A thermocouple is a thermoelectric sensor for measuring temperature. It consists of two metal current conductors welded together. Depending on the materials used, they have different levels of thermoelectric voltage and are suitable for use at different temperatures.
The most commonly used thermocouple types are K and J.
The operation of a thermocouple is based on the Seeback effect. It relies on the fact that when a closed circuit is formed by connecting two different metals at two locations, and these junctions are held at different temperatures (one at the measurement location, the other at a fixed reference temperature location), an electromotive force is induced in the circuit.
The magnitude of the induced electromotive force varies for different combinations of metals and is proportional to the temperature difference between the junctions.
Type K thermocouple probes are made of chromel and alumel materials and can be categorised as general-purpose sensors. These thermoelectric sensors are popular due to their wide temperature measurement range and high technical potential. However, these thermocouples are not universally applicable - we advise against using them in rooms with a high sulphur content in the air, as this adversely affects the performance of both sensor electrodes.
K-type thermocouples are based on a chromel-aluminium alloy. They are among the most popular temperature sensors for a wide range of applications. Their wide measuring temperature range is from -200°C to +1350°C. In non-oxidising and inert environments, K-type sensors are used up to 1260°C (maximum temperature range).
The accuracy of K thermocouples is up to 41 μV/°C.
can operate in harsh environments and different atmospheres
can be used in a variety of applications due to their widest operating temperature range
NiCr-Ni thermocouple has a higher resistance to oxidation than "E" and "J" thermocouples and is therefore better for high temperatures above 500°C
affordable price
fast response
small size
reliability
accurate temperature measurements can be obtained
not suitable for reducing atmospheres but can withstand metallic fumes
faster aging of the electromotive characteristics, compared to thermocouples made of noble materials (B, R and S)
The K-type thermocouple sensor is constructed, like any thermocouple, from two wires made of different materials. In the K-type, one wire is made up of 90% nickel and 10% chromium and the other is made up of 95% nickel, 2% aluminium, 2% manganese and 1% silicon.
Type J (Fe-CuNi) thermocouple, compared to type K, has a lower lifetime at higher temperatures and a smaller temperature range. Type K tends to be more durable than type J because the JP wire oxidises quickly, especially in the high temperature range. Therefore, we recommend type J thermoelectric sensors for lower temperatures down to 0°C to 600°C. They have a similar price and are equally reliable and accurate.
The properties of these temperature sensors allow them to be used to monitor temperature in a wide variety of environments, such as water, mild chemical solutions, gases or dry areas.
They are most commonly used for applications above 550°C up to the maximum working pressure of the thermocouple. Type K, due to its relative radiation hardness, is also commonly used in nuclear applications.