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Infrared Thermal Imaging to Detect Water Content in Damp Walls
It is a matter of fact that moisture excess highly contributes to the decay of a building. Actually, most historical buildings are affected by such a problem. It is well known that the evaporation process, with the migration of salts within the material, represents the main reason for the deterioration of surfaces. Tensions and stresses due to salt crystallisation on the surface and the mechanical stress and micro-cracks due to freezing–thawing cycles are very dangerous. Chemical and physical phenomena are also activated by moisture, such as the growth of bacteria, algae, fungi and lichens.
Many methods have been proposed for this purpose, but up to the present, the ultimate solution is far from being found. The measurement of moisture content is difficult because it is an indirect measurement using as informative parameters quantities such as, the mass, electrical conductivity, electromagnetic wave absorption in specific bands, propagation speed of elastic waves, dielectric constant of material or magnetic resonance of hydrogen atoms. Actually, it is difficult even defining what the moisture content of a wall is, because moisture has a concentration profile that is variable with depth. Therefore, different wall thicknesses have different moisture content.
Figure 1. Thermogram of a moist wall.
Among these methods, temperature may also be employed as an indicator of the presence of moisture and humidity. However, the complexity of the structure of a building, and the dependence of its thermal history from climatic conditions and from the use for which it is intended for, make the interpretation of the data complex and not always reliable. This paper gives a new key based on an optical analysis to solve an old problem, that is, how to monitor the moisture content of a wall. The detector is a thermographic device that is an imaging radiometer working in the IR band (8–13 mm). IR thermography is a valid tool for locating a moist area, because the water content of a porous material is strongly related to the surface temperature. On the other hand, such an optical testing is very productive and definitively non-destructive. These features make it the most suitable for inspecting very large and precious surfaces and preserving effectively our cultural patrimony.
Figure 2． Thermogram recorded during the test.
The sharp cost reduction of IR thermography makes a new powerful optical tool for many diagnosis purposes available. A new inspecting procedure is designed to be effective, reliable and simple for moisture detection. In order to estimate the moisture, laboratory studies demonstrate that it is possible to follow and quantify the evaporation of a surface by quantitative thermography. Basically, the test merges two important features: the decreasing temperature due to the evaporation and the intensity of the evaporation rate, which is related to the speed of temperature change. The surface labelling according to the moisture content is very indicative of the cause of the problem because any moisture pathology has its own characteristic pattern. Actually, the imaging of the moisture distribution may be more important than the water content measurement itself. Specifically, these aspects are of vital importance in the conservation of artistic heritage including works of art on walls such as frescos and the like.
E. Grinzato, G. Cadelano and P. Bison. Moisture map by IR thermography. Journal of Modern Optics. 57(18):1770-1778, 2010.