WO2007132009A2 - Procédé et appareil pour la détection thermographique des conditions thermohygrométriques de surfaces étendues - Google Patents

Procédé et appareil pour la détection thermographique des conditions thermohygrométriques de surfaces étendues Download PDF

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Publication number
WO2007132009A2
WO2007132009A2 PCT/EP2007/054713 EP2007054713W WO2007132009A2 WO 2007132009 A2 WO2007132009 A2 WO 2007132009A2 EP 2007054713 W EP2007054713 W EP 2007054713W WO 2007132009 A2 WO2007132009 A2 WO 2007132009A2
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thermographic
air
anemolr
hygrometric
irpsicro
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WO2007132009A3 (fr
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Ermanno Grinzato
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Consiglio Nazionale delle Richerche CNR
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Consiglio Nazionale delle Richerche CNR
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/56—Investigating or analyzing materials by the use of thermal means by investigating moisture content
    • G01N25/62—Investigating or analyzing materials by the use of thermal means by investigating moisture content by psychrometric means, e.g. wet-and-dry bulb thermometers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/56—Investigating or analyzing materials by the use of thermal means by investigating moisture content
    • G01N25/66—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
    • G01P5/00—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft
    • G01P5/10—Measuring speed of fluids, e.g. of air stream; Measuring speed of bodies relative to fluids, e.g. of ship, of aircraft by measuring thermal variables

Definitions

  • the present invention relates to a method and a thermographic ancillary equipment for the detection of the thermo-hygrometric conditions of porous means by using infrared thermography and, more precisely, to a method for the thermographic detection of the thermo-hygrometric conditions of environmental parameters and vast surfaces, such as buildings, frescos and the like.
  • STATE OF THE ART In civil buildings and in mundane and historical buildings, including works of art on walls such as frescos and the like, it is known that the evaporation process, with the migration of salts within the material, represents the main reason for the deterioration of the surfaces. Specifically, these aspects are of vital importance in the conservation of the artistic and historical heritage. In the analysis of the moisture of such surfaces, the key points are the knowledge of the water content in the material forming them and the vapour exchange between such a surface and the surrounding atmosphere.
  • the separate measurement of the hygrometric quantities within the material or in the air is not sufficient to know and quantify the whole process. Indeed, only in theory may the mere evaluation of the content of humidity of the atmosphere surrounding the surface be sufficient in the case in which the internal atmosphere and all of the surfaces have the same thermal level, a condition which is rarely achieved as a matter of fact.
  • two kinds of monitoring are required: one directed to know the evolution of the microclimatic state of the air, and one to know the corresponding features of the materials and especially of the surfaces. For instance, in a crowded environment, it may happen that the concentration of vapour increases as well as subsequently the dew point, or that the temperature of a surface decreases by diffusion of the heat towards the outside. In the case in which the temperature of a surface is below the dew point, condensation occurs in this surface. Therefore, the risk of surface condensation may only be evaluated by monitoring at the same time the temperature of the air and the object and by continuously detecting the boundary conditions, even when the latter is not immediately visible with a naked eye, because the water is, for instance, absorbed within the pores of the surface under analysis. Indeed, to estimate the evaporation flow of the surface it is required that the content of vapour in the air is determined, as well as the air speed and the atmospheric pressure.
  • the measurement of the moisture is generally complex because it is an indirect measurement, quantities such as the mass, the electrical conductivity, the propagation speed of elastic waves or microwaves and the temperature as well may be employed as an indicator of the presence of moisture.
  • thermography is an optical measurement method, that in virtue of a radiometric calibration allows obtaining a time sequence of maps of temperature, provided that some parameters are known, such as the emissivity of the surface and the instantaneous and angular radiance, or that the effects thereof are negligible.
  • some parameters such as the emissivity of the surface and the instantaneous and angular radiance, or that the effects thereof are negligible.
  • the complexity of the structure of a building and the dependence of its thermal history from the climatic conditions and from the actual use make the interpretation of the thermograms difficult and not always reliable.
  • the IR thermography may potentially be used for the detection of the moisture contained in the surfaces and the main modes of use are based on the following physical phenomena: a) the selective absorption of the infrared radiation (optical method); b) the influence of the water content on the thermophysical properties of the porous means (active thermal method); c) evaporation cooling (passive thermal method).
  • thermographic camera which "images and records" the distribution of the surface temperature of an opaque object, such as for instance the wall of a building.
  • the field of view depends on the objective used and on the shooting distance.
  • the thermal and spatial resolution depend on the features of the particular thermographic camera used.
  • the spectrum working band may vary and is generally in the range between 1.3-2.5 ⁇ m, 3-5.5 ⁇ m, 8-13 ⁇ m or 8-9 ⁇ m, but it may be a part of these if appropriate filters are used.
