Thermographic express-diagnostics by thermovisor and corresponding processing software
The measurement of surface radiation in diapason of infrared rays (l=1-15 mm) can be effectivelly used in measurement of surface temperature from distance and effortless making of color pictures. The thermographic measurements are performed from distance. Therefore, the measurement accuracy can depend on air humidity, rain or snow, radiation from surrounding objects reflected on the surface of inspected object, etc. The most important source of radiation is the sun which causes the mentioned reflection and heats up the surface of inspected building construction. Hence, the measuremnets cannot be performed for objects irradiated by sun. Wet surfaces of building constructions and strong wind also causes unfriendly climat for thermography. In general, the conditions for detecting of isolation deffects for buildings by infrared radiation are deffined by norms EN 13187 un ISO 6781.
The accuracy of surface temperature measurements essentially depends on correct coefficient of emmision e and coefficient of penetrability of radiation a. For idealised absolutly black source of radiation e=1, while the most of objects can be considered only as grey objects with e=const<1. The values of this coefficient can be found in tables or experimentally. The analysis of temperature distribution is cumbersome if the surfaces in obtained picture differs very much on coefficient of emmision, e.g., (e<0,05) and concrete (e~0,95).
The different transparencies a of building constructions in visible and infrared light should be included as they can differ significantly. For example, the usual glass is transparent to visible light; the transparency remains also for small infrared wavelength (l<5 font style="font-family: Symbol">mm), but for l>8 font style="font-family: Symbol">mm it becomes prractically inpenetrable to radiation. Special, so called selective coating (e.g., Ag layer with atom-size thickness) that are effectively used for decrease of heat transfer coefficient in construction of windows. Such a coating significantly decreases the coefficient e on the inner sufface of window packet (for usual glass e~0,85, while with coating <0,05).
Moreover, the use of thermography for analysis of heat and humidity isolation is possible only for sufficiently high temperature diffrence between outer and inner surface of building construction. This diffence should exceed 15 °C. If the temperatures are constant inside and outside of the building construction, then the inhogenity of building constructions (walls, roofs, windows, etc.) shows the respective variation of temperature on the surfaces. In our climat conditions and colder season, the infrared ray diagnostics show higher temperature of the outer surface at areas with higher heat conductivity (vice versa on inner surfaces). There are many cases in praxis with essentially unhomogeneous distribution of temperature near the surface. A shinning example is heat radiator built in thermic contact with outer wall; the thickness of the gap between heater and outer wall is insufficient. The survey on the outer side shows marked areas with higher temperature particularly for high heat transfer coefficient of the outer wall (the pannels of reinforced concrete with heat transfer coefficient U>0,7 W/m2K and brick outer walls with U>1,0 W/m2K are common for buildings in Latvia). In this case, it shows incorrect construction of heating system.
Moreover, the obtained diagrams displaying the opennings and unpacked areas in outer building constructions (e.g., between jamb and frame of the window) can be very different depending on the air circulation, ventilation and direction of wind (determines pressure on the outer walls). The lowered temperatures will be shown for openings with air influx. On other hand, the air outflux can be detected by areas with higher temperature, if the measurements are performed from outside.
However, the thermography (particularly because of the unstacionar character of heat exchange) does not allow quantitative measuremnts nor of the heat transfer coefficient U, nor of the degree of packing of buildings n even if the thermography is performed additionally from inside of the building.