FAQ’s

Detection of water using thermal imaging

 

Thermal imaging does not detect water per se. It has been seen that energy interaction on the surface of a material is complex. Furthermore damp surfaces have an even more complex relationship with energy transfer due to the increased variable such as time, spatial factors and air movement.

There are two areas that thermal imaging can detect the moisture. Firstly is the detection of cooling by evaporation (D Jenkins, n.d.). As the thermal imaging camera has less than 0.050 C sensitivity (FLIR, 2010), the evaporative effects of the water can theoretically be seen. This effect can either work with the thermal capacity of water, or against it.

The second area is, as mentioned, the thermal capacity of water. It is so much higher that the dry material that it leads to a temperature differential. This effect is a function of the energy flux over time and the spatial distribution of the water over the dry substrate (E Grinzato, 1999).

The porosity of the substrate allows water to enter the pores which increases its weight as well as creating a greater “thermal inertia” as the water increases the thermal capacity of the material. (E Grinzato, 1999).

The camera will pick up the different heat signatures and is capable of effectively mapping out the moisture. Some texts, mainly based in the Mediterranean countries imply that the mapping can lead to a feel of how deep the moisture has penetrated the substrate. This in turn will give an indication as to the physical properties the material. (N. Avdelidis, 2003).This form of detection mapping, the qualitative approach, is a useful tool and has arguably a greater breadth of use than the more specialised quantitative approach. This is a specialist area that involves ‘high end physics’ (E. Grinzato, 1998) (P. Bison, 1994) and many additional measuring instruments as there are a great deal of variables such as air movement, temperature, porosity and humidity.

The qualitative mapping surveys enable the surveyor to theoretically “see” areas of moisture that is invisible to the naked eye. However most damage from moisture that is seen initially visually and can then be remedied without the need for IR. Therefore a potential cost by having the survey has been removed. However it is arguable that the damaged area could be rectified sooner and at less cost if an IR survey was conducted initially (Lyberg, 1990). Lyberg suggests that a survey would take 1- 2 hours and this is supported by the author. However a skilled operator is required especially when identifying moisture and therefore is a barrier for the wide uptake of thermal surveys.

 

 

Emissivity

Radiation acts on an object (or body) in a number of ways.  This creates a problem for gaining information on the ‘real’ heat radiated from a body. There are four methods of radiation that affects bodies-

  • Emission -is the radiation that is given off by the body
  • Absorption- is radiation that is taken in by the body
  • Reflection- is reflected radiation from another source
  • Transmission- is radiation that has passed through the body

From this it is important to know where the radiation comes from in order to identify the relevant source of radiation.

The incident radiation is the total amount of radiation that is radiated into a body from a source. From that total some travels through the body (transmission), some is reflected and some absorbed. Figure 5 shows how the incident radiation is broken down.

 

Figure 5 – Incident radiation components (Infrared Training Center, 2010)

Wα =absorbed radiation

Wρ = reflected radiation

W τ =transmitted radiation

 

It can be equally described by the equation-

Wα +Wρ+ Wτ = W incedent = 100% or 1

 

Exitant radiation is the radiation that leaves the body irrespective of the original source. This radiation is split up into reflected (Wρ), transmitted (W τ) and most importantly emitted radiation (Wε). This last component is the body’s capacity to emit its own radiation. It is the component that is used by the thermographer. Figure 6 illustrates how the exitant components are formed.

 

Figure 6– The components of exitant radiation (Infrared Training Center, 2010)

Wε = emitted radiation                                   {These Three radiation

Wρ =reflected radiation                                 elements

Wτ= transmitted radiation                            make up Exitant Radiation}

 

Tε= Heat that could be emitted

Again this can be broken into equation form:

Wε +Wρ+ Wτ = W EXIT = 100% or 1

Or

ε+ ρ+ τ =1

Stepping back  a moment; theoretically one describes a body that has no reflective or transmitted radiation as black body and this only produces emitted radiation. Black bodies are used to calibrate thermal imaging equipment as it cancels the two unknown radiations.

In reality transmitted radiation can be negated as most objects are considered “opaque” so in effect τ = 0. Therefore the only problem is splitting the emitted radiation from that of the reflected radiation (or ε+ ρ =1). Emissivity values for bodies are calculated and made in table form for thermographers. These values show a percentage of the body that produces radiation. For example common red brick has an emissivity value of 0.92. This shows that the reflectivity in the infrared range of the brick is 0.08 or 8%. Thermographers can now deduct this to gain the emitted radiation from the brick and hence the temperature.

Here it must be noted that the emissivity of the body is influenced by other factors, mainly the surface.  If there is a wall that has two different paints, their emissivity values would be different although the thermal mass is the same. Therefore the emissivity is a directly affected by wavelength. There is a great deal of work done on wavelength and emissivity (Avdelidis and Moropoulou, 2003) in order to help reduce the errors in thermal imaging. There are standard values that are used internationally in order to rationalise the subject these such as ASTM E1933-97 created for the US market (E133-97, 1997), have helped the thermographers negate one of the variables that have to be contended with.

