Chinese name: thermometer
English name: thermometer; Thermometer; Heat indicator; temperature indicator
Introduction: Thermometers can accurately judge and measure the temperature, and are divided into pointer thermometers and digital thermometers.
Working principle According to different purposes, various thermometers are designed and manufactured. The basis of its design is: using the phenomenon that solids, liquids and gases expand when heated and contract when cooled under the influence of temperature; Under the condition of constant volume, the pressure of gas (or steam) changes with different temperatures; The role of thermoelectric effect; Resistance varies with temperature; The influence of thermal radiation, etc.
Generally speaking, any physical property of all substances can be used to mark the temperature and make a thermometer as long as it changes monotonously and significantly with the change of temperature.
Working principle of various thermometers 1. Gas thermometer: Hydrogen or helium is often used as temperature measuring material, because the liquefaction temperature of hydrogen and helium is very low, close to absolute zero, so its temperature measuring range is very wide. This kind of thermometer has high accuracy and is mostly used for precise measurement.
2. Resistance thermometer: it can be divided into metal resistance thermometer and semiconductor resistance thermometer, both of which are made according to the characteristics of resistance 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: It is a temperature measuring instrument widely used in industry. Composed of thermoelectric phenomena. Two different metal wires are welded together to form a working end, and the other two ends are connected with a measuring instrument to form a circuit. When the working end is placed at the temperature to be measured, when the temperatures of the working end and the free end are different, electromotive force will appear, so there is current in the circuit. By measuring the electric quantity, we can use the temperature of a known place to measure the temperature of another place. It is suitable for measuring high temperature and low turbidity between two substances with large temperature difference. Some thermocouples can measure high temperatures as high as 3000℃, while others can measure low temperatures close to absolute zero.
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 thermometer uses the principle of thermal expansion and cold contraction to measure temperature. Because the expansion coefficient of temperature measuring medium is different from boiling point and freezing point, our common glass tube thermometers mainly include kerosene thermometer, mercury thermometer and red pen water thermometer. Its advantages are simple structure, convenient use, high measurement accuracy and low price. The disadvantage is that the upper and lower limits and accuracy of measurement are limited by the quality of glass and the properties of temperature measuring medium. And can't spread very far, fragile.
7. Pressure thermometer: A pressure thermometer uses liquid, gas or saturated steam in a closed container to generate volume expansion or pressure change after being heated as a measuring signal. Its basic structure consists of three parts: temperature bag, capillary and indicator. The advantages of pressure thermometer are simple structure, high mechanical strength and no fear of vibration. The price is low and no external energy is needed. Disadvantages are: the temperature measuring range is limited, generally at-80 ~ 400℃; Large heat loss and slow response time.
8. Mercury thermometer: Mercury thermometer is an expansion thermometer. The freezing point of mercury is -38.87℃ and the boiling point is 356.7℃. Used to measure the temperature in the range of 0- 150℃ or 500℃. Can only be used as a tool for local supervision. Using it to measure temperature is not only simple and intuitive, but also can avoid the error of external remote thermometer.
When using a mercury thermometer, we should first see its measuring range, and then see its minimum dividing value, which is the value represented by each battery. Choose a suitable thermometer to measure the temperature of the measured object. When measuring, the bubble of thermometer should fully contact with the measured object, and the glass bubble should not touch the side wall or bottom of the measured object; When reading, the thermometer should not leave the measured object, and the line of sight of the eyes should be level with the liquid level in the thermometer.
1. It should be calibrated before use (it can be calibrated by multi-branch comparison method of standard liquid temperature or by thermometer with higher accuracy).
2. It is not allowed to use the measured value whose temperature exceeds the maximum scale value of the thermometer.
3. The thermometer has thermal inertia and should be read after the thermometer reaches a stable state. When reading, you should read in the highest tangent direction of the temperature convex meniscus and look straight.
Never use it as a stirring rod.
5. Mercury thermometer should be vertical or inclined to the flow direction of the measured working medium.
6. Mercury thermometers often break the mercury column, and the elimination methods are as follows:
① Cold patching method: insert the temperature measuring bag of the thermometer into the mixed liquid of dry ice and alcohol (the temperature shall not exceed -38℃) for cold shrinkage, so that all mercury in the capillary tube will shrink into the temperature measuring bag.
