A temperature sensor refers to a sensor that can sense temperature and convert it into a usable output signal. Temperature sensors are the core part of temperature measuring instruments and come in a wide variety. Based on the measurement method, they can be divided into two main categories: contact and non-contact. According to the characteristics of sensor materials and electronic components, they can be classified into two types: thermistors and thermocouples.
Main Classifications of Temperature Sensors
Based on the measurement method, temperature sensors can be divided into contact temperature sensors and non-contact temperature sensors. According to the characteristics of sensor materials and electronic components, they can be classified into thermistor temperature sensors and thermocouple temperature sensors.
1. Contact: Contact temperature sensors have their detection part in good contact with the object being measured, also known as thermometers. Thermometers achieve thermal equilibrium through conduction or convection, allowing the thermometer’s reading to directly represent the temperature of the measured object. Generally, the measurement accuracy is relatively high. Within a certain temperature range, thermometers can also measure the internal temperature distribution of objects. However, for moving bodies, small targets, or objects with very small thermal capacity, significant measurement errors may occur. Common thermometers include bimetallic thermometers, glass liquid thermometers, pressure thermometers, resistance temperature detectors, thermistors, and thermocouples.
2. Non-contact: Its sensitive element does not come into contact with the object being measured, also known as non-contact temperature measuring instruments. These instruments can be used to measure the surface temperature of moving objects, small targets, and objects with small thermal capacity or rapid temperature changes (transient), and can also be used to measure the temperature distribution of temperature fields.
Common non-contact temperature measuring instruments are based on the fundamental laws of black body radiation and are called radiation thermometers. Radiation temperature measurement methods include brightness method (see optical pyrometer), radiation method (see radiation pyrometer), and colorimetric method (see colorimetric thermometer). Each type of radiation temperature measurement method can only measure the corresponding photometric temperature, radiation temperature, or colorimetric temperature. Only the temperature measured for a black body (an object that absorbs all radiation and does not reflect light) is the true temperature.
Advantages of non-contact temperature measurement: The upper limit of measurement is not restricted by the temperature tolerance of the sensitive element, thus there is theoretically no limit to the maximum measurable temperature. For high temperatures above 1800°C, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation temperature measurement has gradually expanded from visible light to infrared, being used down to room temperature and below 700°C, with very high resolution.
Main Uses of Temperature Sensors
Temperature is a physical quantity that characterizes the hotness or coldness of an object and is a very important and common measurement parameter in industrial and agricultural production processes. The measurement and control of temperature play a crucial role in ensuring product quality, improving production efficiency, saving energy, ensuring production safety, and promoting the development of the national economy. Due to the universality of temperature measurement, the number of temperature sensors ranks first among various sensors, accounting for about 50%.
Temperature sensors indirectly measure temperature by changing a certain characteristic as the temperature changes. Many materials and components have characteristics that change with temperature, so there are many materials that can be used as temperature sensors. The physical parameters that change with temperature in temperature sensors include expansion, resistance, capacitance, electromotive force, magnetic properties, frequency, optical characteristics, and thermal noise, etc. As production develops, new types of temperature sensors will continue to emerge.
Due to the wide range of temperature measurements in industrial and agricultural production, from several hundred degrees below zero to several thousand degrees above zero, temperature sensors made from various materials can only be used within a certain temperature range.
Temperature sensors are classified into two main categories based on their contact methods with the measured medium: contact and non-contact. Contact temperature sensors need to maintain thermal contact with the measured medium to achieve sufficient heat exchange and reach the same temperature. This category mainly includes resistance temperature detectors, thermocouples, and PN junction temperature sensors. Non-contact temperature sensors do not need to contact the measured medium but measure temperature through the thermal radiation or convection of the measured medium.
This category mainly includes infrared temperature sensors. The main feature of this measurement method is that it can measure the temperature of substances in motion (such as the temperature of bearings on a slowly moving train, or the temperature of a rotating cement kiln) and objects with small thermal capacity (such as temperature distribution in integrated circuits).
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