Classification of temperature measuring instruments

There are many kinds of temperature measuring instruments, but they can be divided into the following types according to the principle of action, measuring methods and measuring range: temperature measurement is realized by using the principle that some properties of objects change with temperature. However, the change of a certain physical property cannot be arbitrarily selected to make a thermometer. The physical properties of the object used for temperature measurement require continuous single-value change with temperature, which has nothing to do with other factors, and has good repeatability and is convenient for accurate measurement.

At present, the thermometers made according to the principle of action mainly include expansion thermometer, pressure thermometer, resistance thermometer, thermocouple pestle thermometer and radiation pyrometer. They are made by using the principle that the expansion, pressure, resistance, thermoelectric potential and radiation properties of objects change with temperature respectively. Temperature measurement can be divided into contact temperature measuring instruments (expansion thermometer, pressure thermometer, resistance thermometer and thermocouple pyrometer) and non-contact temperature measuring instruments (such as radiation pyrometer) according to whether the temperature sensing element directly contacts the measured temperature field (or medium).

The characteristic of contact temperature measurement method is that the temperature measuring element is in direct contact with the measured object, and there is sufficient heat exchange between them, and finally the thermal balance is reached. At this time, the value of the physical parameter of the temperature sensing element represents the temperature value of the measured object. The advantage of this temperature measurement method is intuitive and reliable, but the disadvantage is that the temperature sensing element affects the distribution of the measured temperature field, and poor contact will bring measurement errors. In addition, too high temperature and corrosive medium will adversely affect the performance and life of temperature sensing elements.

The characteristic of non-contact temperature measurement method is that the temperature sensing element does not contact with the measured object, but exchanges heat through radiation, so the shortcomings of contact temperature measurement method can be avoided and the upper limit of temperature measurement is high. In addition, the thermal inertia of non-contact temperature measurement method is small, up to one thousandth of a second, which is convenient for measuring the temperature of moving objects and rapidly changing temperatures. Due to the influence of the emissivity of the object, the distance between the measured object and the instrument, smoke, water vapor and other media, this temperature measurement method generally has a large temperature measurement error. Mercury thermometer has many advantages: simple structure, convenient use, high precision, low price, and mercury does not contact with glass, so it is easy to get high-purity mercury, and the liquid temperature range is relatively large (-38 ~ +356.66℃). In addition, the volume expansion of mercury is almost linear with temperature below 200℃. The temperature measuring range of mercury thermometer is generally -30~ +600℃. Because the boiling point of mercury at atmospheric pressure is 356.966℃, the upper limit of pressureless mercury thermometer can only reach 300℃. If pressurized nitrogen is filled, the upper limit of measurement can reach above 600℃ by using a timely glass tube with small thermal deformation. Recently, China has successfully developed a high-temperature mercury thermometer for measuring 1200℃. Its disadvantage is that the measurement temperature is not high enough, and the measurement results can not be transmitted and recorded remotely.

Mercury thermometer usually consists of a glass bulb filled with liquid, a capillary tube, a scale and a glass shell, as shown in the figure below. Thermometers made of thermoelectric characteristics of thermocouples are called thermocouple latitude meters. The following figure is the simplest composition diagram of thermocouple thermometer. The thermocouple in the picture is a kind of temperature sensing element, which is formed by welding (or twisting) one end of two conductors A and B of different materials. The two ends of the solderless conductor are connected with the display instrument through connecting wires to form a temperature measuring system. When measuring the temperature, the welding end of the thermocouple is in contact with the measured object, and the temperature of the measured object is converted into a corresponding electrical signal by using the thermoelectric characteristics of the thermocouple and transmitted to the display instrument.

Thermocouple thermometer is the most widely used thermometer in industry, and it is also the most widely used thermometer in heat treatment production. Thermocouple temperature measurement has the following characteristics:

L high temperature measurement accuracy. Because it is easy to achieve good thermal contact between thermocouple and the measured object, it can truly reflect the temperature of the measured object.

L simple structure. After two different conductors are connected at one end, they are insulated and mechanically protected, which is the available thermocouple. It can be seen that the thermocouple is simple in structure and easy to assemble and maintain.

Wide temperature measuring range. The temperature measuring range of common thermocouple is 100 ~ 1600℃. Generally, the heat treatment temperature of metal materials is within this range, so it can meet the temperature measurement requirements of heat treatment. Thermocouples made of special materials can also measure temperatures as low as 2K(-27 1℃) or as high as 2800℃.

L the dynamic response speed is faster. Thermocouple can be made into small contacts, so it has small heat capacity and fast dynamic response.

L signal can be transmitted remotely, which is convenient for centralized detection and automatic control.

There are many kinds and types of thermocouples, among which ordinary thermocouples are the most widely used.

In actual temperature measurement, thermocouples with only two hot electrodes are rare. An ordinary thermocouple usually consists of four parts: a hot electrode, an insulating tube, a protective sleeve and a junction box. As shown in the figure below.