(2) Thermal repair method: slowly insert the thermometer into a thermostatic bath with a temperature slightly higher than the upper limit of measurement, so that the broken part of mercury is connected with the whole mercury column, then slowly take out the thermometer and gradually cool it to room temperature in the air.
The earliest invented and improved thermometer was invented by Italian scientist Galileo (1564 ~ 1642) in 1593. His first thermometer was a glass tube with an opening at one end and a glass bulb the size of a walnut at the other. When in use, the glass bulb is heated first, and then the glass tube is inserted into water. With the change of temperature, the water surface in the glass tube will move up and down, and the temperature change and temperature level can be judged according to the movement amount. Thermometers expand with heat and contract with cold, so this kind of thermometer is greatly influenced by environmental factors such as external atmospheric pressure, so the measurement error is large.
Later, Galileo's students and other scientists repeatedly improved on this basis, such as turning the glass tube upside down, putting the liquid in the tube and sealing the glass tube. The most prominent one is the thermometer made by the Frenchman Bliaux in 1659. He reduced the size of the glass bulb and changed the temperature measuring substance into mercury, so the prototype of the thermometer was formed. Later, the Dutchman Warren Hite used alcohol in 1709 and mercury as the measuring substance in 17 14 to make a more accurate thermometer. He observed the boiling temperature of water, the temperature when water and ice are mixed, and the temperature when salt water and ice are mixed. After repeated experiments and approval, the solidification temperature of a certain concentration of brine is finally set at 0℉, the solidification temperature of pure water is set at 32℉, the boiling temperature at standard atmospheric pressure is set at 2 12℉, and the Fahrenheit temperature is expressed in ℉. This is a Fahrenheit thermometer.
At the same time as the Fahrenheit thermometer appeared, the Frenchman Lemuel (1683 ~ 1757) also designed and manufactured the thermometer. He thinks that the expansion coefficient of mercury is too small to be used as a temperature measuring material. He focused on the advantages of using alcohol as a temperature measuring substance. He repeatedly found that the volume expansion of alcohol containing 1/5 water increased from 1000 unit volume to 1080 unit volume between the freezing temperature and boiling temperature of water. So he divided the freezing point and boiling point into 80 parts and set them as the temperature scale of his thermometer, which is Liebherr thermometer.
More than 30 years after Fahrenheit thermometer came out, Swedish Poerxiusi improved the calibration of Warren Hite thermometer in 1742. He set the boiling point of water at 0 degrees and the freezing point of water at 100 degrees. Later, his colleague Schloemer reversed the values of two temperature points (namely, boiling point 100 degree and freezing point 0 degree), and it became a percentage temperature, that is, the temperature in degrees Celsius, which was expressed in degrees Celsius. The relationship between Fahrenheit and Celsius is = 9/5℃+32, or℃ = 5/9 (-32).
In Britain and the United States, Fahrenheit temperature is widely used, in Germany, Liebherr temperature is widely used, while in the world of science and technology, industrial and agricultural production, and in most countries such as China and French, Celsius temperature is widely used.
Temperature unit
Degrees-days: including Celsius (temperature) and K Kelvin (thermodynamic temperature).
The temperature law of Celsius: the temperature of ice-water mixture is 0℃, and the temperature of boiling water at standard atmospheric pressure is 100℃.
Thermodynamic temperature: The lower limit of the temperature in the universe is -273. 15℃, which is called absolute zero. The temperature starting from absolute zero is called thermodynamic temperature. -273. 15℃=0K
The relationship between them: t (thermodynamic temperature) =t (centigrade temperature) +273.6438+05.
Electronic thermometer, digital thermometer, uses temperature sensor to convert (temperature) into digital signal, and then displays the temperature in digital form through display (such as liquid crystal, digital tube, LED matrix, etc.). ), can quickly and accurately measure the highest value of human body temperature. Compared with the traditional mercury thermometer, it has the advantages of convenient reading, short measuring time, high measuring accuracy, memory and prompt sound. In particular, electronic thermometers do not contain mercury, which is harmless to human body and surrounding environment and is especially suitable for hospital use.