  • thermographic detection of the thermo-hygrometric conditions of vast surfaces which employs IR thermography and simplified mathematical models of the evaporative processes, to study the hygrometric conditions of such surfaces and the surrounding air at the same time.
  • means to measure the main thermo-hygrometric quantities related to the atmosphere, which affect the evaporation of the surfaces said measuring means being arranged near the wall to be analysed.
  • thermographic detection means for the thermographic detection, which are made to cooperate with said measuring means in order to determine and record the time evolution of the environmental temperature, the relative humidity and the air speed in one or more points near the surface to analyse. Furthermore, the content of moisture of the solid surface is evaluated spoiling peculiarities of porous materials drying.
  • the evaporation rate is evaluated without performing traditional measurements, by means of thermographic detections, which are processed through automatic and reliable data reduction algorithms. To this purpose, new specific parameters are used, which are defined as: "evaporation thermal index” (ETI), and its time derivative (dETI/dt, also indicated as ETI).
  • ETI evaporation thermal index
  • w c represents a fundamental value in the activation of the degradation processes triggered by moisture and represents a peculiar and extremely important datum from the practical point of view.
  • the present invention provides a method to measure the main thermo-hygrometric quantities related to the atmosphere and solid porous bodies, which affect the evaporation from the surfaces using accurate surface temperature measurements by IR thermography and an apparatus related to the actuation of such a method according to the accompanying claims.
  • thermo-hygrometric conditions of each point included in vast surfaces may be evaluated in a totally non invasive or destructive manner, as well as may the boundary conditions with the same experimental apparatuses and at the same time. Therefore, the accuracy of the measurement may be considerably improved, thus obtaining images, which are immediately interpretable to identify the causes of the abnormal accumulation of moisture.
  • Another advantage of the method of the present invention is given by the rapid analysis provided by an optical sensor in comparison with any other contact measurement instrument, and by the same absence of errors due to the contact and to possible damages of the surface.
  • a further advantage of the method of the present invention is given by the option of using simplified and automatic analysis models for the quantitative evaluation of the physical phenomenon of coupled heat and mass transfer associated to the corresponding thermo-hygrometric flows. The clear advantage of this approach is the possibility to very simply obtain the required information, by maintaining a degree of accuracy comparable to the traditional technical measurements (indicatively ⁇ 10 %).
  • Another advantage of the method of the present invention resides in that it may be used autonomously, thus guaranteeing ease of use and reducing the measurement error; or with a mode combined with other instrumentation.
  • the latter allows to couple the measurements obtained by traditional measurements, by means of which a recording may be obtained for very long time intervals (months or years) to few optical detections at predetermined times. This allows to give a virtually continuous spatial distribution, as well as it being possible to estimate not only the amount of moisture in the walls, but also and above all to easily identify the cause of its accumulation.
  • the method therefore consists in a very useful instrument in the diagnosis of the dampness, especially in mundane buildings and the estimate of the recovery time.
  • thermo-hygrometric conditions of vast surfaces of the present invention given by way of non-limitative example, referring to the accompanying drawings, in which:
  • Figure 1 shows a device for the determination of the temperature of the air, the relative humidity and the dew point by IR thermography, which is part of the apparatus of the present invention
  • Figure 2 is a plot showing the measurement of the air temperature and temperatures detected on a variety of wet surfaces with the device in figure 1 , as the relative humidity (RH) varies;
  • Figure 3 is a graph which shows the measurement of the relative humidity detected with the device in figure 1 using data of fig.2 and compared with a state of the art reference hygrometer;
  • Figure 4 schematically shows a device for the detection of the gradient of the humidity exchange ratio between wall and atmosphere by means of IR thermography;
  • Figure 5 schematically shows a device for the detection of the surface thermal exchange coefficient and the estimation of the speed of the air by means of IR thermography
  • Figure 6 shows to the plot of the evaporation rate, measured with the gravimetric method on samples of "S. Marco" brick displaying different thickness, as a function of time;
  • Figure 7 shows the evolution of the specific evaporation versus time, measured by means of the gravimetric method on samples of S. Marco bricks displaying variable thickness
  • Figure 8 shows the standard measurements of the evaporation rate of the same samples of S. Marco brick in Figure 6 and Figure 7, according to RILEM 25-
  • Figure 9 shows different phases of the evaporating process versus time
  • Figure 10 shows the temperature difference between air and wet surface ( ⁇ T as ) measured by thermography on S. Marco brick samples, with various thickness;
  • Figure 11 shows the trend of the evaporation rate measured according to the RILEM standard and estimated by the Evaporation Thermal Index simplified method versus the residual average moisture content
  • Figure 12a shows the trend vs. time of function K (t) for the S. Marco brick, and the proportionality coefficient KQ,
  • Figure 12b shows the values of ER and ETI as T a and RH vary for samples of S. Marco brick;
  • Figure 12d shows the experimental values of the coefficient Ko for various samples of S. Marco brick displaying different thickness and other materials, as T 3 , RH and v a vary;
  • Figure 13 shows the estimate of the evaporation flow versus time computed for two samples of S. Marco brick of different thickness using different approximations of the K function, with respect to the standard measurement;
  • Figure 14a represents the function (dETI/dt) in time, for samples of S. Marco brick having different thickness;
  • Figure 14b shows the of the characteristic times versus thickness, obtained from Figure 14a and the linear fitting parameters
  • Figure 15 shows the absolute evaporation versus time measured by gravimetry and calculated using ETI per the factor Ko, for samples of S. Marco brick having different thickness;
  • Figure 16a shows the specific evaporation versus the average remaining content of water (w) parametric for relative humidity
  • Figure 16b shows the specific evaporation of samples made of different building materials, as a function of the average remaining moisture content
  • Figure 16c shows the specific evaporation of samples of S. Marco brick displaying different thickness versus the average remaining moisture content
  • FIG 17 shows another embodiment of the apparatus according to the invention.