Furthermore, it has been found that the emittance of radiation is different between the wet and dry surfaces in building materials (Moropoulou et al., 2002). This will make thermal imagery a potentially effective means of mapping moisture.

 

Heat theory

 

In order to understand the capabilities of thermal imaging, a knowledge of basic physics is required.

There are four major forms of heat transfer-

  • Conduction,
  • Convection,
  • Latent Heat (Evaporation/Condensation)
  • Radiation

 

Of these, generally standard thermal imaging in buildings is mainly concerned with conduction and to a lesser extent, convection.  However all of the energy transfer mechanisms are encountered in various ways. In this paper, which concentrates on moisture, latent heat is an important element.

Conduction, where the proximity of atoms or molecules are said to “touch” each other, pass on the heat from the hotter one to the cooler. This is the only method of heat transfer that is found in a solid.

The heat transfer rate can be verbally expressed as-

The rate of heat flow under steady state conditions is directly proportional to the thermal conductivity of the object through which the heat flows, and the temperature difference between the two ends of the object. It is inversely proportional to the length or thickness of the object. (Infrared Training Center, 2010).

From this the mathematical formula can be constructed as-

Q/t = kA (T1-T2)

L

 

In this case Q is energy (J) and t is time (s). The conductivity of the material, k is measured in the expression (W/m*K). The cross sectional area A is in m2 with the temperature difference T1-T2 in (K). Finally the length of the conductive path is L (m). (Infrared Training Center, 2010)

The conductivity of water is considered to be 0.6W/m*K compared to brick which is 1. (Infrared Training Center, 2010). This has a bearing on how thermal imagery can identify moisture. However most important is thermal capacity, which is the ability of a substance to hold stored energy.

The thermal capacity (measured in kJ/kg*K) of water is extremely high, as seen in table 1, when compared to other substances. This is the main reason for thermal imaging being able to detect moisture. As water heats up within the bounds of atmospheric conditions, it becomes higher.

 

Material Specific Heat Capacity  (kJ/kgK)
Water @ 40 C 4
Water @ 200 C 4.183
Limestone 0.84
Sandstone 0.92
Brick 0.9

Table 1 – Specific heat capacities (Engineeringtoolbox.com, n.d.)

The higher the specific heat capacity, the harder it is for the substance to gain energy as well as to lose energy. Therefore it can be seen that water will require a far greater amount of energy to heat up to the same temperature as brick, for example. Therefore for this investigation, water will heat up and cool down more slowly than the dry brick, stone or other substrate. This is how thermal imaging identifies moisture. This can be seen in figure 1 where the silos containing liquid show the level of the liquid inside due to the differing heat capacities of the liquid interior and the metallic exterior.

 

Figure 1 – Thermal image of Silos containing a liquid

A reasonable definition of convection is-

….. a heat transfer mode where a fluid is brought into motion, by either gravity or another force, thereby heat ( is transferred) from one place to another. (Infrared Training Center, 2010)

For convection to take place it must be within liquid or gas. The movement is caused by the differing densities of the molecules or atoms. The higher the temperature the less dense the fluid as the molecules are moving faster and are further away from each other (and taking up more volume). Gravity will have greater effect on the slower/ cooler bodies causing them to sink thus creating circulation. Thermal imagery can look at the patterns on solids that have been created by the convection currents via the boundary layer where the heat transfer is through conduction. Figure 2 shows convective warm air at the top of the wind falling down the window as it cools by conduction. By the time it reached the bottom of the window we can see via the boundary layer conduction, how the now cooler air falls, illustrated by the darker swirling areas below the windowsill.

 

Figure 2 – Thermal image of convection currents of cold air near a window

Latent heat is the heat transfer associated with evaporation and condensation. We all understand that evaporation cools down the body as it extracts heat. An example is sweating. We can see, once a kettle is boiling all further energy from the heat source increases evaporation and therefore the water cannot heat the water any further due to the increased cooing effect of the evaporation.  The opposite is true with condensation.

Figure 3 shows a flat roof with a patch of wet. The evaporation is cooling the surface of the roof giving a lower temperature. It can be noted that sometimes the area that has evaporation might be hotter than the dry contiguous area but the evaporative effect is greater than the underlying heat and therefore will be seen in the thermograph as cooler.

Figure 3 – Thermal image of evaporating water on a roof

Finally, radiation-

Heat transfer by emission and absorption of thermal radiation is called radiation heat transfer. (Infrared Training Center, 2010)

 

This is a factor that needs to be taken into account. However the main affect for thermography is incidental reflective radiation that skews the temperature of a body within the picture taken. This is the emissivity effect and will be mention later.

The electromagnetic spectrum  encompasses all of the radiation wavelengths between the shortest waves of gamma radiation at around 10-14 m to the longest radio waves at around 107 m with the infrared sector or the spectrum roughly in the centre and will be explained in more detail later on page 8.