Application mode
1. Before using the thermometer, the head of the thermometer should be disinfected with alcohol.
2. Press the switch, and the buzzer will beep immediately, and the display will be as shown in Figure A for about 2 seconds.
3. Then the monitor displays the temperature of the last lateral vector, as shown in Figure B (if the last measurement is 36.5℃), and the well lasts for about 2 seconds. The display may display as shown in figure C. The symbol "℃" flashes, indicating that the thermometer is in standby state. (When the room temperature is higher than 32℃, the thermometer will show the room temperature instead of as shown in Figure D, and the symbol "℃" will keep flashing).
Take your temperature with a thermometer. When measuring body temperature, the temperature value rises gradually and the symbol "℃" flashes continuously.
5. When the temperature rise rate is less than 0. 1℃ in 16 seconds, the "℃" symbol stops flashing, and at the same time, the thermometer gives a buzzer sound for about 5 seconds. At this time, the thermometer can read the displayed temperature value after measurement.
Instrument type
With the development of science and technology and the needs of modern industrial technology, temperature measurement technology has been continuously improved and improved. Because the temperature measuring range is wider and wider, different temperature measuring instruments are manufactured according to different requirements. Here are a few.
Rotating thermometer
The rotary 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.
semiconductor thermometer
The resistance change of semiconductor is different from that of metal. When the temperature rises, 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.
thermocouple thermometer
A thermocouple thermometer consists of two different metals connected to a sensitive voltmeter. Metal contacts will produce different potential differences at both ends of the metal at different temperatures. The potential difference is so small that a sensitive voltmeter is needed to measure it. We can know what the temperature is from the reading of the voltmeter.
Thermal radiation pyrometer
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 called 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.
liquid crystal thermometer
The liquid crystals made of different formulations have different phase transition temperatures, and the optical properties will also change during phase transition, which makes 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.
Precision and dividing value
Scale value of instrument name accuracy grade,℃ (Celsius)
Bimetal thermometer 1,1.5,2.5 0.5 ~ 20
Pressure thermometer 1, 1.5, 2.5 0.5~20
Glass liquid thermometer 0.5~2.5 0. 1~ 10
Thermal resistance 0.5~3 1~ 10
Thermocouple 0.5 ~ 1.5 ~ 20
Optical pyrometer 1~ 1.5 5~20
Radiation thermometer (thermopile) 1.5 5~20
Partial radiation thermometer 1~ 1.5 1~20
Colorimetric thermometer 1~ 1.5
Use of laboratory thermometer
When measuring the liquid temperature with a thermometer, the correct method is as follows:
1. First observe the range, dividing value and zero point, and the temperature of the measured liquid cannot exceed the range;
2. All the glass bubbles of the thermometer are immersed in the liquid to be measured, and do not touch the bottom or wall of the container;
3. After the glass bulb of the thermometer is immersed in the liquid to be measured, wait for a period of time, and then read after the pointer of the thermometer is stable;
4. When reading, the glass ball of the thermometer should be kept in liquid, and the line of sight should be flush with the upper surface of the liquid column in the thermometer.
Note: Don't shake it before measuring the temperature.
Mercury spilled by mercury thermometer must be collected immediately with dropper and brush, covered with water (preferably glycerin), and then sulfur powder is sprinkled on the polluted place, and it can only be cleaned after there is no liquid (usually about a week).
The reading of this thermometer has no estimated value. In other words, the last digit of the reading is an accurate value, so there is no need to estimate the number after dividing the value.
Related knowledge of infrared thermometer
The infrared thermometer consists of optical system, photoelectric detector, signal amplifier, signal processing and display output. The optical system collects the infrared radiation energy of the target in its field of view, and the infrared energy is focused on the photodetector, converted into corresponding electrical signals, and then converted into the temperature value of the measured target.
Benefits of using infrared thermometer
Convenient: Infrared thermometer can provide temperature measurement quickly, and it can read the temperature of almost all connection points within the time of reading a leakage connection point with thermocouple. In addition, infrared thermometers are sturdy and light (all lighter than 10 ounce), and can be easily put into a holster when not in use. So you can take it with you during factory inspection and daily inspection.
