Thermocouples Overview

A thermocouple is a temperature sensor that converts temperature variations into voltage.

Thermocouples are active transducers because they don't require an external power source to operate.

They are constructed from two wire leads made of different metals.

The two wire leads are welded together to create a junction called the hot junction.

As temperature changes between the junction and the ends of the wire leads, a voltage develops across the junction.

Different combinations of metals produce different voltage responses, leading to various types of thermocouples suitable for different temperature ranges and accuracies.

The Seeback Effect states that when two dissimilar metals are joined at two junctions and maintained at different temperatures, an electromotive force (EMF) is induced in the closed circuit.

The amount of EMF induced is proportional to the temperature difference between the two junctions.

The junction exposed to the heat source is called the measuring junction or hot junction.

The other junction is called the reference junction or cold junction and is kept at a constant temperature.

When both junctions are at the same temperature, the EMF is zero.

The EMF generated in a thermocouple is given by the equation E = a(ΔΘ) + b(ΔΘ)^2, where ΔΘ is the temperature difference, and a and b are Seebeck constants specific to the thermocouple.

Generally, 'a' is much larger than 'b', so we can simplify the equation to E = a(ΔΘ).

Theoretically, any pair of dissimilar materials can be used as a thermocouple, but in practice, only a few materials are commonly used due to factors such as sensitivity, stability, inertness, and reproducibility.

The upper temperature range of a thermocouple is often limited by the surrounding atmosphere.

A thermocouple with exposed leads has no protective sheath, offering fast response but making it vulnerable to damage.

A grounded thermocouple has a protective sheath welded to the sensor, providing extra protection but allowing for electrical contact and potential ground loops.

An ungrounded thermocouple has a layer of insulation separating the sensor from the sheath, minimizing electrical contact but resulting in a slower response.

Thermocouples have a rapid response to temperature changes, making them appropriate for applications with fast temperature variations.

Thermocouples are effective for point measurements, allowing you to determine the temperature at a specific location in a system.

Thermocouples have relatively low accuracy, making them unsuitable for high-precision measurements.

Thermocouples require protection against contamination to ensure a long service life.

Large distances between the thermocouple and the measuring device can introduce errors in the measurement.

The J-type thermocouple is used for temperatures from -210°C to 1200°C and is composed of iron and constantan.

The K-type thermocouple is made of Chromel and Alumel and is used for temperatures from -270°C to 1370°C.

The T-type thermocouple uses copper and constantan and is suitable for temperatures from -200°C to 400°C.

The E-type thermocouple, made of Chromel and constantan, has a wider temperature range from -270°C to 1000°C.

The S-type thermocouple is made of platinum and 10% rhodium, and is utilized for temperatures from -50°C to 1768°C.