Temperature sensor thermocouple is one of the most commonly used temperature detection elements in industry. Its advantages are:

① High measurement accuracy. Because the thermocouple of the temperature sensor is in direct contact with the measured object, it is not affected by the intermediate medium.

② Wide measuring range. The thermocouple of ordinary temperature sensor can continuously measure -50~+ 1600℃, and the thermocouple of some special temperature sensors can measure as low as -269℃ (such as Au-Fe-Ni-Cr) and as high as +2800℃ (such as W-Re).

③ Simple structure and convenient use. Thermocouple of temperature sensor is usually composed of two different metal wires, and it is not limited by size and beginning, and there is a protective sleeve outside, which is very convenient to use.

1. Basic principle of thermocouple temperature measurement of temperature sensor

Weld two conductors or semiconductors A and B of different materials into a closed loop, as shown in Figure 2- 1- 1. When there is a temperature difference between the two persistent points 1 and 2 of conductors A and B, electromotive force will be generated between them, thus forming a certain amount of current in the loop. This phenomenon is called thermoelectric effect. The thermocouple of temperature sensor works by this effect.

2. Type and structure of thermocouple of temperature sensor

(1) Thermocouple type of temperature sensor

Commonly used temperature sensor thermocouples can be divided into standard temperature sensor thermocouples and non-standard temperature sensor thermocouples. The so-called standard temperature sensor thermocouple refers to the temperature sensor thermocouple whose relationship between thermoelectric potential and temperature and allowable error are stipulated in national standards, with a unified standard graduation table and supporting display instruments to choose from. Non-standardized temperature sensor thermocouples are not as good as standardized temperature sensor thermocouples in application scope or order of magnitude, and generally there is no unified graduation table, which is mainly used for measurement in some special occasions. From June 1 988 65438+1October1,all temperature sensor thermocouples and temperature sensor thermal resistors were produced according to IEC international standards, and seven kinds of standardized temperature sensor thermocouples, S, B, E, K, R, J and T, were designated as temperature sensor thermocouples designed in China.

(2) Structural Form of Thermocouple of Temperature Sensor In order to ensure the reliable and stable operation of Thermocouple of Temperature Sensor, its structural requirements are as follows:

① The welding of the two hot electrodes of the thermocouple of the temperature sensor must be firm;

(2) the two hot electrodes should be well insulated to prevent short circuit;

③ The connection between the compensation line and the thermocouple free end of the temperature sensor should be convenient and reliable;

(4) The protective sleeve should ensure that the hot electrode is completely isolated from harmful media.

3. Temperature compensation of thermocouple cold end of temperature sensor

Because the thermocouple material of temperature sensor is generally expensive (especially when expensive metal is used), and the temperature measuring point is far away from the instrument, in order to save thermocouple material and reduce the cost, compensation wires are usually used to extend the cold end (free end) of the thermocouple of temperature sensor to the control room with stable temperature and connect it to the instrument terminal. It must be pointed out that the compensation line of the thermocouple of the temperature sensor only plays the role of prolonging the hot electrode, so that the cold end of the thermocouple of the temperature sensor moves to the instrument end of the control room, which itself cannot eliminate the influence of the temperature change of the cold end on the temperature measurement, and has no compensation function. Therefore, other correction methods are needed to compensate the influence of cold end temperature t0≠0℃ on temperature measurement.

When using the thermocouple compensation line of temperature sensor, we must pay attention to the matching of models, and the polarity should not be wrong. The temperature at the connecting end of the compensation line and the thermocouple of the temperature sensor shall not exceed 100℃.

Second, the application principle of temperature sensor thermal resistance

Thermal resistance of temperature sensor is the most commonly used temperature detector in middle and low temperature area. Its main features are high measurement accuracy and stable performance. Among them, platinum resistance has the highest measurement accuracy, which is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.

1. principle and material of temperature sensor thermal resistance temperature measurement

The thermal resistance temperature measurement of temperature sensor is based on the characteristic that the resistance value of metal conductor increases with the increase of temperature. Most of the thermal resistances of temperature sensors are made of pure metal materials, and platinum and copper are the most widely used at present. In addition, materials such as platinum, nickel, manganese and rhodium have been used to manufacture the thermal resistance of temperature sensors.

2. Thermal resistance structure of temperature sensor

(1) Familiar with the structure and characteristics of thermal resistance temperature sensing elements (resistors) of temperature sensors commonly used in industry are shown in Table 2-1-1. According to the temperature measurement principle of the thermal resistance of the temperature sensor, the change of the measured temperature is directly measured by the change of the resistance of the thermal resistance of the temperature sensor. Therefore, the resistance changes of various wires such as the lead-out wires of the thermal resistance of the temperature sensor will affect the temperature measurement. In order to eliminate the influence of lead resistance, three-wire system or four-wire system is adopted. For details, please refer to the first section of Chapter III of this article.