  • the method of the present invention is based on. Measurement of the thermohvgrometric state of the humid air
  • thermo- hygrometric parameters of the humid air surrounding the porous means must be detected, as they affect the thermo-hygrometric processes occurring between the surface and the surrounding air. Therefore, according to the method of the present invention, there is provided the optical measurement of these quantities with a time continuity and strategic spatial position, by using IR thermography combined with a special new device which is an object of the present invention and is intended for the detection of the air temperature, the relative humidity and the dew point.
  • Figure 1 represents this device, from now on designated as IRpsicro.
  • the operation principle thereof is based on the well-known technique of psychrometric measurement, but is carried out by means of a radiometric detection of the surface temperatures, which are made equivalent to the classical thermohygrometric measurements, which are defined as dry bulb temperature (T a ) and wet bulb temperature (T b ). Therefore, by analysing a thermogram it will be possible to know the main microclimatic parameters at the same time as the distribution of temperature on the examined surface. It must be specified here that the positioning and the distance of this accessory device, which in general is preferably positioned near the wall subjected to the detection, is especially important.
  • the IRpsicro essentially consists of two surfaces S a and S b , as shown in figure 1.
  • T a the temperature of the air
  • S b consists instead of a special hydrophilic hose, which absorbs by capillarity the demineralised water contained in the underlying vessel, in which it is immersed below.
  • the hose is placed within a constant section channel and is rearly lapped by a forced air flow having an average speed of 5 m s "1 .
  • This detection allows determining the microclimatic conditions with a suitable spatial distribution.
  • the temperature measurement accuracy is potentially better by 0.1 K, as the emissivity of this surface is known.
  • the precision depends on the particular thermographic apparatus used. In the case the instrument is employed outdoors it will be advisable to place a reflecting screen consisting of a thin aluminium sheet so as to screen the direct solar radiation, otherwise incident on S a and S b .
  • the relative humidity measurement As far as the relative humidity measurement is concerned, it has already been specified that the measurement of the average temperature on the surface S 3 provides with a very good precision the temperature of the humid air. It may also be considered that this value coincides with the dry bulb temperature of a psychrometric measurement.
  • the average temperature thermographically detected on S b represents the wet bulb temperature 7 b .
  • the channel is appropriately provided with baffles so as to allow an appropriate air flow which is on the average uniform on the surface S b , though neither affecting S a nor the wall.
  • the temperatures T a and 7 b may be respectively defined as the temperature to which the water vapour is taken in conditions of equilibrium of convective heat and mass exchange with air subjected to forced convection. In other terms, it may be stated that T b (detected on the wet surface) is the lowest temperature to which air may be cooled by the evaporation of water, when it is air providing the vaporisation latent heat required for the change of status.
  • the wet bulb temperature obtained with the psychrometer in natural convection moderately differs from this.
  • the precision of the measurement and some mathematical reductions adopted in the equations describing the thermal exchange are not allowed.
  • each step corresponds to a value of relative humidity set in the climatic chamber so as to respectively correspond to 20%, 40%, 60% and 80%, by maintaining the temperature set point of the chamber at 20 0 C.
  • the time constant for the IRpsicro may also be determined, which is mainly connected to the thickness of the material forming the surface S b .
  • the small deviations between the values detected by IRpsicro and those set in the climatic chamber are due to the non-uniformity of the conditions within the cell.
  • the comparison of the wet temperature surfaces (T b ) and the dry (T 3 ) one by means of IRpsicro is measured inside a climatic chamber on different materials to setup the most suitable S b surface.
  • the pressure of the saturated vapour (e s ) may be determined by the Murray relation:
  • T a the air temperature
  • the saturation pressure (Ps) may also be calculated with the following equation (2):
  • the dew point (Td) may be calculated by means of equation (3). - ⁇ )
  • thermogram which contains IRpsicro therein
  • the temperature of the surface will be known at the same time as the dew point, and therefore the risk of condensation may be evaluated for each point.
