Principle of Thermocouples

A Thermocouple consists of two different metals joined together to create a closed circuit between two points. An electromotive force (voltage) will occur according to temperature difference, as known as the Seebeck effect. The following schematic illustration consists of the sensor, which ariseｓ electromotive force (voltage) and the measurement equipment which converts electromotive force to temperature. The sensor is a combination of two different metal wires labeled below as, Metal A and Metal B.

When the measuring equipment is kept at a constant temperature, any temperature change of the sensor will result in a voltage. This voltage is called electromotive force (EMF). Therefore, by using the electromotive force, it is possible to measure the temperature. The difference in thermal electromotive power occurs when it is combined with different type of metals, and therefore, will have different characteristics. Although the standards are regulated by JIS and IEC, the most commonly used thermocouple is the K-Type which uses Chromel-Alumel. In addition, there are many types of sensor tips and it is important to select the correct type according to the object and the environment which you are measuring in. In order to measure the temperature accurately, the tip shape and contact area needs to be kept into consideration.

Basic shapes of the sensor tip Type Picture Explanation 13 or 25μm dia. Fine Perfect for measuring small components. Able to measure with a fast response and high accuracy. Since it is fine and flexible, it can be fixed by using an Ag paste so that the tip will not move. more info 40 μm thick Sheet Suitable for measuring surface temperature. Able to measure with a fast response and high accuracy. more info 10 μm thick Fine Sheet Able to measure with a very fast response and high accuracy. Recommended to measure the surface temperature of a silicon wafer. more info Multi-purpose A very robust thermocouple. Suitable for measurements where there isn’t a rapid change of temperature. Unlike the conventional thermocouple, the tip is not a ball so it is easier to measure fine areas compared to the conventional thermocouple. more info Innovative thermocouples and its uses