What is the temperature and how to measure it?
In meteorology, the physical quantity representing the degree of air heat and cold is called temperature, which is referred to as temperature for short. The international standard unit for temperature measurement is Celsius (℃).
The temperature mentioned in the weather forecast refers to the air temperature measured in the open air without direct sunlight (usually measured by blinds). The highest temperature is the highest temperature in a day, generally appearing in14-15; The lowest temperature is the lowest temperature in a day, usually before sunrise. China uses centigrade scale, which is expressed in degrees Celsius. Generally, it is observed four times a day, at 02: 08: 00,/kloc-0: 4: 00 and 20: 00: 00 respectively. According to the actual situation, some stations observe three times a day, at 08: 00, 14 and 20: 00 respectively.
It can be measured with a thermometer.
Introduce several commonly used thermometers:
1. gas thermometer: a thermometer with a certain mass of gas as its working substance. The temperature scale used by gas thermometer to reflect ideal gas is standard temperature scale. The temperature measured by the gas thermometer is consistent with the thermodynamic temperature. Gas thermometers are filled with hydrogen or nitrogen (hydrogen or helium are usually used as temperature measuring substances, because their liquefaction temperatures are very low, close to absolute zero, so their temperature measuring range is very wide), and their properties can be extrapolated to ideal gases. There are two kinds of thermometers: constant volume gas thermometer and constant pressure gas thermometer. Constant volume gas thermometer is that the volume of gas remains constant and the pressure changes with temperature. Constant pressure gas thermometer is that the pressure of gas remains constant and its volume changes with temperature.
2. Resistance thermometer: a thermometer that measures temperature according to the law of conductor resistance changing with temperature. The most commonly used resistance thermometers are made of wire wound temperature sensing elements, mainly platinum resistance thermometers and copper resistance thermometers, and carbon, germanium and rhodium iron resistance thermometers at low temperatures. Precision platinum resistance thermometer is the most accurate thermometer at present, its temperature coverage is about 14 ~ 903 K, and the error can be as low as110000 degrees Celsius. It is a reference thermometer which can reproduce the international practical temperature standard. Our country also uses the first-class and second-class standard platinum resistance thermometers to transfer the temperature scale, and uses this as the standard to verify mercury thermometers and other types of thermometers. It can be divided into metal resistance thermometer and semiconductor resistance thermometer, both of which are made according to the characteristics of resistance value changing with temperature. Metal thermometers are mainly made of pure metals such as platinum, gold, copper and nickel and alloys of rhodium, iron and phosphor bronze. Semiconductor thermometers mainly use carbon and germanium. Resistance thermometer is convenient and reliable to use, and has been widely used. The measuring range is about -260℃ to 600℃.
3. Thermocouple thermometer: a thermometer that measures temperature with a thermocouple. Connect the two ends of two different metal conductors to form a closed loop. One end is heated and the other end is cooled. Because the temperatures of the two contact points are different, electromotive force will be generated and current will be generated in the conductor. Because this thermoelectric electromotive force is a function of the temperature difference between two contact points, a thermometer is made by using this characteristic. If one or more wires of different metals are connected in the loop of thermocouple, the temperature of connecting wires and contact points is uniform, which has no effect on the original electromotive force. By measuring the thermoelectric electromotive force, the measured temperature is obtained, thus forming a thermocouple thermometer. This thermometer has a wide temperature range. For example, the temperature measurement range of thermocouple composed of copper and constantan is between 200 ~ 400℃; Iron and constantan are used between 200 ~ 1000; Thermocouple composed of platinum and platinum-rhodium alloy (rhodium 10%) can measure the temperature above 1000℃. Iridium and iridium rhodium (rhodium 50%) can be used at 2300℃; If tungsten and molybdenum (25% molybdenum) are used, it can be as high as 2600℃.
4. High temperature thermometer: refers to the thermometer specially used for measuring the temperature above 500℃, including optical thermometer, colorimetric thermometer and radiation thermometer. The principle and structure of high temperature thermometer are complicated, so I won't discuss it here. Its measuring range is above 500℃ to 3000℃, so it is not suitable for measuring low temperature.
5. pointer thermometer: it is a kind of thermometer shaped like a dashboard, also known as thermometer, which is used to measure room temperature and is made by using the principle that metals expand when heated and contract when cooled. It uses bimetal as a temperature sensing element to control the pointer. Bimetal sheets are usually riveted together with copper sheets and iron sheets, with copper sheets on the left and iron sheets on the right. Because the thermal expansion and contraction of copper is much more obvious than that of iron, when the temperature rises, the copper sheet pulls the iron sheet to bend to the right, and the pointer deflects to the right (pointing to high temperature) driven by bimetal; On the contrary, when the temperature becomes low, the pointer deflects to the left (pointing to low temperature) driven by bimetal.
6. Glass tube thermometer: Glass tube liquid thermometer is the most widely used thermometer with simple structure, convenient use, high precision and low price. According to the purpose, it can be divided into three types: industrial use, standard use and laboratory use. Standard glass thermometers are supplied in complete sets, which can be used to verify other thermometers, and the accuracy can reach 0.05 ~ 0. 1 Celsius. In order to prevent industrial glass thermometers from being broken in use, glass tubes are usually protected by metal sleeves, and only the scale part is exposed for operators to read. The glass tube thermometer used in the laboratory is similar to the standard in form and has high accuracy.
