An RTD (Resistance Temperature Detector) is a precision temperature sensor that calculates temperature by measuring the predictable, linear change in electrical resistance of a pure metal element—almost always platinum—as it heats up or cools down.
In a real circuit, an RTD fundamentally changes your hardware design requirements: unlike passive sensors, it demands a stable excitation current source or a Wheatstone bridge to convert resistance into a readable voltage, and it forces you to mathematically account for the resistance of the copper lead wires connecting the sensor to your microcontroller or PLC.
Makers and junior technicians commonly confuse RTDs with thermocouples (which generate their own millivolt signal via the Seebeck effect and require no excitation current) and NTC thermistors (which use ceramic semiconductors with a highly non-linear, negative temperature coefficient). If you need ±0.1°C accuracy in a sub-400°C environment, you need an RTD.
The Core Principle: Platinum, Resistance, and the ITS-90 Standard
The physics of an RTD relies on the positive temperature coefficient of pure metals. As thermal energy increases, the metal lattice vibrates more intensely, scattering electrons and increasing electrical resistance. Platinum is the industry standard because it remains chemically inert, does not oxidize easily, and maintains a highly repeatable resistance-to-temperature curve.
The global standard for this curve is defined by the NIST ITS-90 (International Temperature Scale of 1990). The most common industrial variant is the 'Pt100' sensor, which has a nominal resistance of 100 Ω at 0°C. The 'alpha' coefficient (α) defines the average change in resistance per degree Celsius between 0°C and 100°C. For standard industrial RTDs, α = 0.00385 Ω/Ω/°C.
- RTD (Pt100/Pt1000): High accuracy (±0.1°C), highly linear, slow response time (seconds), requires excitation current. Best for HVAC, lab baths, and food processing.
- Thermocouple (Type K/J): Wide range (-200°C to +1250°C), non-linear, fast response (milliseconds), generates its own voltage. Best for kilns, exhaust gas, and soldering irons.
- NTC Thermistor (10k/100k): High sensitivity, extremely non-linear, cheap, fast response. Best for 3D printer hotends and basic battery pack monitoring.
The Math in the Metal: A Worked Numeric Example
To understand why RTD wiring topology matters, we have to look at the actual numbers. Let us calculate the resistance of a standard Pt100 sensor at 100°C, and then see what happens when we add real-world copper wire to the circuit.
R_0 = 100.00 Ω
Alpha (α) = 0.00385
R_100 = R_0 * [1 + (α * 100)]
R_100 = 100 * [1 + 0.385] = 138.50 Ω
At 100°C, the sensor's resistance is exactly 138.50 Ω. The delta is 38.50 Ω, which gives us a sensitivity of roughly 0.385 Ω per °C.
Now, imagine you install this Pt100 in a commercial brew kettle, and your microcontroller is mounted 10 meters away. You use standard 24 AWG copper wire. According to the Omega Engineering RTD Guide, 24 AWG copper has a resistance of approximately 84.2 mΩ (0.0842 Ω) per meter at 20°C.
- Distance to sensor: 10 meters
- Return path to ground: 10 meters
- Total wire length in a 2-wire loop: 20 meters
- Total wire resistance: 20 m * 0.0842 Ω/m = 1.684 Ω
Your microcontroller measures the total loop resistance: 138.50 Ω (sensor) + 1.684 Ω (wire) = 140.184 Ω. Because the system assumes 0.385 Ω equals 1°C, it calculates the wire resistance as temperature: 1.684 Ω / 0.385 Ω/°C = 4.37°C of error. Your 100°C boiling wort will read as 104.37°C, completely ruining your automated mash profile.
Where You Meet RTDs in Practice
You will rarely find RTDs in consumer electronics; they are the backbone of industrial and commercial thermal management. You will encounter them in:
- HVAC Chillers and Boilers: Immersion-style Pt1000 probes measure supply and return water temperatures to calculate delta-T for building management systems (BMS).
- Food and Beverage Pasteurization: Sanitary tri-clamp RTDs are used in dairy and brewing because their stainless steel sheaths can withstand CIP (Clean-In-Place) caustic washes while maintaining FDA-traceable calibration.
- Plastics Injection Molding: Embedded nozzle RTDs monitor the exact melt temperature of polymers like ABS or polycarbonate, where a 5°C deviation can cause flash or short shots.
- High-End 3D Printing: While hobbyist printers use thermistors, industrial FDM printers printing PEEK or ULTEM use Pt100 RTDs to maintain chamber temperatures above 150°C with zero drift.