  • a fundamental purpose of the environmental measurements is represented by the need to identify the interactions which occur between environment and the body. For instance, a measurement of the gradient of the exchange ratio (o) made near a wall may indicate if and when this is liable to condensation or evaporation. For instance, if on the wall the level of vapour exchange is greater than in free air at a certain distance (Ao ⁇ 0) it means that the local enrichment is due to the evaporation by the wall; if instead it is smaller ( ⁇ o >0), it means that the condensation has taken away molecules from the air closest to the wall.
  • the same material and technology employed in the above-described first embodiment is employed to manufacture the surfaces S t> i and S b 2-
  • the crucial point consists of the distances and size of the evaporating surface.
  • the evaporating surfaces are arranged parallelly to the wall and at a distance of 5 mm for S b i and a 100 mm for S b2 .
  • the two surfaces are obviously contained in the visual field of the thermographic camera and are such as to represent a minimum obstacle for the natural circulation of the air adjacent to the examined surface.
  • a surface of the S a type must be placed near the surface S b2 so that the temperature of the air in that area is also measured.
  • thermographic detection allows to use the thermographic detection for the determination of the surface thermal exchange coefficient.
  • a third embodiment of the IRpsicro device which is designated as AnemolR, is employed to carry out this measurement, as shown in Figure 5.
  • the accessory device is adapted to detect the surface exchange coefficient and/or the estimate of the speed of the air by means of IR thermography.
  • this third embodiment of the device provides that the same consists of a reservoir containing demineralised water, an evaporating surface, saturated with water by capillarity (S b ), connected to the reservoir, a waterproof surface with a high emissivity pigment (S 3 ) and a high radiometric contrast calibration ruler.
  • the water reservoir will be closed and have a regular section (Av [m 2 ]), with a thin wall in a plastic material and a low section/height ratio, so that the detection of the variation of the volume of water is precise enough, on the basis of a level measurement (/ [m]).
  • the instrument sensitivity will increase inversely with the cross section Av [m 2 ]), whereas the autonomy will increase proportionally to A w times the height [m 3 ].
  • the reservoir will be refilled when the level will reach a minimum value, with a system which may be manual or automatic.
  • a cylindrical reservoir having a diameter of 6 mm and a working lenght of a 100 mm has been made.
  • This configuration provides a resolution of 0.056 [g/mm].
  • the evaporating surface (S b [m 2 ]) displayed a base perimeter of 20 mm and a height of 50 mm.
  • An improvement aiding the measurement of the level consists in the activation at regular intervals of a heating circuit, which coincides with the ruler divided into millimetres which will slightly heat the outside of the reservoir (about 1 °C).
  • the heating may, for instance, be generated by a normal printed circuit to which a low voltage current pulse is provided (generated for instance by a normal dry cell) just before carrying out the thermographic reading.
  • equation (5) shows that in steady state, most of the energy required to support the water evaporation from the surface of the hydrophilic material, forming S b of IRpsicro is provided by the air flow which laps it, which is at a temperature T a .
  • the coefficient h thus plays a fundamental role, as will be explained in greater detail hereinafter and is an important parameter to be determined.
  • C L may be considered a known constant (2.257 10 6
  • Equation (5) therefore shows that from the measurement of the variation of the level of the distilled water contained in the reservoir, coefficient h may be derived by measuring T 3 and T b .
  • thermographic system as water has a very high thermal capacity and therefore a vertical temperature gradient is generated outside the reservoir just at the surface of the water, each time the surface of the reservoir is thermally stressed, even because of small variations of T a .
  • thermographic camera the thermal gradient imposed on the surface with the further heating device may be stressed as shown in figure 5.
  • the algorithm shown hereinafter which automatically processes the thermographic pictures, is used for the detection of the level, the calculation of the vapour flow and the calculation of the convection coefficient.
  • thermographic sequence is processed with the following algorithm implemented in a Matlab ® environment:
  • the thermal stress device will not be required as shown in figure 5.
  • a simple way to make the calibration ruler consists in placing by the side of the surface S 3 (in the visual field of the thermographic camera) a stripe of reflecting aluminium (S r ) having known length, delimitated at the ends by edges of black opaque paint.
  • FIG. 17 shows a reference wet surface, added to an aluminium cavity, working as a reference for the air temperature measurement and equipped with a portable data logger recording main environment quantities, for the needed time.
  • This calibrated reference allows to enhance at easy the measured temperatures by thermography to 0.1 °C.
  • the frame 1 supports IRpsicro targets 3 (one of which is shown in enlarged scale in the centre of the figure) for the measure of the boundary conditions, including air speed, the close up of one target 3 and the reference IRpsicro for the temperature calibration 2 equipped with a data logger for the long run recording of the environment conditions.