(1) hot electrode

Hot electrode is the core of thermocouple. The hot electrode of a common thermocouple is usually processed into a wire with one end welded. The diameter of wire-like hot electrode is mainly determined by the price of material, mechanical strength, the use of thermocouple and the temperature measurement range. The hot ends of thermocouples are usually connected by welding. The forms of solder joints are spot welding, butt welding and torsion spot welding. In order to reduce the heat transfer error, the size of the solder joint should be as small as possible, usually not exceeding twice the diameter of the hot electrode.

(2) Insulating tube

Insulating tube, also known as insulator, has a through hole on the hot electrode, which is used to isolate the two electrodes and the isolated electrode from the metal protective sleeve, otherwise it will lead to thermoelectric potential loss due to short circuit and cause measurement error. Insulating tubes are usually made of high temperature resistant insulating materials, such as ceramics, timely, alumina, magnesium oxide, etc. The cross section is round or oval, with single hole, double hole and four holes.

(3) protection tube

A hot electrode sheathed with an insulating tube is installed in a protective tube with one end closed. The function of the protective tube is to prevent or reduce the direct erosion of various harmful gases and substances on the hot electrode and the direct erosion of high temperature flame or airflow; Prevent direct contact between conductive medium and hot electrode: in addition, it has the function of fixing and supporting hot electrode. Therefore, the thermocouple protection tube plays an important role in prolonging the service life of the hot electrode and ensuring the measurement accuracy.

(4) Junction box

The junction box is where the cold end of the thermocouple is connected with the connecting wire (compensation wire). It is made of aluminum alloy. In the junction box, the cold ends of thermocouples are fixed on the terminals with screws in advance. When wiring, lead the connecting wire into the junction box from the outlet hole, open the junction box, fix the wire on the two terminals marked with the sign with screws, and then cover the junction box. In order to prevent harmful gases from entering the thermocouple protection tube, the outlet hole and cover of the junction box are usually sealed with gaskets and washers. According to the degree of sealing, junction boxes are common, sealed (or splash-proof), waterproof, explosion-proof and flameproof.

There are different types of thermocouple thermometers, and different types can be selected for different occasions to meet the use requirements. Radiation pyrometer is an instrument to measure the temperature of an object by using the thermal radiation phenomenon of the object. The most obvious difference between this thermometer and thermal resistance, thermocouple and expansion thermometer is that the radiation pyrometer does not directly contact the measured object when measuring temperature, and belongs to a non-contact temperature measuring instrument. The main features of radiation pyrometer are as follows:

(1) temperature measurement will not destroy the temperature field of the measured medium, which is of special significance for measuring the temperature of small temperature field.

(2) Theoretically, the upper limit of temperature measurement of the instrument is infinite. However, due to the limitation of temperature sensing element or protective tube material, contact thermometer can not measure high temperature.

(3) Because it is thermal radiation, there is no problem of thermal balance between the temperature sensing element and the measured object, so the heat transfer speed is fast and the thermal inertia is small.

(4) The output signal can be very large, so the sensitivity of the instrument is high.

(5) Since it is not in contact with the measured object, the radiation pyrometer is suitable for measuring the temperature of strongly corrosive media and moving objects.

(6) Because it is a non-contact instrument, the factors affecting the measurement results are more complicated. Therefore, the measurement error of general industrial radiation pyrometer is greater than that of contact thermometer.

Radiation pyrometer is often used to measure the temperature of high temperature salt bath furnace, ion nitriding furnace and induction heating workpiece in heat treatment production. At present, there are four common types of such instruments, namely total radiation pyrometer, optical pyrometer, photoelectric pyrometer and photoelectric colorimetric pyrometer.

The radiation pyrometer is designed and manufactured according to the functional relationship between the radiation energy of an object and its temperature in the whole wavelength range. The radiation temperature sensor is used as a primary instrument, and the electronic potentiometer is used as a secondary instrument. It belongs to a lens-focused temperature sensor, with an aluminum alloy shell and an objective lens in front. The shell is equipped with the compensation diaphragm of the thermopile, and there is a displacement plate on the field diaphragm near the thermopile. The function of moving plate is to adjust the radiation energy irradiated on the thermopile, so that the product has a unified index value.

The radiation temperature sensor focuses the radiation energy of the measured object on the heat-sensitive element through the lens, and the heat-sensitive element converts the radiation energy into electrical parameters. The secondary instrument measures the thermoelectric potential and displays the temperature value according to the known relationship between the thermoelectric potential and the object temperature. This temperature value must be corrected by the total radiation blackbody coefficient of the object, or directly inserted into a high-temperature salt bath furnace with a platinum-rhodium 10- platinum thermocouple to measure the temperature, and then compared with the temperature displayed by the instrument to calibrate the pyrometer measurement.

The following figure is the physical schematic diagram of the radiation pyrometer.