A thermoelectric sensor is a device that converts temperature changes into electrical changes. It utilizes the property of certain materials or components that change with temperature for measurement. For example, it converts temperature changes into changes in resistance, thermoelectric potential, thermal expansion, or magnetic permeability, and then uses appropriate measuring circuits to detect the temperature. A thermoelectric sensor that converts temperature changes into electrical potential is called a thermocouple; a thermoelectric sensor that converts temperature changes into resistance is called a resistance temperature detector (RTD).
Features of thermoelectric sensors
1. Characteristics of thermocouple sensors:
High measurement accuracy: Because the thermocouple is in direct contact with the object being measured, it is not affected by the intermediate medium.
Wide measurement range: Commonly used thermocouples can continuously measure from -50 to +1600℃, and some special thermocouples can measure as low as -269℃ (such as gold-iron-nickel-chromium) and as high as +2800℃ (such as tungsten-rhenium).
Simple in construction and easy to use: Thermocouples are usually composed of two different metal wires and are not limited by size or opening. They are covered by a protective sheath and are very convenient to use.
2. Features of thermistor sensors:
Thermocouples have a larger signal output and are easier to measure; RTDs require an external power supply, while thermocouples can generate their own potential; RTDs have a slower temperature response; RTDs made of the same materials do not have the same upper temperature limit as thermocouples.
Working principle of thermoelectric sensors
A thermocouple is a temperature sensor that utilizes the thermoelectric effect. The thermoelectric effect refers to the phenomenon where an electromotive force (EMF) is generated in a closed circuit when two conductors (or semiconductors) of different materials form a closed loop and the temperatures T and T0 at their junctions are different. The EMF generated by the thermoelectric effect includes contact EMF and thermoelectric EMF.
1. Contact electromotive force (EMF) is the electromotive force generated at the contact point due to the difference in free electron density between two different conductors. Its value depends on the material properties of the two different conductors and the temperature of the contact point.
2. Thermoelectric electromotive force is an electromotive force generated at the two ends of the same conductor due to the temperature difference.
The mechanism is as follows: the electron energy at the high-temperature end is greater than that at the low-temperature end, and the number of electrons moving from the high-temperature end to the low-temperature end is greater than the number moving from the low-temperature end to the high-temperature end. As a result, the high-temperature end becomes positively charged due to the loss of electrons, while the low-temperature end becomes negatively charged due to the gain of excess electrons, thus forming a thermoelectric potential at the two ends of the conductor.
Resistance temperature detectors (RTDs) measure temperature based on the principle that the resistance of a conductor changes with temperature. They are widely used to measure temperatures in the range of -200 to 850°C, and in some cases, can measure temperatures down to 1K and up to 1000°C. Standard platinum resistance thermometers offer high accuracy and serve as the standard instrument for reproducing the international temperature scale.