  • the anemometer is not required, because the estimation of the speed of the air (v a [m s "1 ]) may be obtained by equation (6) which derives it from the value of h in a laminar regime of natural convection, i.e. when the movement of the air only occurs by gravitational and by thermal field effects between the surface and fluid.
  • ANN artificial neural networks
  • the "back-propagation” architecture for neural networks is the simplest and is used in this case. It consists of a first layer of neurons, which contains the input units, the second layer contains the "hidden” units that process information and transfer it to the following output units. Each neuron of a layer is connected to all units of the previous layer, but has no connections with neurons of the same layer. The signal propagates in one direction from the input to the output through the hierarchy of the intermediate layers.
  • the dynamics of the system is represented by two laws: an activation law, which updates the state of the neurons; a learning law, which modifies the strength of the connections.
  • ANN therefore processes data by providing an answer similar to the behaviour used for the training phase.
  • the learning phase the information passes from an input neuron to an output neuron ("Forward Propagation").
  • the network stores the experiences in the form of thresholds and weights and initialises the procedure again. This procedure is continuously repeated by using a great number of models.
  • a complete model consists of input data provided by speed measurements carried out with accurate anemometric instruments, in controlled conditions.
  • the learning cycle is completed when all of the models have passed the network. The sequence of the models within a cycle is chosen randomly.
  • the training is interrupted, when many cycles one after the other have provided no improvements and the results are satisfactory.
  • the ANN is ready for its "true” processing. Therefore, after having "fed” the actual data, the expected estimates will be obtained.
  • the training step for the ANNs may be fairly long, but the processing speed is high.
  • a network may be trained to determine v a .
  • the training must include the RH whole range, as well.
  • the training is obtained by comparison of the measurement given by a reference anemometer.
  • the iterative process leads to the minimisation of a function of positive cost, which measures the difference between answers provided by the neural network, fed with the data of IRpsicro and the reference anemometer.
  • T 3 , RH and the speed of the air is locally monitored by a network of IRpsicro held by the frame, as the picture 17 shows. Just one of these targets 3 must be devoted to the humidity measurement, the others to T a and v a ,
  • IRpsicro acts as a low pass filtering of the data and it is indicated for analysis of stationary or slowly-varying flows as needed in the study of the evaporation phenomena, in the environmental study. Monitoring of the evaporation processes
  • the evaluation of the dampness within the solid material is also provided. It is known that the thermo-hygrometric phenomena are extremely complex when the concentration of water starts to differentiate between the damp surface and the adjacent atmosphere or within the porous material. Indeed, up today there is not an ultimate monitoring procedure.
  • the method of the present invention is also based on the analysis of particular conditions and characteristic of porous materials, which are significant and important in the practice, when, some of these phenomena prevail on the others. In this manner, simple and robust models may be used to estimate with good accuracy the fundamental quantities in the evaluation of the risk of degradation of the surfaces, triggered by moisture.
  • thermohygrometric processes acting as quasi-stationary processes.
  • the evaporation mainly concerns the most superficial layers of the porous material, progressing in time, so that the deeper layers are concerned only when the environmental conditions allow this.
  • the main factors, which affect evaporation may listed as follows:
  • the concentration of other substances in the air - the temperature of the water (in a porous means, it depends on the temperature of the surface and the air, and the higher the temperature, the greater the evaporation); - the- flow rate of the air (in an open environment, it is connected to the speed of the boundary layer). If the air stills over the surface, the concentration of water will tend to saturate, thus progressively reducing evaporation. If air freely circulates, this is not the case. Furthermore, the evaporation will tend to further increase, because the air molecules in motion have a greater kinetic energy;
  • the Evaporation Rate "ER” [g s "1 ] is defined as the net water vapour time derivative, i.e. the flow rate density, which evaporates from the surface (A) of the porous means in the time interval ⁇ f.
  • Equation (7) quantitatively expresses the evaporation rate and the calculation thereof carried out on the basis of a sequence of measurements of the mass of material (M) in time, where t ⁇ and fn are consecutive moments in time. In this manner, if a constant time interval is used, At, the weight variation AM will be multiplied by the constant MA At.
  • the methodology suggested is potentially capable to also study this aspect, but for the main aim of estimating the average content of water the measurements will have to be made independent from the thickness. This result is made easier when the thickness of the material is relevant. Specific Evaporation
  • E(t) (M s -M)/A [g m "2 ], which provides the amount of water evaporated from the surface and is given by the decrease in weight, i.e. the difference between the mass of material at a certain
  • Figure 7 shows the evolution of the specific evaporation as a function of time, measured with the weight method on samples of S. Marco brick having variable thickness (between 3, 12, 28 e 45 mm). As may be noted in the figure, during the 10 saturated phase the relation is linear. The slope of the line, independent from the thickness of the material, varies with the boundary conditions. Furthermore, it may be noted in the figure that the evaporation of the total amount of the water contained in the material, obviously depends on the volume of the sample and is given by the area subtended by the curves in Figure 6, i.e. by the integral thereof.
