A temperature sensor is a sensor that can sense temperature and convert it into a usable output signal. Temperature sensors are the core component of temperature measuring instruments and come in a wide variety. They can be broadly classified into two categories based on the measurement method: contact and non-contact. Based on the sensor materials and electronic component characteristics, they are further divided into resistance temperature detectors (RTDs) and thermocouples.
Main classifications of temperature sensors
Temperature sensors can be divided into two main categories based on their measurement method: contact temperature sensors and non-contact temperature sensors. They can also be divided into two categories based on their sensor materials and electronic component characteristics: resistance temperature sensors and thermocouple temperature sensors.
1. Contact type: The detection part of a contact type temperature sensor has good contact with the object being measured, and is also known as a thermometer.
A thermometer achieves thermal equilibrium through conduction or convection, allowing its reading to directly represent the temperature of the object being measured. Generally, they offer high measurement accuracy. Within a certain temperature range, thermometers can also measure the internal temperature distribution of an object. However, they can produce significant measurement errors for moving objects, small targets, or objects with very low heat capacity. Commonly used thermometers include bimetallic thermometers, liquid glass thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples.
2. Non-contact type: Its sensing element does not contact the object being measured, hence it is also called a non-contact temperature measuring instrument. This type of instrument can be used to measure the surface temperature of moving objects, small targets, and objects with small heat capacity or rapid temperature changes (transient), and can also be used to measure the temperature distribution of a temperature field.
The most commonly used non-contact temperature measuring instruments are based on the fundamental law of blackbody radiation and are called radiation thermometers. Radiation thermometry includes the luminance method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer). Each type of radiation thermometry can only measure the corresponding photometric temperature, radiation temperature, or colorimetric temperature. Only the temperature measured on a blackbody (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 limited by the temperature resistance of the sensing element, so there is no limit to the highest measurable temperature in principle. For high temperatures above 1800℃, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation thermometry has gradually expanded from visible light to infrared light, and it has been used for temperatures below 700℃ up to room temperature, with very high resolution.
Main applications of temperature sensors
Temperature is a physical quantity that characterizes the degree of hotness or coldness of an object, and it is a very important and ubiquitous measurement parameter in industrial and agricultural production. Temperature measurement and control play a vital role in ensuring product quality, improving production efficiency, saving energy, ensuring production safety, and promoting national economic development. Due to the widespread use of temperature measurement, temperature sensors rank first in number among all types of sensors, accounting for approximately 50%.
Temperature sensors indirectly measure temperature by observing how an object changes its properties with temperature. Many materials and components exhibit temperature-dependent properties, making them suitable for temperature sensors. The physical parameters of a temperature sensor that change with temperature include: expansion, resistance, capacitance, electromotive force, magnetic properties, frequency, optical characteristics, and thermal noise. As production advances, new types of temperature sensors will continue to emerge.
Because the temperature measurement range in industrial and agricultural production is extremely wide, from hundreds of degrees below zero to thousands of degrees above zero, temperature sensors made of various materials can only be used within a certain temperature range.
Temperature sensors can be categorized into two main types based on their contact method with the measured medium: contact and non-contact. Contact temperature sensors require thermal contact with the measured medium to ensure sufficient heat exchange and reach the same temperature. Examples of this type include resistive sensors, thermocouples, and PN junction temperature sensors. Non-contact temperature sensors do not require physical contact with the measured medium. Instead, temperature is measured through the thermal radiation or convection of the measured medium. Infrared temperature sensors are a primary example of this type. The main advantage of this method is its ability to measure the temperature of moving materials (such as the bearing temperature of a slowly moving train or the temperature of a rotating cement kiln) and objects with small heat capacity (such as the temperature distribution within an integrated circuit).
