Reading temperature with a PT100 sensor and an industrial PLC

This guide covers how a PT100 works, the difference between 2-wire, 3-wire and 4-wire probes, how to wire a head-mounted transmitter to an Industrial Shields PLC, and the code to convert the analog reading to °C.

What you need

• An Industrial Shields PLC with a 0-10 V analog input: Arduino based PLC (M-Duino, Ardbox), ESP32 PLC or Raspberry PLC
• A 24 Vdc power supply for the PLC and the transmitter
• A PT100 probe (2-wire, 3-wire or 4-wire)
• A PT100 transmitter with 0-10 V output. In this guide: 24 Vdc supply, range -50 to 100 °C, accuracy 0.2 % of full scale, as printed on its label

How does a PT100 sensor work?

A PT100 is a resistance temperature detector (RTD). Inside there is a platinum element whose resistance rises with temperature. "PT" stands for platinum and "100" for its resistance at 0 °C: 100 Ω. The standard curve of IEC 60751 gives a mean slope of 0.00385 Ω/Ω/°C between 0 and 100 °C, about 0.385 Ω per °C.

The curve is close to linear but not exactly linear. IEC 60751 describes it with the Callendar-Van Dusen equation. These values come from that equation:

A PT1000 works the same way with 1000 Ω at 0 °C. Its larger change per degree makes the lead wire resistance less important, but the transmitter must be made for PT1000.

2-wire, 3-wire and 4-wire PT100: which one to use

The wires between the probe and the transmitter add their own resistance, and the transmitter reads it as temperature. One ohm of extra resistance is about 2.6 °C of error. For example, 10 m of 0.5 mm² copper cable has about 0.69 Ω in the loop of a 2-wire probe, close to 1.8 °C of error.

The probe also has its own tolerance class. According to IEC 60751, class B allows ±(0.3 + 0.005 × |t|) °C and class A ±(0.15 + 0.002 × |t|) °C, so at 100 °C a class B probe can read up to ±0.8 °C off.

From resistance to an analog signal: the PT100 transmitter

The analog inputs of the PLC measure 0-10 V, not resistance. A PT100 transmitter feeds a small current through the probe, measures the voltage across it and gives an output proportional to the temperature over its range. The transmitter in this guide is a head-mounted unit with 24 Vdc supply, PT100 input, range -50 to 100 °C and 0-10 V output.

Other ranges and outputs exist, for example 0 to 300 °C, or a 4-20 mA output. A 4-20 mA loop is better for long cables and noisy plants. To read it with a 0-10 V input, see how to convert a 4-20 mA signal to 0-10 V in an Arduino PLC.

Wiring the PT100 and the transmitter to the PLC

1. Supply. Connect 24 Vdc to the two top terminals of the transmitter, + and -.
2. Output. Connect the 0-10 V output terminal to an analog input of the PLC, here I0.7, and connect the 0 V of the transmitter supply to the GND of the PLC.
3. Probe. Connect the PT100 to the terminals under the resistor symbol. In a 3-wire probe, the two wires of the same colour (red in this probe) go to the same end of the element; connect them to the paired terminals, and the single wire (white) to the other one. For a 2-wire probe, bridge the paired terminals. Follow the drawing of your own transmitter.

Convert the analog reading to temperature

The analog inputs of the Industrial Shields PLCs measure 0-10 V, with a different resolution in each family. The datasheets give these values:

The transmitter output is linear over its range, so two steps give the temperature:

voltage     = raw × 10 / maximum count
temperature = (T_max - T_min) / 10 × voltage + T_min

For -50 to 100 °C, (100 - (-50)) / 10 = 15 °C per volt, so temperature = 15 × voltage - 50. For example, 5 V gives 25 °C.

This sketch reads I0.7 once per second without blocking the loop and prints the raw value, the voltage and the temperature:

// PT100 transmitter -50 to 100 C, output 0-10 V, on analog input I0.7
// ADC_MAX: M-Duino and Ardbox 1023 (10 bit), ESP32 PLC 2047 (11 bit), Raspberry PLC 4095 (12 bit)
const float ADC_MAX = 1023.0;
const float V_MAX = 10.0;      // full scale of the PLC input, V
const float T_MIN = -50.0;     // transmitter range, C
const float T_MAX = 100.0;
const unsigned long PERIOD = 1000;  // ms between readings
unsigned long lastRead = 0;

void setup() {
  Serial.begin(115200);
  pinMode(I0_7, INPUT);
}

void loop() {
  unsigned long now = millis();
  if (now - lastRead >= PERIOD) {
    lastRead = now;
    int raw = analogRead(I0_7);
    float volt = raw * V_MAX / ADC_MAX;
    float temp = (T_MAX - T_MIN) / V_MAX * volt + T_MIN;  // 15 C per volt for -50 to 100 C
    Serial.print("Raw: ");
    Serial.print(raw);
    Serial.print("  Voltage: ");
    Serial.print(volt, 2);
    Serial.print(" V  Temperature: ");
    Serial.print(temp, 1);
    Serial.println(" C");
  }
}

We compiled it for the M-Duino 21+ with avr-gcc and the Industrial Shields AVR board package 1.2.1, without errors or warnings. We did not run it with a probe for this edition. For an ESP32 PLC, set ADC_MAX to 2047, as the first version of this post did. For another transmitter range, change T_MIN and T_MAX.

Troubleshooting a PT100 reading

FAQ about PT100 sensors and PLCs

Can I connect a PT100 directly to a PLC analog input?

Not to a 0-10 V or 4-20 mA input. The PT100 changes its resistance, not a voltage or a current, so you need a transmitter or an RTD input module in between.

What is the resistance of a PT100 at 0 °C and at 100 °C?

100.00 Ω at 0 °C and 138.51 Ω at 100 °C, according to the IEC 60751 curve.

2-wire or 3-wire PT100?

Use 3-wire for most industrial installations. It compensates the resistance of the cable if the three wires are equal. A 2-wire probe is fine only with short cables or when an error of about one degree is acceptable.

0-10 V or 4-20 mA transmitter?

0-10 V is simpler to read with the analog inputs of the Industrial Shields PLCs. 4-20 mA resists noise better on long cables and shows a broken loop as 0 mA.

Why is the formula linear if the PT100 curve is not?

Because the transmitter gives an output proportional to the temperature over its range. The PLC only scales the voltage. If you measure the resistance yourself, use the Callendar-Van Dusen equation.

More about analog sensors on industrial PLCs

• How to convert a 4-20 mA signal to 0-10 V in an Arduino PLC
• DS18B20 temperature on a Raspberry PLC via 1-Wire
• Arduino millis() vs delay() in industrial PLCs
• Choosing an industrial controller