Temperature measurement is fundamental to almost every industrial process — and the choice between a PT100 RTD sensor and a thermocouple is one of the most common decisions an instrumentation engineer faces. Both measure temperature, but they work on completely different physical principles, have different strengths and limitations, and suit different applications. This guide explains both technologies clearly so you can make the right choice for your specific requirements.

How a Thermocouple Works

A thermocouple consists of two wires made from different metals joined at one end — the measurement junction. When the junction is exposed to heat, a small voltage is generated across the open ends of the wires. This voltage — typically in the range of millivolts — is proportional to the temperature difference between the hot measurement junction and the cold reference junction (the instrument end of the thermocouple).

Different metal combinations produce different voltage characteristics and suit different temperature ranges. The most common types are J (iron-constantan), K (chromel-alumel), R (platinum-rhodium), and T (copper-constantan). The instrument must know which type of thermocouple is connected to apply the correct conversion table (thermocouple linearisation).

How a PT100 RTD Works

A PT100 is a Resistance Temperature Detector (RTD) made from pure platinum wire or film. The electrical resistance of platinum changes predictably with temperature — at 0°C the resistance is exactly 100 ohms (hence PT100), and it increases by approximately 0.385 ohms for every 1°C rise in temperature. The instrument measures this resistance and converts it to a temperature reading using the standard PT100 calibration curve.

The "PT" stands for Platinum and "100" refers to the resistance at 0°C. This precise and stable relationship between resistance and temperature makes PT100 one of the most accurate temperature measurement technologies available.

PT100 vs Thermocouple — Direct Comparison

Thermocouple Types and Temperature Ranges

When to Choose PT100

PT100 sensors are the right choice when accuracy and stability are more important than temperature range or cost:

• Pharmaceutical manufacturing — GMP requires precise temperature records; PT100 provides ±0.1°C accuracy
• Food processing and cold storage — HACCP compliance demands reliable, accurate temperature measurement
• Laboratory and research — where measurement uncertainty must be minimised
• HVAC and building management — room temperature monitoring where accuracy matters
• Humidity instruments — the dry and wet bulb temperature measurement in classical psychrometers uses PT100
• Any application below 600°C where accuracy is critical

When to Choose a Thermocouple

Thermocouples are the right choice when temperature range, response time, or cost takes priority:

• Industrial furnaces and ovens — temperatures above 600°C make thermocouple the only option
• Heat treatment and annealing — K-type covers 0–1250°C reliably
• Kilns and ceramics — R-type covers up to 1700°C
• Exhaust gas and combustion — high temperature, fast response needed
• High vibration environments — thermocouples are more mechanically robust
• Cost-sensitive multi-point applications — thermocouples cost less per point

3-Wire PT100 — Why Three Wires?

When a PT100 sensor is connected to an instrument over a long cable, the resistance of the cable itself adds to the measured resistance and creates an error. A 2-wire PT100 connection cannot compensate for this. A 3-wire connection uses a third wire to measure the cable resistance separately and subtract it from the reading — eliminating the error. For accurate PT100 measurement over cable runs longer than a few metres, always use 3-wire connection. Most industrial instruments including Countronics data loggers support 3-wire PT100 input.

Cold Junction Compensation in Thermocouples

A thermocouple measures the temperature difference between its hot junction (at the process) and its cold junction (at the instrument terminals). If the instrument terminal temperature changes — due to ambient temperature variation — the reading shifts even if the process temperature is constant. Cold junction compensation (CJC) uses a separate temperature sensor built into the instrument to measure the terminal temperature and automatically correct the reading. All quality thermocouple instruments and data loggers include automatic CJC.