The purpose and classification of various thermometers

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

Optical pyrometer:

If the temperature of the object is so high that it emits a lot of visible light, the temperature can be determined by measuring the amount of its thermal radiation. This kind of thermometer is a light thermometer. This thermometer is mainly made of a telescope equipped with a red filter and a set of circuits with small bulbs, galvanometers and variable resistors. Before use, establish the relationship between the temperature corresponding to the different brightness of the filament and the reading on the ammeter. When using, align the telescope to the object to be measured and adjust the resistance so that the brightness of the bulb is the same as that of the object to be measured. At this time, the temperature of the object to be measured can be read from the galvanometer.

Gas thermometer:

The purpose and classification of various thermometers

Hydrogen or helium is often used as temperature measurement materials, 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.

Resistance thermometer:

Divided into metal resistance thermometers and semiconductor resistance thermometers, they are all made according to the characteristic that the resistance value changes with temperature. Metal thermometers mainly use pure metals such as platinum, gold, copper, nickel and rhodium iron, phosphor bronze alloys; semiconductor thermometers mainly use carbon, germanium, etc. Resistance thermometers are easy to use and reliable, and have been widely used. Its measuring range is about -260℃ to 600℃.

High temperature thermometer:

Refers to thermometers specifically used to measure temperatures above 500°C, including optical thermometers, colorimetric thermometers and radiation thermometers. The principle and structure of high temperature thermometers are more complicated, so I won't discuss them here. Its measurement range is 500°C to 3000°C or more, which is not suitable for measuring low temperature.

Pointer thermometer:

It is a thermometer shaped like a dashboard, also called a cold-heat meter. It is used to measure room temperature. It is made by the principle of thermal expansion and contraction of metal. It uses bimetal as the temperature sensing element to control the pointer. The bimetal is usually riveted with copper and iron, and the copper is on the left and the iron is on the right. Since the thermal expansion and contraction effect of copper is 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) under the drive of the bimetallic sheet; , The temperature becomes lower, and the pointer deflects to the left (pointing to low temperature) driven by the bimetal.

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 measuring medium is different from the boiling point and freezing point, our common glass tube thermometers mainly include: kerosene thermometer, mercury thermometer, red pen water thermometer. Its advantages are simple structure, convenient 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. And it cannot be transmitted remotely, it is fragile.

Thermocouple thermometer:

It is a temperature measuring instrument widely used in industry. Made using thermoelectric phenomenon. 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. Put the working end at the temperature to be measured. When the temperature of the working end and the free end are different, electromotive force will appear, so a current flows through the loop. Through the measurement of electrical quantity, the temperature of another place can be determined by using the temperature of a known place. This kind of thermometer is mostly composed of copper-constantan, iron-constantan, nickel inscription-constantan, gold cobalt-copper, platinum-rhodium, etc. It is suitable for two substances with a large temperature difference, and is mostly used for high temperature and low turbidity measurement. Some thermocouples can measure high temperatures as high as 3000℃, and some can measure low temperatures close to absolute zero.

Thermocouple thermometer:

The thermocouple thermometer is composed of two different metals connected to a sensitive voltmeter. Metal contacts will produce different potential differences at the two ends of the metal at different temperatures. The potential difference is very small, so it needs a sensitive voltmeter to measure it. From the reading of the voltmeter, you can know the temperature.

LCD thermometer:

Liquid crystals made with different formulations have different phase transition temperatures. When their phase transitions, their optical properties will also change, making the liquid crystals look discolored. If liquid crystals with different phase transition temperatures are painted 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. It is often used in ornamental fish tanks to indicate water temperature.

Rotating thermometer:

The rotating thermometer is made of a rolled bimetallic sheet. One end of the bimetal is fixed, and the other end is connected to the pointer. Due to the different expansion degree of the two metal sheets, the degree of curling of the bimetallic sheet is different at different temperatures, and the pointer points to different positions on the dial. The temperature can be known from the reading on the dial.

Semiconductor thermometer:

The resistance change of a semiconductor is different from that of a metal. When the temperature rises, its resistance decreases, and the range of change is larger. Therefore, a small amount of temperature change can also cause a significant change in resistance. The made thermometer has a high degree of precision and is often called a temperature sensor.

Pressure thermometer:

The pressure thermometer uses the liquid, gas or saturated vapor in a closed container to produce volume expansion or pressure change as a measurement signal after being heated. Its basic structure is composed of three parts: temperature bulb, capillary tube and indicator meter. It is one of the earliest temperature control methods used in the production process. The pressure type temperature measurement system is still a very widely used measurement method for in-situ indication and control of temperature. The advantages of the pressure thermometer are: simple structure, high mechanical strength, not afraid of vibration. The price is low and no external energy is needed. The disadvantages are: the temperature measurement range is limited, generally -80~400℃; the response time is slow when the heat loss is large; the instrument sealing system (temperature bulb, capillary tube, spring tube) is damaged and difficult to repair and must be replaced; the measurement accuracy is affected by the ambient temperature, The installation position of the bulb has a great influence, and the accuracy is relatively low; the transmission distance of the capillary tube is limited.