I. Classification
Contact
A contact temperature sensor, also known as a thermometer, has its sensing part in good contact with the object being measured.
Thermometers achieve thermal equilibrium through conduction or convection, allowing the thermometer reading to directly represent the temperature of the object being measured.
Generally, thermometers offer high measurement accuracy. Within a certain temperature range, they 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, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples. They are widely used in industry, agriculture, commerce, and other sectors. People also frequently use these thermometers in daily life. With the widespread application of cryogenic technology in defense engineering, space technology, metallurgy, electronics, food, medicine, and petrochemical industries, and with research into superconducting technology, cryogenic thermometers for measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, cryogenic resistance thermometers, and cryogenic thermocouples. Cryogenic thermometers require small-sized sensing elements with high accuracy, good repeatability, and stability. Carburized glass resistance thermometers, made by carburizing and sintering porous high-silica glass, are a type of sensing element in cryogenic thermometers and can be used to measure temperatures in the range of 1.6–300K .
Non-contact
Its sensing element does not contact the object being measured, hence it is also known as 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 (transient) temperature changes. It 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 temperature measuring instruments.
Radiation thermometry includes the luminance method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer). Each radiation thermometry method can only measure the corresponding photometric temperature, radiation temperature, or colorimetric temperature. Only the temperature measured for a blackbody (an object that absorbs all radiation and does not reflect light) is the true temperature. To determine the true temperature of an object, corrections for the material's surface emissivity must be made. However, the surface emissivity depends not only on temperature and wavelength but also on surface condition, coating, and microstructure, making precise measurement difficult. In automated production, radiation thermometry is often used to measure or control the surface temperature of certain objects, such as the rolling temperature of steel strips, rolls, forgings, and the temperatures of various molten metals in furnaces or crucibles in metallurgy. In these specific cases, measuring the surface emissivity of an object is quite difficult. For automatic measurement and control of solid surface temperature, an additional reflector can be used to form a blackbody cavity with the surface being measured. The effect of the additional radiation can increase the effective radiation and effective emissivity of the measured surface. By using the effective emissivity to correct the measured temperature with an instrument, the true temperature of the measured surface can be obtained. The most typical additional reflector is a hemispherical reflector. Diffuse radiation energy from the measured surface near the center of the sphere is reflected back to the surface by the hemispherical mirror, forming additional radiation, thereby increasing the effective emissivity. In the formula , ε is the emissivity of the material surface, and ρ is the reflectivity of the reflector.
For radiation measurements of the true temperature of gaseous and liquid media, a method can be used to insert a heat-resistant material tube to a certain depth to form a blackbody cavity. The effective emission coefficient of the cylindrical cavity after reaching thermal equilibrium with the medium can be calculated. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (i.e., the medium temperature) to obtain the true temperature of the medium.
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.
II. Working Principle
Sensor designed based on the principle of metal expansion
Metals expand when the ambient temperature changes, and sensors can convert this response into signals in different ways.
Bimetallic strip sensor
A bimetallic strip consists of two metals with different coefficients of thermal expansion bonded together. As the temperature changes, material A expands more than the other metal, causing the strip to bend. The curvature of the bend can be converted into an output signal.
Bimetallic rod and metal tube sensor
As the temperature rises, the length of the metal tube (material A) increases, while the length of the non-expanding steel rod (metal B ) does not increase. Thus, the linear expansion of the metal tube due to the change in position can be transmitted. Conversely, this linear expansion can be converted into an output signal.
Sensors designed based on the deformation curves of liquids and gases
When the temperature changes, both liquids and gases will change in volume accordingly.
Various types of structures can convert this expansion change into a position change, thus producing a position change output (potentiometer, sensing deviation, baffle, etc.).
Resistance sensing
The electrical resistance of a metal changes with temperature.
For different metals, the change in resistance is different for every degree of temperature change, and the resistance value can be directly used as an output signal.
There are two types of resistance changes.
Positive temperature coefficient
Temperature increases = resistance increases
Temperature decrease = resistance decrease
Negative temperature coefficient
Temperature increases = resistance decreases
Temperature decrease = resistance increase
Thermocouple sensing
A thermocouple consists of two metal wires of different materials, which are welded together at the ends. By measuring the ambient temperature of the unheated part, the temperature of the heated point can be accurately determined. Because it must have two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivities are also different. The sensitivity of a thermocouple is the change in output potential difference when the temperature of the heated point changes by 1 °C. For most thermocouples supported by metal materials, this value is approximately between 5 and 40 microvolts / °C. [1]
Because the sensitivity of thermocouple temperature sensors is independent of the material's thickness, temperature sensors can be made from very fine materials. Furthermore, due to the excellent ductility of the metal materials used to make thermocouples, these tiny temperature-sensing elements have extremely high response speeds, enabling them to measure rapidly changing processes.
III. Development Status
In recent years, the modernization of China's industry and the continuous high-speed growth of its electronic information industry have driven the rapid rise of the sensor market. Temperature sensors, as a crucial category of sensors, account for over 40% of the total sensor demand. Temperature sensors utilize the characteristic of NTC ( nitrile metal detector) resistance changing with temperature to convert non-electrical physical quantities into electrical quantities, enabling precise temperature measurement and automatic control. Temperature sensors have a wide range of applications, including temperature measurement and control, temperature compensation, flow rate, airflow and wind speed determination, liquid level indication, ultraviolet and infrared light measurement, and microwave power measurement. They are widely used in color TVs, computer color monitors, switching power supplies, water heaters, refrigerators, kitchen equipment, air conditioners, and automobiles. The rapid growth of the automotive electronics and consumer electronics industries in recent years has further fueled the rapid growth in demand for temperature sensors in China.
IV. Main Uses
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).
V. Application Areas
Temperature sensors are among the earliest developed and most widely used types of sensors. Their market share far exceeds that of other sensors. Temperature measurement began in the early 17th century. With the support of semiconductor technology, semiconductor thermocouple sensors, PN junction temperature sensors, and integrated temperature sensors have been developed in this century .
When two conductors of different materials are connected at a point, heating that connection point will create a potential difference in the unheated areas. The value of this potential difference depends on the temperature of the unheated area and the materials of the two conductors. This phenomenon can occur over a wide temperature range. By accurately measuring this potential difference and the ambient temperature of the unheated area, the temperature of the heated point can be accurately determined. Because it requires two conductors of different materials, it is called a "thermocouple." Thermocouples made of different materials are used in different temperature ranges, and their sensitivities also vary.
Thermocouple sensors have their own advantages and disadvantages. They have relatively low sensitivity, are easily affected by environmental interference signals, and are also susceptible to preamplifier temperature drift, making them unsuitable for measuring minute temperature changes. Furthermore, the sensitivity of thermocouple temperature sensors is independent of the material's thickness.
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