Thermocouples

A thermocouple is a temperature-measuring device made from two dissimilar metal wires joined at one end, forming a junction. When this junction experiences a temperature change, it produces a small voltage due to the Seebeck effect. This voltage varies with temperature and can be measured to determine the temperature at the junction. 

Thermocouples are widely used in various industries due to their durability, low cost, and the ability to measure a broad temperature range. Common types include Type K, J, T, and N, each are suited for specific temperature ranges and applications. They are often used in furnaces, engines, ovens and other industrial processes where temperature monitoring is essential. 

Thermocouples do not require an external power source, making them easy to use and reliable. However, their accuracy can be influenced by electrical noise or incorrect calibration. Despite this, thermocouples remain one of the most popular tools for temperature sensing across many fields.

A thermocouple works based on the Seebeck effect, which states that when two dissimilar metals are joined at one end and there is a temperature difference between the joined end (hot junction) and the other ends (cold or reference junction), a small voltage is generated. This voltage is directly related to the temperature difference. Many temperature displays have a thermocouple circuit that measures this voltage and converts it into a temperature reading.

The hot junction is placed where temperature measurement is needed, while the cold junction is kept at a known reference temperature. Because different metal combinations produce different voltage responses, various types of thermocouples (such as Type K, J, or T) exist for different applications. Thermocouples are popular due to their wide temperature range, fast response time, and rugged design. 

Thermocouples come in various types, classified by the combination of metals used and their temperature ranges, sensitivities, and applications. The most common types include Type K, J, T, E, N, R, S, and B.

Type K (Nickel-Chromium/Nickel-Alumel) is the most widely used, suitable for general-purpose use up to about 1300°C. Type J (Iron/Constantan) works well in lower temperatures (up to 750°C) but oxidizes quickly in moist atmospheres. Type T (Copper/Constantan) offers excellent accuracy at low temperatures,  and they are often used for cryogenic applications. Type E (Chromel/Constantan) provides a stronger signal and is good for low-temperature applications. Type N (Nicrosil/Nisil) is stable and accurate at high temperatures, making it suitable for industrial use.

For high-temperature applications, Type R, S and B (all using platinum alloys) are used. These types are often used in laboratories or in the metal processing industries.

When using thermocouples, several important considerations ensure accurate and reliable temperature measurement. Proper installation is key—secure contact between the thermocouple tip and the surface or environment being measured is essential for accuracy. If you are just measuring the voltage output then cold junction compensation must be applied, as thermocouples measure temperature differences, not absolute temperatures. Using a reference junction or an electronic compensator helps correct this.

Wire type and extension matter: using the same thermocouple material or proper extension wire prevents voltage errors. Electrical noise can also affect readings, so shielding and grounding are often necessary, especially in industrial environments. Thermocouple Calibration is crucial for accuracy. Regular checks ensure the sensor maintains its performance, especially in high-temperature or harsh conditions that can degrade materials. Additionally, choose the correct thermocouple type for the temperature range and environment—for example, use corrosion-resistant types in oxidizing atmospheres. 

A thermocouple and a Pt100 temperature sensor are both used to measure temperature, but they operate on different principles and are suited for different applications. A thermocouple consists of two dissimilar metals joined at one end, generating a voltage that varies with temperature due to the Seebeck effect. It is rugged, inexpensive, and can measure a wide temperature range (up to 1700°C), but it is less accurate and more prone to electrical noise.

In contrast, a Pt100 sensor is a type of resistance temperature detector (RTD) made from platinum, with a resistance of 100 ohms at 0°C. As temperature increases, its resistance changes in a predictable way. Pt100 sensors are more accurate and stable over time than thermocouples but are more expensive and generally limited to temperatures below 650°C.

In summary, thermocouples are ideal for high-temperature, rough environments, while Pt100 sensors are preferred where accuracy and stability are more important.