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The hollowing and even falling of the decorative layer of the outer wall of the house has always been an "invisible destroyer at high altitude", which seriously threatens the safety of people's lives and property. The application of infrared thermography technology in house detection is a relatively new subject, which has been studied at home and abroad. The basic principle of infrared thermal imager and its application in exterior wall detection are briefly analyzed below.
1. Introduction to infrared thermography detection technology
Infrared thermal imager is a kind of detection equipment to calculate the temperature value. It can accurately quantify the detected heat. It can not only observe the thermal image, but also accurately identify and strictly analyze the heating fault area.
Infrared is wavelength 0.78-1000 μ M. The infrared thermal imager for measuring high-temperature objects adopts 3~5 μ M-band infrared thermal imager for measuring room temperature and low temperature objects adopts 8-14 μ M-band. Any object above absolute temperature zero is an infrared radiation source. When there is a defect inside the object, it will change the heat conduction of the object and change the temperature distribution on the surface of the object. The infrared detector can measure the change of surface temperature distribution and detect the location of defects.
Characteristics of infrared detection technology: the focal length of the detector is 20cm~ infinity, which is suitable for non-contact telemetry of large area; The detector only responds to infrared ray, so it can work in day and night; The temperature resolution of infrared thermal imager is up to 0.1-0.02 ℃, and the accuracy of detecting changing temperature is high; The temperature measurement range is -50~2000 ℃, and the application field is wide; The camera speed is 1~30 frames / s, which can detect the temperature changes of static and dynamic targets.
1.1 Basic principle of infrared thermography detection technology
Infrared nondestructive testing is a method to identify the quality of an object by measuring its heat and heat flow. The theory of infrared thermography is based on the law of thermal radiation and the differential equation of heat conduction.
According to the law of thermal radiation, the infrared radiant energy of an object can be expressed as follows:
After using the infrared thermal imager to convert the power signal radiated by the object into an electrical signal, the output signal of the imaging device can completely simulate the corresponding thermal image of the heat distribution on the surface of the scanned object one by one. By using this method, the long-distance thermal state imaging and temperature measurement of the target can be realized, and the state of the measured object can be analyzed and judged
1.2 Technical requirements for infrared thermal image detection of building exterior wall facing layer
The infrared thermal imager suitable for the safety detection of building exterior wall finishes should have an infrared sensor to detect the infrared radiation energy emitted by the object and a device to visualize the measured signal, including image processing, analysis, storage, output devices and other equipment. The infrared thermal imager used to detect the bonding defects of the decorative layer of the building exterior wall should be 8~13 μ M-band long wave machine.
(1) The performance index of infrared thermal imager should meet the following requirements: 1) the detection range should be -20-100 ℃; 2) The minimum resolution temperature difference shall be less than 0.1 ℃; 3) The detection accuracy should be within ± 0.5% of full scale; 4) The pixel range of the obtained image should not be less than 300dpix200dpi; 5) The instantaneous visible area shall not be less than 2.5mrad.
(2) The operating environment of the infrared thermal imager shall meet the following requirements: (1) the ambient temperature is 0-40 ℃; (2) The allowable humidity RH is less than or equal to 90%, without condensation; (3) The lens must not be exposed to direct sunlight; (4) The measurement position and angle should not affect the accuracy of image processing.
(3) Advantages of infrared thermal image detection: infrared thermal image is a non-contact measurement technology, which will not damage the measured temperature field. Visually display the temperature field on the surface of the object; High temperature resolution, generally up to 01 ℃; It can use a variety of signal display, data storage, computer processing and geometric processing; Easy to operate and carry.
2. Application of infrared thermography technology in the detection of building exterior wall finishes
Peeling off the mortar powder layer of the new and old building walls will lead to leakage and large-area falling off, which may lead to major accidents. The existence of wall defects and damages reduces the thermal conductivity. The thermal radiation and conduction after solar irradiation make the temperature distribution at the defects and damages significantly different from that of the surface layer with good quality. Through the high-precision temperature resolution of infrared imaging, the defects and damages can be visually detected.
The infrared thermal image detection of the exterior wall finish should be carried out under the environmental conditions of no rain and low wind speed. During the detection, factors such as the orientation of the building, sunshine and whether the surrounding environment is blocked should be considered, and the best shooting time should be selected to process and analyze the infrared image to judge the damage of the exterior wall finish.
3. Conclusion
Infrared thermal imaging technology mainly uses infrared thermal imager to capture the temperature field on the surface of the object, and then convert it into image signals. Its important feature is that it can measure the temperature remotely, and has the advantages of non-contact, long-distance, real-time, fast, full field measurement and so on. Infrared ray detects large areas through non-contact, and thermal images can be recorded and displayed in a direct visual way. The detection results can be analyzed with high accuracy by analyzing the thermal image. The application of infrared thermography technology in building detection is a relatively new discipline, which is widely used in many fields, such as wall and roof leakage detection, pipeline leakage detection, post fire building detection and so on.
