A Brief Talk on Temperature Sensors

Temperature is a basic physical quantity. All processes in nature are closely related to temperature. Temperature sensors are the earliest developed and most widely used sensors. The market share of temperature sensors far exceeds that of other sensors. People began to use temperature for measurement 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 successively in this century. Correspondingly, according to the law of interaction between waves and matter, acoustic temperature sensors, infrared sensors and microwave sensors have been developed successively.

If two conductors of different materials are connected together at a certain point and this connection point is heated, a potential difference will appear in the unheated part. The value of this potential difference is related to the temperature of the measurement point in the unheated part and the materials of the two conductors. This phenomenon can occur in a wide temperature range. If this potential difference is accurately measured and the ambient temperature of the unheated part is measured, the temperature of the heated point can be accurately known. Since 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 refers to the change in the 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.

Thermocouple sensors have their own advantages and disadvantages. They have low sensitivity and are easily affected by environmental interference signals and preamplifier temperature drift, so they are not suitable for measuring small temperature changes. Since the sensitivity of thermocouple temperature sensors has nothing to do with the thickness of the material, temperature sensors can be made of very fine materials. Also, because the metal material used to make thermocouples has good ductility, this fine temperature measuring element has an extremely high response speed and can measure rapidly changing processes.

Temperature sensor is the most commonly used one among various sensors. Modern temperature sensor is very small in size, which makes it 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 sensors include analog output and digital output types.

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

Thermometers achieve thermal equilibrium through conduction or convection, so that the indication of the thermometer can directly represent the temperature of the object being measured. Generally, the measurement accuracy is high. Within a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object. However, for moving objects, small targets or objects with very small heat capacity, large measurement errors will occur. 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 departments. People also often use these thermometers in daily life. With the wide application of cryogenic technology in defense engineering, space technology, metallurgy, electronics, food, medicine and petrochemical industries and the research of superconducting technology, low-temperature thermometers that measure temperatures below 120K have been developed, such as low-temperature gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, low-temperature thermal resistors and low-temperature thermocouples. Low-temperature thermometers require small temperature sensing elements, high accuracy, good reproducibility and stability. The carburized glass thermal resistor made by carburizing and sintering porous high-silicon oxide 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 sensitive element of the non-contact temperature sensor does not contact the object being measured, so it is also called a non-contact temperature measuring instrument. This instrument can be used to measure the surface temperature of moving objects, small targets, and objects with small heat capacity or rapid temperature changes (transients). It can also be used to measure the temperature distribution of temperature fields. The most commonly used non-contact temperature measuring instrument is based on the basic law of blackbody radiation and is called a radiation temperature measuring instrument. The radiation temperature measurement method includes the brightness method (see optical pyrometer), the radiation method (see radiation pyrometer) and the colorimetric method (see colorimetric thermometer). Various radiation temperature measurement methods 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. If you want to determine the true temperature of an object, you must correct the surface emissivity of the material. The surface emissivity of the material depends not only on the temperature and wavelength, but also on the surface state, coating and microstructure, so it is difficult to measure accurately. In automated production, it is often necessary to use radiation temperature measurement to measure or control the surface temperature of certain objects, such as the rolling temperature of steel strips, the temperature of rolling rollers, the temperature of forgings, and the temperature of various molten metals in smelting furnaces or crucibles in metallurgy. In these specific cases, it is quite difficult to measure the surface emissivity of objects. For the automatic measurement and control of solid surface temperature, an additional reflector can be used to form a black body cavity together with the measured surface. The influence of additional radiation can increase the effective radiation and effective emission coefficient of the measured surface. The actual temperature can be corrected accordingly by the instrument using the effective emission coefficient, and the real temperature of the measured surface can be obtained. The most typical additional reflector is a hemispherical reflector. The diffuse radiation of the measured surface near the center of the sphere can be reflected back to the surface by the hemispherical mirror to form additional radiation, thereby increasing the effective emission coefficient: where ε is the surface emissivity of the material, and ρ is the reflectivity of the reflector. As for the radiation measurement of the real temperature of gas and liquid media, the method of inserting a heat-resistant material tube to a certain depth to form a black body cavity can be used. The effective emission coefficient of the cylindrical cavity after reaching thermal equilibrium with the medium is calculated. In automatic measurement and control, this value can be used to correct the measured cavity bottom temperature (i.e. medium temperature) to obtain the actual temperature of the medium.

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