  • ER and E are defined as a function of the average remaining content of water (w) .expressed as a
  • Figure 8 and Figure 9 show the standard measurements of the evaporation rate of the same samples of S. Marco brick having various thickness (3, 12, 28 and 45 mm) of Figure 6 and Figure 7, according to RILEM standards 25-5432/1980, as a function respectively of the average remaining content of humidity and of time.
  • the critical moisture content (w c ) is an extremely important parameter for the evaluation of the water content of a porous material and the method focuses on its measurement.
  • w c represents the transition threshold between the condition near saturation and the progressive desiccation. The practical importance of a rapid and reliable technique as that described, capable of identifying the possible regions of the evaluated surface which are above or around this critical point, may easily be understood.
  • the evaporation will proceed at a rate which is close to maximum. This process is mainly controlled by the boundary conditions of the atmosphere. However, the evaporation is also affected by the cohesion forces and by the surface tension. For a determined material, the flow of evaporating water will therefore be a function of the microclimatic factors, such as: air speed, energy available for the surface, shortage of the vapour pressure.
  • the evaporation generally proceeds at a constant rate in this first step of the desiccation process, at least in so far as such microclimatic conditions are maintained. Therefore, having reached the critical moisture content, if the water inflow is less than the evaporation rate, the moisture content will progressively decrease until the pores of the material start to dry up, obviously starting from the surface. When the average remaining content of water will be below w c , the evaporation flow is supported by the diffusion of water within capillaries, progressively decreasing the concentration of water of increasingly deeper layers. Therefore, in this second phase it will be the characteristics of the material which determine the intensity of the evaporation flow from the surface, which will very rapidly decrease. This peculiarity of porous material is spoiled by this invention.
  • the evaporation process may be distinguished in a phase in which it occurs at the maximum degree possible, as the surface porous are saturated with water in a liquid phase and then, after having overcome w c , with a progressively decreasing intensity.
  • This method is focused to the detection of this particular condition, which is linked to a moisture content of interest for the analysed material, instead of an arbitrary moisture value.
  • a third phase may be distinguished in which the actual desiccation of the material will be reached, or rather when the so-called physiological water content (vi/ f ) is reached.
  • the transition to the second and third stage occurs when the porous material in not capable of providing enough water to the surface to satisfy the evaporation requirement.
  • the phenomenon is controlled by the hydraulic conductivity of the porous means. As the wall dries up, the evaporation plane shifts inwards and, accordingly, the net evaporation decreases.
  • the shape of the graph curve in this second phase is closely connected to the type of material (for instance, bricks tend to lose more water by evaporation than tuff, which drains faster).
  • thermographic methodology for the determination of the content of moisture and obviously, provide a validation of the results obtained.
  • thermohygrometric conditions may be evaluated by using simplified models, based on the estimate of the heat needed for the phase change. In practice, it is assumed that the thermal conduction only has a moderate effect. In this manner, the flow of evaporating mass may be estimated from the measurement of a parameter, which is based on the temperature gradient between the evaporating surface and the air outside the boundary layer.
  • FIG 10 there are shown thermal gradients between air and humid surface ( ⁇ T as ) which have been measured thermographically on wet samples of S.
  • Marco brick displaying various thicknesses, placed in a climatic cell, with constant boundary conditions, considered standard (7 a , 20 °C, RH 40%).
  • the figure shows the time evolution of the difference in temperature of the samples and the temperature of the air.
  • the tests have been performed in parallel with the evaporation measurements, according to the RILEM standards, displayed in the previous figures.
  • Evaporation and condensation are isothermal processes which are accompanied by the transfer of latent heat, respectively towards the outside of the water body and towards the surface of the water. They are affected by the amount of water vapour in the air near the evaporating surface, which in turn affects the latter.
  • the latent heat of evaporation, CL [J Kg "1 ] relates the specific heat flux, (Q) [W ⁇ f 2 ], and the evaporation flow rate ER [g rrf 2 s "1 ] in the following manner:
  • ⁇ is the thermal conductivity of the material and h is the surface heat exchange coefficient.
  • Equation (10) establishes that the heat subtracted by evaporation is provided by the adductive thermal exchange, given by the sum of the sensible heat of the air, exchanged by conduction and of the radiative heat flux, plus the conduction of the material itself.
  • the conductive flux is lower by at least one order of magnitude with respect to the convective flux. Therefore, it is reasonable to consider the conductive heat flux as a fraction of the thermal adduction in air. Therefore, the ratio between the surface exchange coefficient h [W m "2 K "1 ] and the latent heat C L may be concentrated at first guess in a proportionality coefficient k.
  • the coefficient k also considers the contribution given by the conductive thermal flux. Generally, it must be reminded that the adductive/conductive flux fraction depends on the particular material and varies in time with the evolution of the desiccation.
