Temperature sensor type and application

As we all know, temperature is a basic physical quantity, and all processes in nature are closely related to temperature. The temperature sensor is the earliest and most widely used type of sensor. The market share of temperature sensors exceeds that of other sensors. From the beginning of the 17th century, people began to use temperature for measurement. With the support of semiconductor technology, semiconductor thermocouple sensors, PN junction temperature sensors and integrated temperature sensors have been developed in this century. Correspondingly, acoustic temperature sensors, infrared sensors and microwave sensors have been developed in succession according to the laws of interaction between waves and matter.

Conductors of two different materials, such as being connected to each other at a certain point, heat the connection point, and a potential difference occurs at the point where they are not heated. The value of this potential difference is related to the temperature of the measurement point of the non-heating part and is related to the material of the two conductors. This phenomenon can occur over a wide temperature range. If the potential difference is accurately measured and the ambient temperature of the unheated area is measured, the temperature of the heated point can be accurately known. Because it must have two different material conductors, it is called a "thermocouple." Thermocouples made of different materials are used in different temperature ranges and their sensitivity varies. The sensitivity of a thermocouple is the amount of change in the output potential difference when the temperature of the heating point changes by 1°C. For most metal-supported thermocouples, this value is approximately between 5 and 40 microvolts/°C.

Thermocouple sensors have their own advantages and disadvantages. They have low sensitivity, are easily affected by environmental interference signals, and are susceptible to preamplifier temperature drift. Therefore, they are not suitable for measuring small temperature changes. Since the sensitivity of the thermocouple temperature sensor is independent of the thickness of the material, a very thin material can also be used as a temperature sensor. Also because of the very good ductility of the metal materials used to make thermocouples, this subtle temperature measurement element has a very high response speed and can measure rapidly changing processes.

Temperature sensors are the most commonly used among a wide variety of sensors. Modern temperature sensors are very small in appearance, which makes it more widely used in various fields of production practice, and provides countless conveniences and functions for our lives. .

There are four main types of temperature sensors: thermocouples, thermistors, resistance temperature detectors (RTDs), and IC temperature sensors. IC temperature sensor includes analog output and digital output.

The detection part of the contact type temperature sensor has good contact with the measured object, which is also called a thermometer.

The thermometer achieves thermal equilibrium through conduction or convection so that the indication of the thermometer can directly represent the temperature of the measured object. The general measurement accuracy is high. Within a certain temperature range, the thermometer can also measure the temperature distribution inside the object. However, for sports objects, small objects or objects with small heat capacity, large measurement errors will occur. Commonly used thermometers include bimetal thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples. They are widely used in industrial, agricultural, commercial and other sectors. People often use these thermometers in daily life. With the wide application of cryogenic technology in defense engineering, space technology, metallurgy, electronics, food, medicine, and petrochemicals, and the study of superconducting technology, cryogenic thermometers measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, steam Pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, low-temperature thermal resistance, and low-temperature thermocouples. Cryogenic thermometers require that the temperature sensing element be small, accurate, reproducible, and stable. The carburizing glass thermal resistance formed by carburizing sintered porous high-silica glass is a temperature sensing element of a low-temperature thermometer and can be used to measure temperatures in the range of 1.6 to 300K.

The non-contact temperature sensor's sensitive components and the measured object do not contact each other, also known as non-contact temperature measurement instrument. This instrument can be used to measure the surface temperature of moving objects, small objects and objects with small heat or rapid temperature changes (transients). It can also be used to measure the temperature distribution of a temperature field. The most commonly used non-contact temperature measurement instrument is based on the basic law of blackbody radiation and is called a radiation thermometer. Radiation thermometry includes the brightness method (see optical pyrometer), radiation method (see radiation pyrometer) and colorimetry (see colorimeter). All kinds of radiation temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured for a black body (a body that absorbs all radiation and does not reflect light) is the true temperature. To determine the true temperature of the object, the material surface emissivity must be corrected. The surface emissivity of the material depends not only on the temperature and wavelength, but also on the surface state, coating film, and microstructure, and it is difficult to measure accurately. In automated production it is often necessary to use radiation thermometry to measure or control the surface temperature of certain objects, such as steel strip rolling temperature in metallurgy, roll temperature, forging temperature, and the temperature of various molten metals in the smelting furnace or crucible . In these specific cases, the measurement of the surface emissivity of the object is quite difficult. For the automatic measurement and control of the solid surface temperature, additional mirrors can be used to form the black body cavity together with the measured surface. The effect of additional radiation can increase the effective radiation and effective emission coefficient of the measured surface. The actual measured temperature is corrected by the instrument using the effective emission coefficient, and the true temperature of the measured surface can be finally obtained. The most typical additional mirror is a hemispherical mirror. The diffuse radiant energy of the measured surface near the center of the sphere is reflected back to the surface by the hemispherical mirror to form additional radiation, thereby increasing the effective emission coefficient: where ε is the material surface emissivity and ρ is the reflectivity of the mirror. For radiometric measurements of the true temperature of the gas and liquid medium, a method of inserting a tube of heat-resistant material to a certain depth to form a black body cavity may be used. The calculation of the effective emission coefficient of the cylindrical cavity after thermal equilibrium with the medium is calculated. In automatic measurement and control, this value can be used to correct the temperature of the measured cavity bottom (ie the medium temperature) to obtain the real temperature of the medium.

Non-contact temperature measurement Advantages: The upper limit of measurement is not limited by the temperature resistance of the temperature-sensing element, so there is no limit to the maximum measurable temperature in principle. For high temperatures above 1800°C, non-contact temperature measurement methods are used. With the development of infrared technology, the radiation temperature measurement gradually expands from visible light to infrared light, and temperatures below 700°C have been used up to room temperature, and the resolution is very high.