Thermocouples are a common type of sensor used to measure temperature. Thermocouples are created when two wires made from different metals are connected together at one end, creating a junction. When the junction changes in temperature, a small voltage is createdas a result of the thermoelectric effect. That voltage is compared to a ’reference’ junction that is created in temperature measuring instruments and converted to a temperature indication.
Thermocouples are widely used in many manufacturing, industrial, and scientific applications. Thermocouples are also used in homes and offices as temperature sensors in everyday items such as thermostats, stoves, and toasters.
Thermocouples are typically selected over other temperature measurement methods because of their lower cost, wider temperature ranges, and durability. The main limitation of thermocouples is accuracy. Thermocouple accuracies better than 2°F are difficult to achieve. There are ‘Special Limit’ thermocouples available that are made with a higher purity of thermocouple wire, which increases their accuracy, and cost. Some temperature measuring instruments are able to compensate for thermocouple inaccuracies and provide a more accurate measurement.
Although a thermocouple junction can be formed by joining any two different metals, certain metal combinations have been recognized as being favorable for temperature measurements.Thermocouples are typically identified by letter designation. In addition to the letter designation, the different thermocouple types are also color coded. The color codes can vary in some countries so it is best to be aware of this if they are sourced from other countries.
The most common thermocouple types are J, K, T, and E and are called ‘Base Metal’ thermocouples. ‘Noble Metal’ thermocouples such as types R, S, and B are designed for high-temperature applications such as the measurement of steel during forming and machining.
Each different thermocouple type has its own unique characteristics.To select the appropriate thermocouple for a specific application, considerations will need to be made of the expected temperature range, the humidity and chemical makeup of the environment, the required accuracy, and the cost. Below are some of the characteristics of the major thermocouple types.
K- Type Thermocouple
The K-type thermocouple is the most common type of thermocouple,due to its low cost and high temperature range. It consists of Chromel and Alumel wires. Its color designation is Yellow. The temperature ranges from -200 to 2300°F. This thermocouple has better accuracy and stability at high temperatures. This thermocouple type is capable of being measured by a number of handheld digital multimeters.
J- Type Thermocouple
The Type J-type thermocouple is one of the most frequently used thermocouples. It consists of wires made of Iron and Constantan. Its color designation is Black. The temperature ranges from –200 to 1400°F. The iron wire of this thermocouple can rust at higher temperatures or humid conditions so it is recommended to be used in dry, lower temperature environments.
The T-type thermocouple consists of Copper and Constantan wires. The temperature ranges from –400 to 750°F. Its color designation is Blue. It has the lowest temperature range and the highest accuracy versus the other major thermocouple types. It is often used in environmental chambers or other systems requiring sub-zero temperatures.
The E-type thermocouple consists of Chromel and Constantan wires. The temperature ranges from –200 to 1600°F. Its color designation is Purple. This thermocouple type resists corrosion and is suitable for most environments, except where sulfur is present.
The N-Type thermocouple consists of wires made from Nicrosil and Nisil. The temperature ranges from –200 to 2300°F. Its color designation is Orange. It has the same general range as a K-type, but it performs better during temperature cycling. It is typically more expensive than other thermocouple types.
B, R & S-Type Thermocouples
The B, R and S-type thermocouples all contain wires with varying amounts of Platinum and Rhodium. The temperature ranges from 32 to 3100°F for B-type and 32 to 2640°F for R&S-types. The color designation is Gray for B-type and Green for R&S-types. These thermocouples are typically used only in extremely high-temperature applications.