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technical article
What are the temperature measuring instruments?
2011-5-26 16:55:46

With the development of science and technology and the needs of modern industrial technology, the temperature measurement technology has also been continuously improved and improved. As the temperature measurement range is getting wider and wider, according to different requirements, different temperature measurement instruments are manufactured. Here are a few.

1. Optical pyrometer : When the temperature of an object is high enough to emit a large amount of visible light, it can be used to determine the temperature by measuring the amount of its thermal radiation. This type of thermometer is an optical thermometer. This thermometer is mainly made of a telescope with a red filter and a set of circuits with a small bulb, an ammeter and a variable resistor. Before use, first establish the relationship between the temperature corresponding to the different brightness of the filament and the reading on the galvanometer. When in use, align the telescope with the object under test and adjust the resistance so that the brightness of the bulb is the same as the object under test. At this time, the temperature of the object under test can be read from the galvanometer. 12. Liquid crystal thermometer: Liquid crystals made with different formulas have different phase transition temperatures. When the phase transition occurs, the optical properties of the liquid crystal will also change, making the liquid crystal look discolored. If liquid crystals with different phase transition temperatures are coated on a piece of paper, the temperature can be known from the change in the color of the liquid crystal. The advantage of this thermometer is that it is easy to read, but the disadvantage is that it is not accurate enough. It is often used in ornamental fish tanks to indicate the water temperature.
2. Gas thermometer : Hydrogen or helium is often used as a temperature measurement substance, because the liquefaction temperature of hydrogen and helium is very low, close to absolute zero, so its temperature measurement range is very wide. This kind of thermometer has high accuracy, and is mostly used for precision measurement.
3. Resistance thermometer : It is divided into metal resistance thermometer and semiconductor resistance thermometer, which are made according to the characteristic of resistance value changing with temperature. Metal thermometers are mainly used for pure metals such as platinum, gold, copper, nickel and rhodium iron, phosphor bronze alloys; semiconductor thermometers are mainly used for carbon, germanium, etc. Resistance thermometers are easy to use and reliable, and have been widely used. Its measurement range is around -260 ° C to 600 ° C.
4. Thermocouple thermometer: It is a temperature measuring instrument widely used in industry. Made by the phenomenon of temperature difference electricity. Two different metal wires are welded together to form a working end, and the other ends are connected to a measuring instrument to form a circuit. When the working end is placed at the temperature to be measured, when the working end and the free end have different temperatures, an electromotive force will appear, and a current will flow through the circuit. Through the measurement of electrical quantity, the temperature at another place can be determined by using the temperature at a known place. This type of thermometer is usually composed of copper-constantan, iron-constantan, nickel-nickel-constantan, gold-cobalt-copper, platinum-rhodium, and the like. It is suitable for two materials with large temperature difference, and is mostly used for high temperature and low turbidity measurement. Some thermocouples can measure high temperatures up to 3000 ° C, and some can measure low temperatures close to absolute zero.
5, high temperature thermometer: refers to a thermometer specifically used to measure temperatures above 500 ° C, there are light thermometers, colorimeter thermometers and radiation thermometers. The principle and structure of high temperature thermometers are relatively complicated and will not be discussed here. Its measurement range is from 500 ° C to 3000 ° C or higher, which is not suitable for measuring low temperatures.
6, pointer thermometer : is a thermometer shaped like a dashboard, also known as the cold and summer meter, used to measure room temperature, is made of metal thermal expansion and contraction principle. It uses bimetal as a temperature sensing element to control the pointer. Bimetals are usually riveted with copper and iron, with copper on the left and iron on the right. Because the thermal expansion and contraction of copper is more obvious than that of iron, when the temperature increases, the copper sheet pulls the iron sheet to bend to the right, and the pointer is deflected to the right (pointing to high temperature) by the bimetal sheet; , The temperature becomes low, the pointer is deflected to the left (pointing to low temperature) driven by the bimetal.
7. Glass tube thermometer: The glass tube thermometer uses the principle of thermal expansion and contraction to achieve temperature measurement. Because the expansion coefficient of the temperature measurement medium is different from the boiling point and the freezing point, our common glass tube thermometers are: kerosene thermometer, mercury thermometer, red pen water thermometer. Its advantages are simple structure, easy to use, relatively high measurement accuracy and low price. The disadvantage is that the upper and lower limits and accuracy of the measurement are limited by the quality of the glass and the nature of the temperature measurement medium. It can't be transmitted far, and it is fragile.
8. Pressure type thermometer : The pressure type thermometer uses the liquid, gas or saturated vapor in a closed container to generate volume expansion or pressure change as a measurement signal after being heated. Its basic structure is composed of a thermostat, a capillary tube and an indicator meter. It is one of the earliest methods used for temperature control in the production process. Pressure-type temperature measurement systems are still widely used in local indication and control of temperature. The advantages of pressure thermometers are: simple structure, high mechanical strength, and not afraid of vibration. Low price and no external energy required. The disadvantages are: the temperature measurement range is limited, generally in the range of -80 ~ 400 ° C; the large heat loss has a slow response time; the instrument sealing system (temperature pack, capillary tube, spring tube) is difficult to repair and must be replaced; the measurement accuracy is affected by ambient temperature, The installation position of the temperature pack has a great influence and the accuracy is relatively low; the capillary transmission distance is limited. The normal working range of the pressure thermometer should be 1 / 2--3 / 4 of the measurement range, and the display meter and the temperature pack should be in a horizontal position as much as possible. The installation temperature of the temperature pack mounting bolts will cause temperature loss and cause inaccurate temperature. During installation, heat insulation treatment should be performed, and the temperature pack should be operated in an environment without vibration as much as possible.
9. Rotary thermometer: The rotary thermometer is made of a curled bimetal. The bimetal is fixed at one end and connected to a pointer at the other end. Due to the different degrees of expansion of the two metal sheets, the bimetal sheet curls differently at different temperatures, and the pointer will then point to different positions on the dial. The temperature can be known from the reading on the dial.
10. Semiconductor thermometer: The resistance change of a semiconductor is different from that of a metal. When the temperature increases, its resistance decreases, and the change range is larger. Therefore, a small amount of temperature change can also cause a significant change in the resistance. The manufactured thermometer has a higher precision and is often called a temperature sensor. Stainless steel electrode 11, thermocouple thermometer: Thermocouple thermometer is composed of two different metals connected to a sensitive voltmeter. At different temperatures, metal contacts will produce different potential differences between the two ends of the metal. The potential difference is so small that it requires a sensitive voltmeter to measure it. From the reading of the voltmeter, you can know the temperature.