7. Pressure thermometer: A new generation of liquid pressure thermometer and its series of temperature measuring instruments have overcome the shortcomings of single performance, poor reliability and large temperature packaging products of the original instrument, reduced the volume of temperature measuring elements to the original 1/30 or 1/60, creatively installed the sensor thermal resistor in the temperature measuring elements, and realized the temperature measuring function of electromechanical integration. A multifunctional serial temperature instrument based on liquid pressure thermometer is formed, such as remote transmission, shock prevention, corrosion prevention, electrical contact and temperature signal transmission. Divided into two series, ordinary and explosion-proof type. The principle of this thermometer is to measure the temperature according to the relationship between the saturated vapor pressure of the evaporated liquid and the temperature in a closed temperature measurement system. When the bulb feels the temperature change, the saturated steam in the closed system generates the corresponding pressure, which causes the curvature change of the elastic element and makes its free end displace, and then the gear amplification mechanism changes the displacement into the indicated value. This thermometer has the characteristics of small bulb size, fast response speed, high sensitivity and intuitive reading, and almost integrates all the advantages of glass rod thermometer, bimetal thermometer and gas pressure thermometer. It can be made into shockproof and anticorrosion type, and can realize remote contact. It is a mechanical temperature measuring instrument with the widest application range and the most comprehensive performance at present.
8. Rotating thermometer: The rotating thermometer is made of curled bimetal. One end of the bimetal is fixed and the other end is connected with the pointer. The expansion degree and temperature of the two metal sheets are different, which leads to the different curling degree of the bimetal. Then the pointer points to different positions on the dial, and its temperature can be known from the readings on the dial.
9. Semiconductor thermometer: The resistance change of semiconductor is different from that of metal. With the increase of temperature, its resistance decreases and changes greatly. Therefore, a small amount of temperature change can also make the resistance change obviously. The thermometer has high precision and is often called a temperature sensor.
10. Thermocouple thermometer: Connect two ends of two conductors with different compositions (called thermocouple wires or hot electrodes) to form a loop. When the junction temperature is different, electromotive force will be generated in the loop. This phenomenon is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouple uses this principle to measure temperature, in which one end directly used to measure the temperature of medium is called working end (also called measuring end) and the other end is called cold end (also called compensation end); The cold end is connected with a display instrument or a supporting instrument, and the display instrument will point out the thermoelectric potential generated by the thermocouple. Thermocouple is actually an energy converter, which converts thermal energy into electrical energy and uses the generated thermoelectric potential to measure temperature. For the thermoelectric potential of thermocouple, we should pay attention to the following problems: ① The thermoelectric potential of thermocouple is the difference of temperature function between the two ends of thermocouple working end, not the function of temperature difference between the cold end and working end of thermocouple; (2) When the thermocouple material is uniform, the thermoelectric potential generated by the thermocouple has nothing to do with the length and diameter of the thermocouple, but only with the composition of the thermocouple material and the temperature difference between the two ends; ③ When the material composition of two thermocouple wires is determined, the thermoelectric potential of thermocouple is only related to the temperature difference of thermocouple; If the temperature at the cold end of the thermocouple remains unchanged, the thermoelectric potential of the thermocouple is only a single-valued function of the temperature at the working end.
1 1. Optical pyrometer: It is an instrument for measuring high temperature by using the relationship between brightness and temperature radiated by heat source. The main part of the instrument includes: installing a red glass filter F and a small light bulb L in the M tube of the telescope. When the optical pyrometer faces the furnace. From the telescope, you can see the black filament of the bulb and the strong light of the fire behind it. The filament is connected in series with the power supply E and the variable resistor R, and the resistance of the variable resistor R is adjusted to make an appropriate current pass through the filament. Until the brightness of the filament is the same as that of the fire. If the known temperature value is engraved on ammeter A in advance, the temperature value can be read directly from the reading of ammeter. When measuring the temperature, there is no need for the instrument to contact with the measured object, so the optical pyrometer can be used to measure the temperature above the melting point of many metals. If the temperature of an object is high enough to emit a lot of visible light, its temperature can be determined by measuring its thermal radiation. This kind of thermometer is an optical thermometer. This thermometer mainly consists of a telescope with a red filter and a set of circuits with a small bulb, an ammeter and a variable resistor. Before use, establish the relationship between the temperature corresponding to different brightness of filament and the reading on ammeter. When in use, aim the telescope at the measured object, and adjust the resistance so that the brightness of the bulb is the same as that of the measured object. At this time, the temperature of the object to be measured can be read from the galvanometer.
12, liquid crystal thermometer: Liquid crystals made of different formulations have different phase transition temperatures, and their optical properties will also change during phase transition, making the liquid crystals 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 of liquid crystal color. The advantage of this thermometer is that it is easy to read, but the disadvantage is that it is not accurate enough. Often used to watch fish tanks, indicating water temperature.
reference data
China instrument network. China instrument network [reference time 2017-12-19]