  • IRpsicro provides the existence of a third functional surface S r , which displays a high degree of diffused reflection and is particularly suitable to indicate the extent of the radiating flow, similarly to what is schematically shown in Figure 5.
  • Evaporation Thermal Index (T a -T s )/T a , where T 5 is the temperature of the wet surface, whereas T a is the temperature of the air (expressed in Kelvin degrees). This adimensional parameter establishes a proportionality between the evaporation rate and the thermal gradients, determined with thermographic detections.
  • Figure 11 shows this aspect, where there are compared the trend of the evaporation rate and its estimate, obtained by means of the method set forth, both of them as a function of the average remaining moisture content. More precisely, Figure 11 shows the evaporation rate measured according to the RILEM standard and estimated with the Evaporation Thermal Index times K 0 vs. the moisture content. The measurements have been performed in standard conditions and low air speed on samples of S. Marco brick of different thickness.
  • thermographic measurement is strictly superficial, whereas the weight measurement is of the volumetric type and the concentration of water varies with depth.
  • Figure 12a shows the function K(t) vs. time and the coefficient K 0 for a sample of S. Marco brick having a thickness of 28 mm.
  • T a 20°C
  • Figure 12d shows the experimental values of K 0 for various samples of S. Marco brick displaying different thickness and other materials, as the temperature, the relative humidity and the air speed are set to different values.
  • thermographic detections Therefore, by using K and ETI the evaporation flow may be estimated with good precision on the basis of thermographic detections.
  • Figure 13 shows the estimate of the evaporation flow versus time for two samples of S. Marco brick displaying different thickness (with boundary conditions of
  • the function becomes negative at time t c , i.e. when the critical moisture content is reached, and the peak corresponds to time t m , whereas the derivative is again zero at time t d ;
  • t ⁇ and ft are respectively the initial and final time of the test.
  • the true critical moisture content virtually assignable to a material having no thickness may be simply calculated.
  • the function K may be constructed, as previously described.
  • the function (dETI/dt) identifies the characteristic times of evaporation, emphasising them. These times characterise the Il phase (shown in Figure 9), i.e. the most interesting state from the point of view of the diagnosis of surfaces deteriorated by a moisture excess. This approach spoils a general characteristic of porous materials. Furthermore, for multilayer materials, the function ⁇ dETIIdt) is useful in the evaluation of the concentration of the moisture in depth. Estimation of the desiccation Pit)
  • Figure 7 shows a plot of the specific evaporation vs. time. E(t) proceeds linearly vs. time, until critical time t c is reached. Therefore, in the saturation phase, the following expression applies:
  • E s and M 5 are respectively the evaporation and the mass of the material at the maximum water content.
  • a function which is complementary to evaporation is desiccation or decrease in the specific weight loss, defined as the water mass contained per unity wall surface and as a function of time, as from the following expression: For small enough ⁇ t, the following relations applies:
  • equations (13) and (14) may also be applied to an estimate obtained on the basis of ETI and the function K, as previously shown.
  • Equation (15) is important as it allows to express the specific desiccation proportionally to the average remaining content of water (w) and therefore to invert these relations.
  • This function is shown in Figure 16a and shows a perfect linearity. In this manner, the value of w may be obtained on the basis of the thermographic data, by using the functions calculated up to now. It must be specified here that the effects of the variations of the boundary conditions have been measured by means of the shifts of the interpolating lines, obtained by least squares regression of the experimental data. For instance, Figure 16a shows the variations due to the variation of the relative humidity between 20 and 80%.
  • Figure 16b shows the specific evaporation of samples of different building materials, as a function of the average remaining moisture content.
  • Figure 16c shows the specific evaporation of samples of S. Marco brick having different versus the average remaining moisture content. More precisely, Figure 16b refers to different samples of building material: S. Marco brick (SMB), yellow brick (Ybrk), white tuff (WTuff), yellow tuff (AhTuff), Pietra Serena limestone (PtrS) and marble (Mrb). It should be noted that all of the materials having similar porosity behave in a similar manner. Pietra Serena stone and marble, which have much lower porosity, make an exception.
  • thermographic method for the thermohygrometric monitoring of the surfaces to be examined, two procedures may be provided: a) initially a segmentation of the inspected area on the basis of the critical moisture content value and, then b) quantifying the moisture content.
  • the first goal will be carried out looking at the variation of the evaporative process that is linked to the critical moisture content trough the concept of the Evaporation Thermal Index time derivative.
  • the procedure comprises the recording of a sequence of thermographic images which is synchronised with the stressing of the surface carried out by increasing the air speed with a flow parallel to the surface itself.
  • PCA Principal Component Analysis
  • thermogram about every few seconds (for instance 10-15 s);
  • thermographic camera processing data with the proprietary or appropriate software for the thermographic camera so as to extract the average temperatures on the surfaces of IRpsicro and process them as described in the first part, thus determining the time profiles of T 3 , RH, v a ; 7) carrying out the analysis of the PCA on the sequence, after having subtracted to all the images the average thermogram obtained before the switching on of the vents;
  • thermographic apparatus is positioned so as to frame them as shown in the principal figure.