(2) Thermal resistance of armored temperature sensor The thermal resistance of armored temperature sensor is a solid composed of temperature sensing element (resistance), lead wire, insulating material and stainless steel sleeve, as shown in Figure 2- 1-7. Its outer diameter is generally φ2~φ8mm, and the minimum can reach φ mm.

Compared with the thermal resistance of ordinary temperature sensor, it has the following advantages: ① small volume, no air gap inside, and small thermal inertia measurement lag; ② Good mechanical properties, vibration resistance and impact resistance; ③ Flexibility, convenient installation and long service life.

(3) End-face temperature sensor Thermal resistance End-face temperature sensor Thermal resistance temperature sensing element is formed by winding specially treated resistance wire, which is close to the end face of thermometer, and its structure is shown in Figure 2- 1-8. Compared with the thermal resistance of the general axial temperature sensor, it can reflect the actual temperature of the measured end face more accurately and quickly, and is suitable for measuring the end face temperature of parts such as bearing shells.

(4) Flame-proof temperature sensor The thermal resistance of the explosion-proof temperature sensor passes through the junction box with special structure, and the explosion of explosive mixed gas in its shell due to the influence of sparks or arcs is confined in the junction box, so that excessive explosion will not occur in the production site. The thermal resistance of flame-proof temperature sensor can be used to measure the temperature of places with explosion danger in Bla~B3c grade area.

3. Composition of temperature sensor thermal resistance temperature measurement system

Temperature sensor thermal resistance temperature measurement system generally consists of temperature sensor thermal resistance, connecting wire and display instrument. We must pay attention to the following two points:

① The thermal resistance of the temperature sensor and the scale number of the display instrument must be consistent.

(2) In order to eliminate the influence of the resistance change of the connecting wire, the three-wire connection mode must be adopted. See Chapter 3 for details.

(2) Thermal resistance of armored temperature sensor The thermal resistance of armored temperature sensor is a solid composed of temperature sensing element (resistance), lead wire, insulating material and stainless steel sleeve, as shown in Figure 2- 1-7. Its outer diameter is generally φ2~φ8mm, and the smallest can reach φ mm. Compared with the thermal resistance of ordinary temperature sensors, it has the following advantages: ① small volume, no air gap inside, and small thermal inertia measurement lag; ② Good mechanical properties, vibration resistance and impact resistance; ③ Flexibility and convenient installation; ④ Long service life.

(3) End-face temperature sensor Thermal resistance End-face temperature sensor Thermal resistance temperature sensing element is formed by winding specially treated resistance wire, which is close to the end face of thermometer, and its structure is shown in Figure 2- 1-8. Compared with the thermal resistance of the general axial temperature sensor, it can reflect the actual temperature of the measured end face more accurately and quickly, and is suitable for measuring the end face temperature of parts such as bearing shells.

(4) Flameproof temperature sensor The thermal resistance of the thermoelectric barrier explosion temperature sensor passes through the junction box with special structure, and the explosive mixed gas in its shell will inevitably change the length of the resistance wire due to the open circuit repair of shadow resistance such as spark or arc, so it is best to replace the resistance. If welding repair is adopted, it can only be used after welding.

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With the development of the times, the demand for humidity sensors in scientific research, agriculture, heating and ventilating, textile, computer room, aerospace, electric power and other industrial sectors is increasing, and the requirements for product quality are also increasing. The control of environmental temperature and humidity and the monitoring and analysis of moisture value of industrial materials have become one of the more common technical conditions. Humidity sensor products and humidity measurement belong to the industries that rose in the 1990s. How to use the humidity sensor well and how to judge the performance of the humidity sensor is still a complicated technical problem for ordinary users.

The following article is for your reference.

1. Classification and humidity sensing characteristics of humidity sensors

Humidity sensors are divided into two types: resistive and capacitive. The basic form of the product is to coat the humidity-sensitive material on the substrate to form a humidity-sensitive film. After the water vapor in the air is adsorbed on the humidity-sensitive material, the impedance and dielectric constant of the element change greatly, thus making the humidity-sensitive element.

Domestic and foreign manufacturers have different levels of humidity sensor products, and the quality and price are quite different. It is really difficult for users to choose the ideal product with the best cost performance, which needs to be deeply understood. The humidity sensor has the following characteristics:

1, accuracy and long-term stability

The accuracy of humidity sensor should reach 2% ~ 5% RH. Without this level, it is difficult to use as a measuring instrument. It is difficult for humidity sensor to reach 2% ~ 3% RH. Generally, the characteristics given in the product data are measured at normal temperature (20℃ 10℃) and in clean gas. In practical use, due to the influence of dust, oil pollution and harmful gases, the humidity sensor will age and its accuracy will decrease after a long time of use. Generally speaking, long-term stability and service life are the primary problems affecting the quality of humidity sensors. There are few products whose annual drift is controlled at 1%RH level, generally around 2% or even higher.

