Installation and usage instructions for temperature sensors

A temperature sensor refers to a sensor that can sense temperature and convert it into a usable output signal. Temperature sensors are the core part of temperature measuring instruments, with a wide variety of types. According to measurement methods, it can be divided into two categories: contact type and non-contact type. According to the characteristics of sensor materials and electronic components, it can be divided into two categories: thermistor and thermocouple.
When installing and using temperature sensors, the following precautions should be taken to ensure measurement effectiveness:
1. Errors introduced by improper installation
If the installation position and insertion depth of the thermocouple cannot reflect the actual 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 gap between the protective sleeve of the thermocouple and the wall is not filled with insulation material, which causes heat to overflow or cold air to invade the furnace. Therefore, the gap between the thermocouple protective sleeve and the furnace wall hole should be blocked with insulation materials such as refractory mud or asbestos rope to prevent cold and hot air convection from affecting the accuracy of temperature measurement; The cold end of the thermocouple is too close to the furnace body, causing the temperature to exceed 100 ℃; The installation of thermocouples should avoid strong magnetic and electric fields as much as possible, so thermocouples and power cable wires should not be installed in the same conduit to avoid interference and errors; Thermocouples cannot be installed in areas with little flow of the measured medium. When measuring the gas temperature inside a tube using a thermocouple, it is necessary to install the thermocouple in the opposite direction of the flow rate and make full contact with the gas.
2. Errors introduced due to insulation deterioration
If the thermocouple is insulated, excessive dirt or salt residue on the protective tube and pull plate can cause poor insulation between the thermocouple and the furnace wall, which is even more severe at high temperatures. This not only causes loss of thermoelectric potential but also introduces interference, and the resulting errors can sometimes reach hundreds of degrees.
3. Thermal resistance error
At high temperatures, if there is a layer of coal ash and dust attached to the protective tube, the thermal resistance increases, hindering the conduction of heat. At this time, the temperature reading is lower than the true value of the measured temperature. Therefore, the external cleanliness of the thermocouple protection tube should be maintained to reduce errors.
4. Error introduced by thermal inertness
Due to the thermal inertness of thermocouples, the indicated values of the instrument lag behind the changes in the measured temperature, which is particularly prominent during rapid measurements. Therefore, thermocouples with thinner thermoelectric electrodes and smaller protective tube diameters should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to measurement lag, the amplitude of temperature fluctuations detected by thermocouples is smaller than that of furnace temperature fluctuations. The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuation, and the greater the difference from the actual furnace temperature. When using a thermocouple with a large time constant for temperature measurement or control, although the temperature displayed on the instrument fluctuates very little, the actual furnace temperature may fluctuate greatly. In order to accurately measure temperature, thermocouples with small time constants should be selected. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermocouple's hot end, material density, and specific heat. To reduce the time constant, in addition to increasing the heat transfer coefficient, an effective way is to minimize the size of the hot end as much as possible.