RTD Sensors: PT100 & PT1000

Working Principle

A Resistance Temperature Detector (RTD) exploits the highly predictable change in electrical resistance of pure platinum as temperature varies. The PT100 has a resistance of 100 Ω at 0 °C, while the PT1000 has 1000 Ω at 0 °C. As temperature rises, resistance increases in a near-linear fashion described by the Callendar–Van Dusen equation.

Why platinum? Platinum has the most linear and stable resistance–temperature relationship of any metal, with excellent long-term stability and resistance to oxidation — making it the international standard for precision thermometry.

• PT100: 100 Ω at 0 °C, ~0.385 Ω/°C. Requires amplifier for small signal changes.
• PT1000: 1000 Ω at 0 °C, ~3.85 Ω/°C. Higher sensitivity reduces lead resistance errors.

Electrical Characteristics

Parameter	PT100	PT1000
Resistance at 0 °C	100.00 Ω	1000.0 Ω
Temperature Coefficient	0.00385 Ω/Ω/°C	0.00385 Ω/Ω/°C
Temperature Range	−200 to +850 °C	−200 to +850 °C
Accuracy (Class A, IEC 60751)	±0.15 °C at 0 °C	±0.15 °C at 0 °C
Accuracy (Class B)	±0.30 °C at 0 °C	±0.30 °C at 0 °C
Wiring	2-wire, 3-wire, or 4-wire	2-wire or 3-wire
Self-Heating	Excitation current dependent	Lower current needed

Interfacing with an MCU

RTDs require a constant excitation current (typically 1 mA for PT100, 0.1 mA for PT1000) passed through the element, and the resulting voltage drop is measured. Dedicated RTD-to-digital converters simplify this:

• MAX31865: SPI-based PT100/PT1000 converter with 15-bit resolution, automatic wire compensation, and fault detection. The recommended approach.
• Wheatstone bridge + instrumentation amplifier: Traditional analogue approach for custom designs.

Calibration

RTDs are the most stable temperature sensors but benefit from periodic calibration:

• Ice-point check: PT100 should read exactly 100.00 Ω in a properly prepared ice bath (0.01 °C)
• Callendar–Van Dusen coefficients: Use A, B, C constants from the sensor certificate for linearisation
• MAX31865 reference resistor: Use a precision Rref (430 Ω for PT100, 4300 Ω for PT1000) with ±0.1% tolerance

Code Example

/*
 * PT100 RTD via MAX31865 — Arduino
 * Requires: Adafruit MAX31865 library
 * Wiring: SPI (CLK, MISO, MOSI, CS → Pin 10)
 * Rref: 430Ω for PT100, 4300Ω for PT1000
 */
#include <Adafruit_MAX31865.h>

#define CS_PIN 10
#define RREF   430.0    /* 430Ω for PT100 */
#define RNOMINAL 100.0  /* 100Ω at 0°C */

Adafruit_MAX31865 rtd = Adafruit_MAX31865(CS_PIN);

void setup() {
    Serial.begin(115200);
    rtd.begin(MAX31865_3WIRE);  /* 2WIRE, 3WIRE, or 4WIRE */
    Serial.println("PT100 via MAX31865 Ready");
}

void loop() {
    uint16_t raw = rtd.readRTD();
    float ratio = (float)raw / 32768.0;
    float resistance = ratio * RREF;
    float tempC = rtd.temperature(RNOMINAL, RREF);

    Serial.print("RTD resistance: ");
    Serial.print(resistance, 2);
    Serial.print(" Ohm  |  Temperature: ");
    Serial.print(tempC, 2);
    Serial.println(" C");

    /* Check for faults */
    uint8_t fault = rtd.readFault();
    if (fault) {
        Serial.print("Fault 0x");
        Serial.println(fault, HEX);
        rtd.clearFault();
    }

    delay(1000);
}

Real-World Applications

Limitations

• Cost: PT100 probes and MAX31865 converters cost significantly more than DS18B20 or LM35 solutions.
• Self-heating: Excitation current through the element generates heat — use the minimum current that gives adequate signal-to-noise ratio.
• Slow response: Probe thermal mass means response times of seconds, not milliseconds.
• Lead resistance: 2-wire connections introduce cable resistance errors — use 3-wire or 4-wire for accuracy.
• Fragile elements: Thin-film PT100s are sensitive to mechanical shock and vibration.