The working principle and classification of the thermometer

A variety of thermometers have been designed and manufactured depending on the purpose of use. The design basis is: the phenomenon of thermal expansion and contraction caused by the influence of temperature on solids, liquids and gases; under constant volume conditions, the gas (or vapor pressure changes due to different temperatures; the effect of thermoelectric effect; resistance with temperature) Change changes; the effects of thermal radiation, etc.

A thermometer is a general term for temperature measuring instruments. Depending on the temperature measurement substance used and the temperature range, there are kerosene thermometers, alcohol thermometers, mercury thermometers, gas thermometers, resistance thermometers, thermocouple thermometers, radiation thermometers and photometric thermometers, and bimetal thermometers.

As the temperature range is wider and wider, different temperature measuring instruments are manufactured according to different requirements. Several types are described below.

1. Gas thermometer: Use hydrogen or helium as the temperature measurement substance. Because the liquefaction temperature of hydrogen and helium is very low, it is close to absolute zero, so its temperature range is very wide. This thermometer is highly accurate and is used for precision measurements.

2, resistance thermometer: divided into metal resistance thermometer and semiconductor resistance thermometer, are based on the characteristics of the resistance value changes with temperature. Metal thermometers are mainly made of pure metals such as platinum, gold, copper, nickel, and bismuth iron or phosphor bronze alloys; semiconductor thermometers mainly use carbon and germanium. The resistance thermometer is easy to use and has been widely used. Its measurement range is from -260 ° C to 600 ° C. 

3, bimetal thermometer: refers to the thermometer used to measure the temperature above 500 ° C, there are light thermometer, colorimetric thermometer and radiation thermometer. The principle and construction of bimetal thermometers are complex and will not be discussed here. Its measurement range is from 500 ° C to 3000 ° C, not suitable for measuring low temperature.

4, pointer thermometer: is a thermometer like a dashboard, also known as the thermometer, used to measure room temperature, is made of metal thermal expansion and contraction principle. It uses a bimetal as a temperature sensing element to control the pointer. Bimetals are usually riveted together with copper and iron, with the copper on the left and the iron on the right. Since the thermal expansion and contraction effect of copper is much more obvious than that of iron, when the temperature rises, the copper piece pulls the iron piece to the right, and the pointer is deflected to the right under the action of the bimetal (pointing to high temperature); The temperature becomes lower and the pointer is deflected to the left (pointing to low temperature) under the action of the bimetal.

5. Thermocouple Thermometer: It is a kind of temperature measuring instrument widely used in industry. Made using thermoelectric phenomena. Two different wires are welded together to form a working end, and the other ends are connected to a measuring instrument to form an electric circuit. When the working end is placed at the temperature to be measured and the working end and the free end temperature are different, an electromotive force will appear, so that a current flows through the circuit. The temperature at another location can be determined by measuring the electrical quantity using the temperature at a known location. This kind of thermometer is mostly composed of copper - Kang copper, iron - Kang copper, nickel Ming - Kang copper, gold and cobalt - copper, platinum - bismuth and so on. It is suitable for use between two substances 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. 

6, glass tube thermometer: glass tube thermometer is the use of thermal expansion and contraction principle to achieve temperature measurement. Because the expansion coefficient of the temperature measuring medium is different from the boiling point and freezing point, our common glass tube thermometers are: kerosene thermometer, mercury thermometer, red pen water thermometer. His advantages are simple structure, easy to use, relatively high measurement accuracy and low price. The disadvantage is that the upper and lower limits of measurement and accuracy are limited by the quality of the glass and the properties of the temperature measuring medium. And can not be passed far, fragile.

7. Pressure thermometer: The pressure thermometer uses the liquid in the closed container, and the gas or saturated vapor is heated to generate volume expansion or pressure change as a measurement signal. Its basic structure consists of three parts: a warm pack, a capillary and an indicator. It is one of the first methods used in the temperature control of production processes. Pressure temperature measurement systems are still a widely used measurement method for local indication and control of temperature. The advantages of the pressure thermometer are: simple structure, high mechanical strength, and no fear of vibration. The price is low and no external energy is needed. The disadvantages are: the temperature range is limited, generally -80~400 °C; the heat loss has a slow response time; the instrument sealing system (warm pack, capillary tube, spring tube) is difficult to repair and must be replaced; the measurement accuracy is affected by the ambient temperature. The temperature package installation position has a large influence and the accuracy is relatively low; the capillary transmission distance is limited. The working range of the pressure thermometer should be in the range of 1/2--3/4 of the measurement range, and the display meter and the temperature pack should be placed in a horizontal position as much as possible. The installation of the thermostat mounting bolts will cause the temperature to be lost and the temperature will be inaccurate. The installation should be insulated and the temperature should be kept in a vibrating environment.

8. Thermocouple Thermometer: The thermocouple thermometer consists of two sensitive metals connected to a sensitive voltmeter. Metal junctions produce different potential differences across the metal at different temperatures. The potential difference is very small, so it needs to be measured by a sensitive voltmeter. From the reading of the voltmeter, you can know the temperature.

9. Photometric pyrometer: If the temperature of the object is high enough to emit a large amount of visible light, the temperature of the object can be determined by measuring the amount of heat radiation. This thermometer is a photometric thermometer. The thermometer is mainly made up of a telescope with a red filter and a set of circuits with small bulbs, ammeters and variable resistors. Before use, first establish the relationship between the temperature corresponding to the different brightness of the filament and the reading on the ammeter. When using, the telescope is aligned with the object to be tested, and the resistance is adjusted so that the brightness of the bulb is the same as the object to be tested. At this time, the temperature of the object to be tested can be read from the ammeter.

10. Liquid crystal thermometer: Liquid crystals made with different formulations have different phase transition temperatures. When their phase changes, their optical properties will also change, making the liquid crystal look colored. If liquid crystals of different phase transition temperatures are applied to a piece of paper, the temperature can be known by the change in the color of the liquid crystal. The advantage of this thermometer is that it is easy to read, but the shortcoming is the lack of precision, which is often used in ornamental fish tanks to indicate.

11. Rotary thermometer: The rotary thermometer is made of a crimped bimetal. The bimetal is fixed at one end and the other end is connected to the pointer. Due to the degree of expansion of the two metal sheets, the degree of curling of the bimetal is different at different temperatures, and the pointer is then pointed to different positions on the dial, and the temperature is known from the reading on the dial.

12. Semiconductor thermometer: The resistance change of semiconductor is different from that of metal. When the temperature rises, its resistance decreases, and the variation range is large. Therefore, a small amount of temperature change can also cause a significant change in the resistance, and the resulting thermometer has a high degree of precision, often referred to as a temperature sensor.

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