard Pt100 sensor, this alpha value is exactly 0.00385 $\Omega/\Omega/^\circ C$. This means for every 1°C rise in temperature, the resistance of a 100-ohm sensor increases by roughly 0.385 ohms.
Standard RTD Materials and Specifications
While platinum dominates the market due to its linear response and chemical stability, other metals are used for specific cost or range constraints. The table below outlines the exact specifications you will find on manufacturer datasheets.
| Sensor Type | Base Resistance ($R_0$ at 0°C) | Alpha Coefficient ($\alpha$) | Usable Temp Range | Primary 2026 Use Case |
|---|---|---|---|---|
| Pt100 | 100 $\Omega$ | 0.00385 | -200°C to +850°C | Industrial HVAC, extruder barrels, CIP systems |
| Pt1000 | 1000 $\Omega$ | 0.00385 | -200°C to +850°C | EV battery thermal management, medical devices |
| Cu100 | 100 $\Omega$ | 0.00427 | -50°C to +150°C | Motor winding temperature monitoring |
| Ni120 | 120 $\Omega$ | 0.00617 | -80°C to +260°C | Consumer appliances, low-cost HVAC |
Note on Pt1000 adoption: In modern embedded systems, the Pt1000 is rapidly replacing the Pt100. Because its base resistance is ten times higher, the impact of lead-wire resistance is reduced by a factor of ten, and it requires only 10% of the excitation current to achieve the same voltage drop, drastically reducing self-heating errors in tight thermal enclosures.
Worked Numeric Example: Calculating Pt100 Resistance
To program a PLC or microcontroller to read an RTD, you must convert the measured resistance back into temperature. Many basic tutorials use the simplified linear equation:
$R_T = R_0 [1 + (\alpha \times T)]$
Let us calculate the expected resistance of a Pt100 sensor at 150°C using this linear formula:
- $R_0 = 100 \Omega$
- $\alpha = 0.00385$
- $T = 150$
- $R_{150} = 100 \times [1 + (0.00385 \times 150)]$
- $R_{150} = 100 \times [1 + 0.5775] = 157.75 \Omega$
Where You Meet RTDs in Practice (And What They Change)
You will encounter RTDs wherever stability and absolute accuracy matter more than raw speed. Common installations include injection molding barrel zones, pharmaceutical clean-in-place (CIP) skids, and commercial chiller plants.
What it changes in your installation: The moment you run an RTD from a sensor head to a control panel, you must account for lead-wire resistance. Let us look at the math for a standard 2-wire Pt100 installation using 22 AWG copper control wire over a 100-foot run.
- 22 AWG copper resistance at 20°C: ~16.14 $\Omega$ per 1,000 ft.
- 100 ft run requires a 200 ft round-trip loop (out and back).
- Lead resistance = $200 \times 0.01614 = 3.228 \Omega$.
- Pt100 sensitivity is ~0.385 $\Omega/^\circ C$.
- Temperature Error = $3.228 / 0.385 = 8.38^\circ C$.
Your PLC will read 8.38°C higher than the actual process temperature simply because of the copper wire in the conduit. This forces the use of 3-wire or 4-wire RTD configurations. A 3-wire setup uses a third identical lead to measure the wire resistance and subtract it via an internal Wheatstone bridge circuit in the transmitter, effectively eliminating the 8.38°C error.
Clearing Up Common Confusions: RTD vs. Thermocouple vs. Thermistor
People commonly confuse RTDs with thermocouples (TCs) and NTC thermistors. While all three measure temperature, their electrical behaviors and circuit requirements are entirely different. Calibration labs treat them as distinct instrument classes.
| Criterion | RTD (Pt100) | Thermocouple (Type K) | NTC Thermistor (10k) |
|---|---|---|---|
| Signal Type | Passive Resistance (Requires Excitation) | Active Voltage (Generates mV via Seebeck effect) | Passive Resistance (Highly Non-Linear) |
| Accuracy | High ($\pm 0.1^\circ C$ to $\pm 0.5^\circ C$) | Moderate ($\pm 1.5^\circ C$ to $\pm 2.2^\circ C$) | Very High in narrow bands ($\pm 0.1^\circ C$) |
| Temp Range | -200°C to +850°C | -200°C to +1260°C | -50°C to +150°C (typically) |
| Response Time | Slow to Medium (0.5s to 5s) | Fast (0.1s to 1s) | Very Fast (milliseconds) |
| Wiring Constraint | Requires 3/4-wire for long runs | Requires specific TC extension wire | 2-wire is fine (high base resistance) |
Frequently Asked Questions
Can I use a standard digital multimeter to read an RTD?
Yes, but you must set the meter to measure resistance (Ohms), not voltage. Keep your test leads short, and be aware that the multimeter's internal test current (often 1mA to 2mA) might cause slight self-heating if left connected to a low-mass Pt1000 element for an extended period.
Do I need a safety barrier for RTDs in hazardous areas?