Accuracy: another advanced feature of infrared thermometer is accuracy, which is usually within 1 degree. This performance is especially important when you perform preventive maintenance, such as monitoring harsh production conditions and special events that may lead to equipment damage or downtime. Because most equipment and factories run for 365 days, stopping work is equivalent to reducing income. In order to prevent this loss, we should scan all the field electronic equipment: circuit breakers, transformers, fuses, switches, buses and switchboards to find hot spots. With the infrared thermometer, you can even quickly detect the slight change of operating temperature, and solve the problem when it is budding, thus reducing the cost and maintenance scope caused by equipment failure.
Safety: Safety is the most important benefit of using infrared thermometer. Unlike contact thermometers, infrared thermometers can safely read the unattainable or unattainable target temperature, and you can read the target temperature within the range allowed by the instrument. Non-contact temperature measurement can also be carried out in unsafe areas or areas where contact temperature measurement is difficult, such as near steam valves or heating furnaces, so that they don't have to risk accidentally burning their fingers by contact temperature measurement. Accurately measuring the supply/return air temperature 25 feet above the head is as simple as measuring it at hand. Raytek infrared thermometers all have laser aiming, which is convenient to identify the target area. With it, your work becomes much easier.
What fields are infrared thermometers mainly used in?
Infrared thermometer has been proved to be an effective tool for detecting and diagnosing electronic equipment faults. You can save a lot of money. Using infrared thermometer, you can continuously diagnose the electronic connection problem and detect the functional status of UPS by finding the hot spot at the connection of output filter on DC battery. You can check the connection between the battery pack and the distribution board terminal, switchgear or fuse to prevent energy consumption. Because loose connectors and assemblies generate heat, infrared thermometers are helpful to identify insulation faults of circuit breakers or monitor electronic compressors. The daily scanning of transformer hot spots can detect the cracks in windings and terminals.
Infrared thermometer measurement
The following are three temperature measurement technologies of Raytek non-contact thermometer:
Point measurement: measure the temperature of all surfaces of objects, such as engines or other equipment;
Temperature difference measurement: compare the measured temperatures of two independent points, such as connectors or circuit breakers;
Scanning measurement: detect the change of the target in a large range or continuous area. Such as refrigeration pipes or distribution rooms.
Infrared radiation thermometer
Temperature range: the temperature range of Raytek products is -50~3000 degrees (segmented), and each type of thermometer has its specific temperature range. The temperature range of the selected instrument should match the temperature range of the specific application.
Target size: When measuring temperature, the target to be measured should be larger than the field of view of the thermometer, otherwise there will be errors in measurement. It is suggested that the size of the measured object should exceed 50% of the field of view of the thermometer.
Optical resolution (D:S): the ratio of thermometer probe to target diameter. If the thermometer is far away from the target and the target is small, you should choose a thermometer with high resolution.
Skills of accurate temperature measurement
When measuring the surface temperature of luminous objects such as aluminum and stainless steel, the reflection of the surface will affect the reading of infrared thermometer. Before reading the temperature, a rubber strip can be placed on the metal surface, and after the temperature is balanced, the temperature in rubber strip area can be measured.
If the infrared thermometer can walk back and forth from the kitchen to the cold storage area and still provide accurate temperature measurement, it will take some time to reach temperature balance in the new environment before measurement. It is best to put the thermometer in a place where it is often used.
Using infrared thermometer to read the internal temperature of liquid food, such as soup or sauce, must be stirred before measuring the surface temperature. Keep the thermometer away from the steam to avoid polluting the lens and causing incorrect reading.
The world's largest thermometer is located in the Flame Mountain Scenic Area in Turpan, Xinjiang. In the center of the underground palace in the Flame Mountain Scenic Area, stands a huge thermometer. This stereoscopic thermometer, which was completed on August 16, 2004, was named "Golden Hoop" and won the Guinness World Records Award.
The giant thermometer has a diameter of 0.65m, a height of12m and a temperature display height of 5.4m. It can measure the surface temperature and air temperature within 100℃ with an error of less than plus or minus 0.5 degrees.