Wiring Topologies: 2-Wire, 3-Wire, and 4-Wire
To eliminate the 4.37°C error calculated above, engineers use different wiring topologies. The choice dictates your hardware cost and accuracy ceiling.
| Topology | Wire Count | How It Compensates | Typical Use Case |
|---|---|---|---|
| 2-Wire | 2 (Red, White) | None. Adds lead resistance directly to the measurement. | Short runs (<1m), low accuracy, or when using high-resistance Pt1000 sensors. |
| 3-Wire | 3 (Red, White, White) | Measures the resistance of one lead wire and subtracts it from the total loop mathematically via a bridge circuit. | Standard industrial default. Runs up to 50m with ±0.2°C accuracy. |
| 4-Wire | 4 (Red, Red, White, White) | Uses a Kelvin connection. Two wires carry excitation current, two separate wires measure voltage drop. Completely eliminates lead resistance. | Laboratory metrology, calibration baths, runs over 100m. |
Decision Tree: Picking Your RTD and Interface for 2026 Builds
Selecting the right sensor and amplifier depends entirely on your microcontroller architecture, run length, and required precision. Use this decision matrix to terminate your part selection.
| If Your Application Is... | Then Choose This Sensor... | And Pair It With This Interface... |
|---|---|---|
| Industrial PLC panel (4-20mA loop) | Pt100, 4-wire, Class A | WIKA T15 DIN-rail RTD Transmitter |
| ESP32/Arduino IoT brew kettle or HVAC monitor | Pt1000, 3-wire | Adafruit MAX31865 Breakout |
| High-temp kiln monitoring (>500°C) | Stop. RTDs max out around 600°C. Use a Type K Thermocouple with a MAX31856. | N/A |
| Short-run battery pack thermal cutoff (<1m) | Pt1000, 2-wire | Simple voltage divider into ESP32 ADC (12-bit) |
If you are building a custom IoT temperature monitor, automated kiln controller (under 500°C), or precision brewing system, buy the Adafruit MAX31865 RTD Amplifier Breakout (Product ID: 3328) and pair it with a 3-wire Pt1000 probe.
Why this exact pick? The MAX31865 handles the constant-current excitation, the Wheatstone bridge, and the 15-bit ADC conversion internally, communicating via SPI. It natively supports 2, 3, and 4-wire configurations and automatically calculates the Callendar-Van Dusen equation in hardware. Choosing a Pt1000 over a Pt100 gives you 10x the base resistance (1000 Ω at 0°C), which drastically reduces the percentage of error introduced by lead wires and makes it far more forgiving on long cable runs. The total BOM cost for the breakout and a food-grade probe is typically under $35 USD. Reference the Adafruit MAX31865 Guide for exact SPI pinouts and library initialization.
FAQ: Real-World Integration Pitfalls
Why is my RTD reading 2°C higher than my reference thermometer when the fluid is perfectly still?
You are experiencing 'self-heating.' An RTD requires an excitation current to measure resistance. That current passing through the platinum element generates heat (I²R). In a stagnant fluid or air, that heat cannot dissipate fast enough, and the sensor measures its own internal temperature. The MAX31865 allows you to lower the excitation current or use a pulsed measurement duty cycle to mitigate this. Always ensure adequate fluid flow across the probe sheath.
Can I use standard CAT5e Ethernet cable to extend my 3-wire RTD?
Yes, but with strict caveats. CAT5e uses 24 AWG solid copper, which is fine for resistance. However, you must use three wires from the same twisted pair (or at least ensure the physical routing is identical) to maintain thermal and electrical balance. More importantly, RTD signals are low-level analog measurements. If your CAT5e cable runs parallel to a VFD (Variable Frequency Drive) or a 3-phase motor feeder, the EMI will induce noise in the unshielded twisted pairs. For industrial environments, always use a dedicated, foil-shielded 3-conductor instrument cable with the drain wire grounded at the PLC panel only.
What does 'Class A' vs 'Class B' mean on the datasheet?
This refers to the IEC 60751 tolerance standard. A Class B Pt100 has a tolerance of ±0.3°C at 0°C, which widens as temperature increases. A Class A Pt100 has a tighter tolerance of ±0.15°C at 0°C. For most DIY and commercial HVAC applications, Class B is perfectly adequate and significantly cheaper. Only specify Class A (or the even tighter 1/3 DIN class) for laboratory calibration or pharmaceutical processing.
