When installing and using the temperature sensor, the following matters shall be paid attention to in order to ensure the best measurement effect:

1. Error caused by improper installation

For example, the installation position and insertion depth of the thermocouple cannot reflect the real temperature of the furnace. In other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least 8 ~ 10 times the diameter of the protective tube; The space between the protective sleeve and the wall of the thermocouple is not filled with insulating materials, resulting in heat overflow or cold air intrusion in the furnace. Therefore, the gap between the protective sleeve of the thermocouple and the hole of the furnace wall shall be blocked with insulating materials such as refractory mud or asbestos rope, so as to prevent the convection of cold and hot air from affecting the accuracy of temperature measurement; The cold end of the thermocouple is too close to the furnace body to make the temperature exceed 100 ℃; The installation of thermocouples should avoid strong magnetic field and strong electric field as far as possible, so thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors; The thermocouple cannot be installed in the area where the measured medium rarely flows. When the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the flow direction and fully in contact with the gas.

2. Error caused by insulation deterioration

For example, if the thermocouple is insulated, there is too much dirt or salt slag on the protective tube and pull plate, resulting in poor insulation between the thermoelectric dipole and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermal potential, but also introduce interference, and the resulting error can sometimes reach up to Baidu.

3. Error introduced by thermal inertia

Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change of the measured temperature, which is particularly prominent in rapid measurement. Therefore, thermocouples with thin Thermoelectrode and small diameter of protective tube should be used as far as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation. The greater the measurement lag, the smaller the amplitude of thermocouple fluctuation and the greater the difference from the actual furnace temperature. When thermocouple with large time constant is used for temperature measurement or control, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to measure the temperature accurately, the thermocouple with small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient and is directly proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end. The inner diameter of the tube is small, and the protective performance of the tube is good. In the more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged and should be corrected and replaced in time.

4. Thermal resistance error

At high temperature, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance increases and hinders heat conduction. At this time, the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce the error.

Development status

In recent years, the process of China's industrial modernization and the continuous rapid growth of electronic information industry have driven the rapid rise of sensor market. As an important category of sensors, temperature sensor accounts for more than 40% of the total demand of the whole sensor. Temperature sensor is a semiconductor device that uses the resistance value of NTC to change with temperature to convert non electrical physical quantities into electrical quantities, so as to accurately measure and automatically control the temperature. Temperature sensor has a wide range of applications. It can be used for temperature measurement and control, temperature compensation, flow rate, flow and wind speed measurement, liquid level indication, temperature measurement, ultraviolet and infrared light measurement, microwave power measurement and so on. It is widely used in color TV, computer color display, switching power supply, water heater, refrigerator, kitchen equipment, air conditioning, automobile and other fields. In recent years, the rapid growth of automotive electronics and consumer electronics industry has driven the rapid growth of the demand for temperature sensors in China.

Main purpose

Temperature is a physical quantity that represents the degree of cold and heat of an object. It is a very important and common measurement parameter in the process of industrial and agricultural production. Temperature measurement and control plays a very important role in ensuring product quality, improving production efficiency, saving energy, production safety and promoting the development of national economy. Due to the universality of temperature measurement, the number of temperature sensors ranks first among all kinds of sensors, accounting for about 50%.

The temperature sensor indirectly measures the temperature by changing some characteristics of the object with the change of temperature. The characteristics of many materials and components change with the change of temperature, so there are quite a lot of materials that can be used as temperature sensors. The physical parameters of temperature sensor change with temperature: expansion, resistance, capacitance, electromotive force, magnetic properties, frequency, optical characteristics, thermal noise and so on. With the development of production, new temperature sensors will continue to emerge.

Due to the wide range of temperature measurement in industrial and agricultural production, from hundreds of degrees below zero to thousands of degrees above zero, temperature sensors made of various materials can only be used in a certain temperature range.

The contact mode between the temperature sensor and the measured medium is divided into two categories: contact type and non-contact type. The contact temperature sensor needs to keep thermal contact with the measured medium, so that they can exchange heat sufficiently to reach the same temperature. This kind of sensor mainly includes resistance type, thermocouple, PN junction temperature sensor and so on. The non-contact temperature sensor does not need to contact with the measured medium, but is transmitted to the temperature sensor through the thermal radiation or convection of the measured medium, so as to achieve the purpose of temperature measurement. This kind of sensor mainly includes infrared temperature sensor. The main feature of this temperature measurement method is that it can measure the temperature of moving substances (such as the bearing temperature of slow-moving trains and the temperature of rotating cement kilns) and objects with small heat capacity (such as the temperature distribution in integrated circuits).

application area 

Temperature sensor is the earliest developed and most widely used sensor. The market share of temperature sensor greatly exceeds that of other sensors. From the early 17th century, people began to measure temperature. With the support of semiconductor technology, semiconductor thermocouple sensor, PN junction temperature sensor and integrated temperature sensor have been developed in this century.

If two conductors of different materials are connected to each other at a certain point, if the connection point is heated, there will be a potential difference where they are not heated. The value of this potential difference is related to the temperature of the measuring point at the unheated part and the material of the two conductors. This phenomenon can occur in a wide temperature range. If the potential difference is accurately measured and the ambient temperature of the unheated part is measured, the temperature of the heating point can be accurately known. Because it must have two conductors of different materials, it is called "thermocouple". Thermocouples made of different materials are used in different temperature ranges, and their sensitivity is also different.

Thermocouple sensor has its own advantages and disadvantages. Its sensitivity is relatively low. It is easy to be affected by environmental interference signal and preamplifier temperature drift. Therefore, it is not suitable for measuring small temperature changes. Because the sensitivity of thermocouple temperature sensor has nothing to do with the thickness of material.