  • the procedure therefore consists in the following steps:
  • thermographic system is placed and focused on the region to be analysed.
  • the spatial distribution of the surface temperature and the air temperature equal to that detected on surface S 3 , is measured.
  • the relative humidity of the air is determined (by means of equation 1 and the corresponding dew temperature is determined by applying equation 3).
  • the frame 1 shown in the figure 17 allows to easily register visual and IR images and eventually to assemble a unique mosaic image when optical limitation do not let to a complete view of the surface.
  • This embodiment is clearly visible on both IR and visual band, indicating the centre of any target 3. It is easy to calibrate both mosaic images into metric units, because the distance between the optical centres in each IRpsicro target 3 is known.
  • IRpsicro targets 3 are submitted to a forced ventilation of about 5 m s "1 achieved through the supporting pipes, or an air compressed reservoir or by means of a fan, RH is measured there; it is also possible to measure local v a values on the others IRpsicro targets 3, according to the described procedure, spoiling ANN. T 3 is measured on each IRpsicro targets 3.

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Abstract

L'invention concerne un procédé et un système pour la détection thermographique des conditions thermohygrométriques de paramètres environnementaux et de surfaces étendues, telles que des bâtiments, des fresques et similaire, au moyen d'un nouveau dispositif de détection psychrométrique (IRpsicro; AnemolR) et par utilisation concomitante de la thermographie infrarouge.
PCT/EP2007/054713 2006-05-15 2007-05-15 Procédé et appareil pour la détection thermographique des conditions thermohygrométriques de surfaces étendues Ceased WO2007132009A2 (fr)

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ITPD2006A000191 2006-05-15
ITPD20060191 ITPD20060191A1 (it) 2006-05-15 2006-05-15 Metodo di rilevazione termografica delle condizioni termoigrometriche di ampie superfici

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US8539818B2 (en) 2010-02-01 2013-09-24 The Boeing Company Methods and systems for evaporative cooling infrared thermographic leak detection
CN107064213A (zh) * 2017-05-18 2017-08-18 金华职业技术学院 一种测量薄膜导热系数的装置
CN108692832A (zh) * 2018-05-24 2018-10-23 博为远方电气（北京）有限公司 一种直接空冷系统散热器管内流体温度的间接测量方法
WO2019081102A1 (fr) * 2017-10-25 2019-05-02 Deutsches Zentrum für Luft- und Raumfahrt e.V. Procédé de détermination de l'humidité sur les parois d'une pièce et système de détermination de l'humidité sur les parois d'une pièce
CN113237796A (zh) * 2021-05-13 2021-08-10 北京建筑大学 基于蒸发介质润湿度的热湿传递特性测试系统及方法
CN114113204A (zh) * 2021-11-11 2022-03-01 南京大学（苏州）高新技术研究院 一种裂隙土体表面优势流定量测试方法
CN114444309A (zh) * 2022-01-27 2022-05-06 中山大学 一种用于区域平面样品的热史模拟方法及相关装置
DK202430503A1 (en) * 2023-09-01 2025-03-06 Biodry Aps A method for identifying one or more moisture types and their corresponding causes in a damp wall

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Publication number Priority date Publication date Assignee Title
US8539818B2 (en) 2010-02-01 2013-09-24 The Boeing Company Methods and systems for evaporative cooling infrared thermographic leak detection
CN107064213A (zh) * 2017-05-18 2017-08-18 金华职业技术学院 一种测量薄膜导热系数的装置
CN107064213B (zh) * 2017-05-18 2024-05-07 金华职业技术学院 一种测量薄膜导热系数的装置
WO2019081102A1 (fr) * 2017-10-25 2019-05-02 Deutsches Zentrum für Luft- und Raumfahrt e.V. Procédé de détermination de l'humidité sur les parois d'une pièce et système de détermination de l'humidité sur les parois d'une pièce
CN108692832A (zh) * 2018-05-24 2018-10-23 博为远方电气（北京）有限公司 一种直接空冷系统散热器管内流体温度的间接测量方法
CN113237796A (zh) * 2021-05-13 2021-08-10 北京建筑大学 基于蒸发介质润湿度的热湿传递特性测试系统及方法
CN114113204A (zh) * 2021-11-11 2022-03-01 南京大学（苏州）高新技术研究院 一种裂隙土体表面优势流定量测试方法
CN114444309A (zh) * 2022-01-27 2022-05-06 中山大学 一种用于区域平面样品的热史模拟方法及相关装置
DK202430503A1 (en) * 2023-09-01 2025-03-06 Biodry Aps A method for identifying one or more moisture types and their corresponding causes in a damp wall

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