I. Temperature sensor error
1. Errors introduced by improper installation
For example, the location and insertion depth of the thermocouple installation may not reflect the true temperature of the furnace. In other words, the thermocouple should not be installed too close to the door or heating element, and the insertion depth should be at least 8 to 10 times the diameter of the protective tube. If the gap between the thermocouple's protective tube and the furnace wall is not filled with insulating material, heat may overflow or cold air may intrude. Therefore, the gap between the thermocouple's protective tube and the furnace wall hole should be sealed with refractory mud or asbestos rope to prevent the convection of hot and cold air from affecting the accuracy of temperature measurement. If the cold junction of the thermocouple is too close to the furnace body, the temperature may exceed 100°C. Thermocouple installation should avoid strong magnetic and electric fields as much as possible. Therefore, thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors. Thermocouples should not be installed in areas where the measured medium has little flow. When using a thermocouple to measure the temperature of gas inside a tube, the thermocouple must be installed against the flow direction and in full contact with the gas.
2. Errors introduced by deterioration of insulation
If the thermocouple is insulated, excessive dirt or salt residue on the protective tube and pull plate can cause poor insulation between the thermocouple electrodes and the furnace wall, which is more serious at high temperatures. This will not only cause the loss of thermoelectric potential but also introduce interference, and the resulting error can sometimes reach hundreds of degrees.
3. Errors introduced by thermal inertia
Because of the thermal inertia of thermocouples, the instrument reading lags behind the change in the measured temperature, a phenomenon particularly pronounced during rapid measurements. Therefore, thermocouples with thinner thermocouple electrodes and smaller protective tube diameters should be used whenever 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 the amplitude of furnace temperature fluctuations. The greater the measurement lag, the smaller the amplitude of the thermocouple fluctuations, and the greater the difference from the actual furnace temperature. When using thermocouples with large time constants for temperature measurement or control, the instrument may display a small temperature fluctuation, but the actual furnace temperature fluctuation may be large. For accurate temperature measurement, 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 hot junction, the density of the material, and the specific heat. To reduce the time constant, besides increasing the heat transfer coefficient, the most effective method is to minimize the size of the hot junction. In practice, protective tubes made of materials with good thermal conductivity, thin walls, and small inner diameters are typically used. In more precise temperature measurements, bare wire thermocouples without protective sheaths are used, but thermocouples are easily damaged and should be calibrated and replaced in a timely manner.
4. Thermal resistance error
At high temperatures, if there is a layer of coal ash or dust on the protective tube, the thermal resistance increases, hindering heat conduction. In this case, the temperature reading will be lower than the true temperature being measured. Therefore, the exterior of the thermocouple protective tube should be kept clean to minimize errors.
II. Applications of Temperature Sensors
(1) Temperature sensor in the refrigerator. When the temperature inside the refrigerator is higher than the set value, the refrigeration system starts automatically; when the temperature is lower than the set value, the refrigeration system stops automatically. The refrigerator temperature is controlled by a temperature sensor.
(2) Temperature Sensors in Automobiles. Automotive sensors are an important component of automotive electronic equipment, responsible for information collection. In automotive electronic fuel injection engine systems and automatic air conditioning systems, temperature is one of the crucial parameters that needs to be measured and controlled. Measuring the engine's thermal state, as well as the temperatures of gases and liquids, all require temperature sensors. Therefore, automotive temperature sensors are indispensable. Because engines operate in harsh environments with high temperatures (engine surface temperatures can reach 150℃, exhaust manifold temperatures can reach 650℃), vibration (acceleration 30g), impact (acceleration 50g), humidity (100%RH, -40℃ to 120℃), and contamination from steam, salt spray, corrosion, and sludge, the technical specifications of sensors used in engine control systems must be 1-2 orders of magnitude higher than those of general industrial sensors. The most critical aspects are measurement accuracy and reliability. Otherwise, measurement errors caused by the sensors will ultimately lead to the engine control system malfunctioning or failing. Temperature sensors are mainly used to detect engine temperature, intake gas temperature, coolant temperature, fuel temperature, and catalytic converter temperature, etc.
(3) Temperature sensors in household appliances. Temperature sensors are widely used in many applications, including temperature measurement and control in household appliances (microwave ovens, air conditioners, range hoods, hair dryers, toasters, induction cookers, frying pans, heaters, refrigerators, freezers, water heaters, water dispensers, dishwashers, disinfection cabinets, washing machines, dryers, and medium- and low-temperature drying ovens, constant temperature chambers, etc.), medical/household thermometers, portable non-contact infrared thermometers, and many other areas.
(4) Temperature sensors in medical instruments and equipment. In medicine, various sensors are used to detect human body temperature, blood pressure and intracavitary pressure, blood and respiratory flow, electrocardiogram, pulse and heart sounds with high accuracy, so as to provide timely feedback on treatment results and realize automatic detection and monitoring of patients.
(5) Temperature sensors in robots. Robots are becoming increasingly intelligent because they use many sensors to control and monitor their behavior, such as position sensors, speed sensors, tactile sensors, vision sensors, and olfactory sensors.
(6) Temperature sensors in aerospace. Aircraft such as airplanes, rockets, and spacecraft use a variety of sensors to detect flight speed, direction, distance, and flight attitude in order to make accurate measurements.
