Industrial Temperature Transmitter: PT100 vs Thermocouple

PT100 RTD vs Thermocouple: Choosing the Right Industrial Temperature Transmitter

Temperature measurement is the most common process variable in industrial automation. Whether you are monitoring furnace temperatures, chiller performance, food processing temperatures, or pipeline heat tracing, choosing between a PT100 (RTD) temperature transmitter and a thermocouple determines your measurement accuracy, range, and cost. This guide compares both technologies and helps you choose the right sensor for your application.

Temperature Sensing Technologies Overview

PT100 RTD (Resistance Temperature Detector)

A PT100 is a platinum resistance element whose resistance changes predictably with temperature. “PT” indicates platinum; “100” means the element has exactly 100 ohms resistance at 0°C. As temperature increases, resistance increases following a known relationship (defined in IEC 60751).

PT100 Resistance Formula (simplified):

R(T) = R0 × (1 + A×T + B×T²)

Where:
R0 = 100Ω (resistance at 0°C)
A = 3.9083 × 10⁻³ /°C
B = −5.775 × 10⁻⁷ /°C²
T = temperature in °C

At 100°C: R = 138.5Ω
At 200°C: R = 175.8Ω

Thermocouple

A thermocouple consists of two dissimilar metals joined at one end. The Seebeck effect creates a small voltage (millivolts) proportional to the temperature difference between the hot junction (measurement point) and the cold junction (reference). Different metal combinations give different characteristics — Type K, Type J, Type T, Type N, and others.

Comparison: PT100 vs Thermocouple

PT100 Wiring Configurations

Lead wire resistance can cause significant measurement errors in PT100 circuits. Three wiring configurations address this:

2-Wire PT100

Simplest but least accurate. Lead resistance (typically 0.1-1Ω per metre) adds directly to the measured resistance, creating a positive temperature error. Use only for very short cable runs (under 1 metre) or where high accuracy is not required.

Error: 1Ω lead resistance ≈ +2.6°C measurement error

3-Wire PT100 (Most Common in Industry)

Three wires from the sensor — two from one side of the element, one from the other. The measuring circuit subtracts the resistance of one lead from the other, compensating for lead resistance. Assumes both leads have equal resistance (true for bundled cables).

This is the most widely used configuration in Indian industry. Most industrial transmitters accept 3-wire PT100 as standard input.

4-Wire PT100 (Highest Accuracy)

Four wires — two carry the excitation current, two measure the voltage drop across the element. Lead resistance is completely excluded from the measurement. Required for laboratory-grade accuracy or where lead resistances differ significantly.

Thermocouple Types and Their Applications

Industrial Temperature Transmitters: Head-Mount vs DIN-Rail

Raw PT100 or thermocouple signals are not directly suitable for PLCs or control systems — the signals are too small or non-linear. A temperature transmitter converts the sensor signal to a standard 4-20mA output suitable for any control system.

Head-Mount Transmitter

• Mounts directly in the thermowell head (sensor connection head)
• Minimises cable run from sensor to transmitter — reduces noise pickup
• IP65/IP67 rated for outdoor and wet environments
• Available for PT100 (2/3/4-wire) and all thermocouple types
• Typical output: 4-20mA (2-wire loop-powered)
• Some models include HART communication for remote configuration

DIN-Rail Mount Transmitter

• Mounts in control cabinet on standard 35mm DIN rail
• May accept multiple input types (PT100 / thermocouple / mV) — configurable
• Lower vibration exposure — suitable for sensitive circuitry
• Often includes additional outputs (relay alarms, serial communications)

Reading PT100 with Arduino

Direct PT100 measurement requires a precision analogue front-end. The MAX31865 is a dedicated PT100/PT1000 to SPI converter IC:

#include <Adafruit_MAX31865.h>

// Use software SPI: CS=10, DI=11, DO=12, CLK=13
Adafruit_MAX31865 thermo = Adafruit_MAX31865(10, 11, 12, 13);

// PT100 reference resistor (typically 430Ω on module)
const float RREF = 430.0;
const float RNOMINAL = 100.0;  // PT100

void setup() {
  Serial.begin(9600);
  thermo.begin(MAX31865_3WIRE);  // 3-wire PT100
}

void loop() {
  float temperature = thermo.temperature(RNOMINAL, RREF);
  
  // Check for faults
  uint8_t fault = thermo.readFault();
  if (fault) {
    Serial.print("FAULT: 0x"); Serial.println(fault, HEX);
    if (fault & MAX31865_FAULT_HIGHTHRESH) Serial.println("RTD High Threshold");
    if (fault & MAX31865_FAULT_LOWTHRESH)  Serial.println("RTD Low Threshold");
    if (fault & MAX31865_FAULT_REFINLOW)   Serial.println("REFIN- > 0.85 x Bias");
    if (fault & MAX31865_FAULT_REFINHIGH)  Serial.println("REFIN- < 0.85 x Bias (FORCE- open)");
    if (fault & MAX31865_FAULT_RTDINLOW)   Serial.println("RTDIN- < 0.85 x Bias (FORCE- open)");
    if (fault & MAX31865_FAULT_OVUV)       Serial.println("Under/Overvoltage");
    thermo.clearFault();
  } else {
    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.println("°C");
  }
  delay(1000);
}

Choosing Between PT100 and Thermocouple: Decision Guide

Choose PT100 when:

• Temperature range is −200°C to +600°C
• Accuracy better than ±1°C is required
• Repeatability and stability are critical (laboratory, pharmaceutical, food processing)
• Slow temperature changes — response time is not critical
• Cable runs are short or 3/4-wire compensation is used

Choose Thermocouple when:

• Temperature exceeds 600°C (furnaces, kilns, molten metal)
• Fastest possible response time is required
• High vibration environment (kilns, engines, forges)
• Cost is a primary concern (thermocouples are cheaper per point)
• Very small probe size is required (bare wire thermocouples)

Frequently Asked Questions

What is a thermowell and do I always need one?

A thermowell is a metal tube that isolates the temperature sensor from the process fluid while allowing heat transfer. It protects the sensor from pressure, flow velocity, and corrosive media. Required in pressurised pipes, corrosive fluids, and any application where the sensor cannot be directly exposed. In low-pressure air measurement or surface temperature, thermowells are often omitted.

Can I use regular copper cable for thermocouple wiring?

No. Thermocouple signals are corrupted wherever the thermocouple wire is joined to different metal (the thermoelectric effect creates an additional unwanted junction). You must use the appropriate extension grade thermocouple wire (same alloy type as the sensor) all the way from the sensor to the transmitter or cold junction compensation point.

What is cold junction compensation in thermocouples?

A thermocouple measures the temperature difference between the hot junction (process) and the cold junction (where it connects to the measuring instrument). To calculate absolute temperature, the instrument must measure its own terminal temperature and add this to the differential reading. This is cold junction compensation — performed automatically by all good thermocouple transmitters and digital meters.

Conclusion

PT100 and thermocouples are complementary technologies — each excels in its appropriate application range. For precise temperature measurement in HVAC, food processing, pharmaceuticals, and industrial processes under 600°C, the PT100 is the preferred choice. For high-temperature furnaces, kilns, and any application requiring very fast response, thermocouples are irreplaceable. Pair either sensor with a proper head-mount transmitter outputting 4-20mA for noise-immune, long-distance signal transmission to your PLC or data acquisition system.

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