2. Temperature coefficient of humidity sensor

Humidity sensors are sensitive not only to environmental humidity, but also to temperature, and their temperature coefficients are generally in the range of 0.2~0.8%RH/℃. Some humidity sensors have different temperature coefficients under different relative humidity. Temperature drift is nonlinear, so it is necessary to add a temperature compensation formula to the circuit. The humidity sensor with or without temperature compensation by single chip microcomputer software can not guarantee the accuracy of the whole temperature range. The linearization of temperature drift curve of humidity sensor directly affects the compensation effect, and nonlinear temperature drift is often not well compensated. Only by using hardware temperature following compensation can the real compensation effect be obtained. The working temperature range of humidity sensor is also an important parameter. Most humidity sensors are difficult to work normally above 40℃.

3. Power supply of humidity sensor

When DC voltage is applied to metal oxide ceramics, polymers, lithium chloride and other humidity-sensitive materials, it will lead to performance changes or even failure, so this kind of humidity sensor can not use DC voltage or AC voltage containing DC component. It must be driven by alternating current.

4. Interchangeability

At present, the interchangeability of humidity sensors is generally poor, and the sensors of the same model cannot be interchanged, which seriously affects the use effect and increases the difficulty of maintenance and debugging. Some manufacturers have made various efforts in this regard, but the interchangeability is still very poor, and good results have been achieved.

5. Humidity correction

Correcting humidity is much more difficult than correcting temperature. Standard thermometer is usually used as the standard of temperature calibration, but the standard of humidity calibration is difficult to achieve. Dry and wet bulb thermometers and some common pointer hygrometers cannot be used for calibration, and their accuracy cannot be guaranteed because their environmental conditions are very strict. Generally, in the absence of perfect verification equipment, simple saturated salt solution verification method is usually used to measure their temperature.

Second, several methods to preliminarily judge the performance of humidity sensor.

When the actual calibration of the humidity sensor is difficult, some simple methods can be used to judge and check the performance of the humidity sensor.

1. Consistency judgment: It is best to buy more than two humidity sensor products of the same model and manufacturer at one time. The more products, the more problems. Put them together to compare the output values, and observe the consistency of the test under relatively stable conditions. If further tests are carried out, they can be recorded at intervals within 24 hours. Generally, there are three kinds of humidity and temperature in a day, which can comprehensively observe the consistency and stability of products, including temperature compensation characteristics.

2. Humidify the sensor by mouth breathing or other humidification methods, and observe its sensitivity, repeatability, humidity-rising and dehumidification performance, resolution and maximum range of products.

3. Test the product with it turned on and off. Compare the consistency and observe the thermal effect.

4. Carry out high-temperature and low-temperature tests on the products (according to the specifications and standards), and compare the tests in normal state with the records before the experiment, so as to investigate the temperature adaptability of the products and observe the consistency of the products.

The performance of the product ultimately depends on the formal and complete testing methods of the quality inspection department. Saturated salt solution can be used for calibration, and brand-name products can also be used for comparative testing. The product should be calibrated for a long time in the long-term use process, so as to judge the quality of the humidity sensor more comprehensively.

Three, some analysis of humidity sensor products in the market

There are many domestic and foreign humidity sensor products in the domestic market, and capacitive humidity sensor is more common. The main types of humidity-sensitive materials are polymers, lithium chloride and metal oxides. Capacitive humidity sensor has the advantages of fast response, small volume, good linearity and stability, and some foreign products also have high temperature performance. The products that achieve the above performance are mostly well-known foreign brands, and the prices are relatively expensive. Some low-priced capacitive humidity sensors sold in the market often fail to reach the above level, and their linearity, consistency and repeatability are not ideal. The moisture sensitive sections below 30%RH and above 80%RH are severely deformed. Some products are compensated and corrected by single chip microcomputer, which makes the humidity jump in a "step-by-step" way, which reduces the accuracy and has the shortcomings of poor consistency and linearity. No matter high-grade or low-grade capacitive humidity sensors, the long-term stability is not ideal, and most of them drift seriously after long-term use. When the capacitance of the humidity sensor changes to pF level, the change of 1%RH is less than 0.5pF, and the drift change of the capacitance often causes an error of several tens of RH%. Most capacitive humidity sensors do not have the performance of working above 40℃, and often fail and are damaged. Capacitive humidity sensors also lack corrosion resistance and usually require high environmental cleanliness. Some products still have lighting failures, electrostatic failures and other phenomena. Metal oxides are ceramic humidity-sensitive resistors, which have the same advantages as humidity-sensitive capacitors. However, in dusty environment, ceramic pores are blocked and components will fail, so the method of electrostatic precipitation is often used, but the effect is not ideal and it can not be used in flammable and explosive environment. Alumina humidity-sensitive materials can not overcome the "natural aging" of their surface structure. The most outstanding advantage of lithium chloride humidity sensitive resistor is that it is extremely stable for a long time. Therefore, instruments and sensor products manufactured by strict technology can achieve high precision. Strong stability means that the product has good linearity, accuracy and consistency, and it is a reliable guarantee for long-term service life. The long-term stability of lithium chloride humidity sensor is irreplaceable by other humidity sensitive materials.

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