Electromagnetic Spectrum (Ibarra-Castanedo, 2007)

Thermal radiation does not just occur in the infrared but across the spectrum. This radiation transfers heat by emission and absorption. The intensity is the important element so that in our surrounding temperatures, the most intense radiation is in the infrared range and the hotter it gets, the intensity moves to shorter wavelengths. An example of this is glowing hot metal has the greatest radiation in the shorter wavelength that become visible. Cooler objects have the radiation intensity further into the long wavelengths. The intensity of the radiation of different wavelengths has differing effects on us, for example we feel the heat from microwaves (in this case it is the energy exciting water molecules that create the heat) or X rays that we cannot feel initially but will cause illness over time.

Thermal infrared radiation wavelengths are longer than that of visible light in the electromagnetic spectrum. Visible light is considered to be between 0.4 and 0.7µm length and at this point it needs to be noted that the wavelength bands are not sharply defined. Thermal infrared wavelengths lie between 1-14µm. This part of the spectrum can be further broken up into near, mid and far infrared.

Within the infrared spectrum that the thermal camera covered there is a “blind spot” due to the effect of the atmosphere. The atmosphere attenuates the radiation between about 5 and 8 µm. Therefore there is no radiation to pick up. THe graph below shows the actual attenuation of radiation at 1km in the infrared band. This occurs between the mid wave infrared (MWIR) and the long wave infrared (LWIR) wavelengths. The result is that this attenuated area cannot be used in infrared imaging technology.

Transmission of air in the infrared part of the electromagnetic spectrum (University of Virginia, 2011)

Water on building materials

 

Arguably the main failure of buildings is the damage caused by water and its interactions on the building and local environment. Whether it is rain runoff or moisture in the air, some consider about 75% of buildings failures are due to water (May, 2005). Ingress of water can occur in either vapour or liquid form but only leave the substrate as a vapour. When in liquid form, the water can not only enter by direct means such as rain or direct water contact but also by capillary action (Karoglou et al., 2005). In vapour form molecular transport, solution diffusion and convection are the methods of water ingress (Kunzel, 1995).Therefore it is important to understand the effects water has on the materials making up the building. Where most of the effects occur is at a microscopic level at the boundary of the water and the substrate. To a lesser extent there are effects at a macro level but these are associated with weather extremes and are less common in the UK.

Generally building materials are hygroscopic in that they take up water and subsequently maintain a dynamic equilibrium of water content by absorbing water from the environment or desorb it. According to Hill et al, this behaviour leads to the expansion and contraction of the material which in turn leads to damage through cracking (Hill and Rizvi, 1982).

Condensation is the chief architect of water damage and creates the most problems both internally and externally. However this is mainly a recent phenomenon as it occurs more commonly in newer structures as the humid warm air is trapped in the building and fabric since new building envelopes are much tighter in order to preserve against heat loss (Thompson, 2000).

Stone has been the mainstay of building construction for centuries. The reason for this is because of the material’s physical robustness and its relative abundance. Brick, coming later proved to be even more useful due to the ease of manufacture and logistical benefits, as well as its regular shape and size.

However like all building materials brick or stone is susceptible to weathering albeit at a relatively slow rate due to its durability. There are three types of damage that weathering can cause and all of these are associated with water. They are :

  • Physical
  • Chemical
  • Biological

The two main areas of physical damage of old buildings in the UK is both the built up of crystals and freezing within the pores of the material. Crystals formed from solution caused by the ingress of water dissolving mineral solutes. With evaporation the crystals are formed and as they grow they increase the pressure of the surrounding substrate to potential destruction (E. Winkler, 1972). Correns in 1949, was able to start the base line for crystalline work by postulating that the ability of a crystal to exert pressure in a restricted environment was based on the function of the super saturation ratio (Correns, 1945). More up to date work has been carried out on detecting water deposits in stone substrate to identify potential crystallisation areas. Standard methodology includes capillary rise tests to identify absorption percentages in specimen rocks under laboratory conditions. However this is fairly restrictive as it does not provide the picture of potential crystallisation in situ. Thermal imaging has the potential of mapping out areas of risk of the build-up of crystals (Avdelides, Moropoulou and Theoulakis, 2003).

The pH of the water can be changed by pollutants and this can also change the rates of crystallisation, dissolution and recrystallisation cycles of the soluble salts (Jones and Wakefield, 1999). Furthermore added to these temporal changes, the structure of the crystals themselves are changed which lead to an additional factor in the understanding of building damage due to crystallisation.

Stone with a larger pore size will attract more water and this, dependent on the shape of pores, will hold more water and for longer and will be less susceptible to temperature change. Additionally, not only is the pore size important but also the roughness of the material as condensation will form more readily on the rough material due to increased surfaced area. The relationship between the two is complex (Camuffo, 1995). Camuffo states that pores with a radius of less than a µm, the physical effects of water dominate whereas with the larger pores, the chemistry of the water will also influence the adsorption of the water onto the stone.

Pore size and shape is important. Water condenses first on the inner walls of any pores and stays there for the longest time as it is subject to less evaporative effects as seen below. The relative humidity required for condensation is less for a curved surface( I.e. less than 100% of the required saturation in the atmosphere) (Camuffo, 1995).

Illustration of internal and external pores in stone showing how water is affected by even the evaporative effects of wind and sun. Adapted from Camuffo (Camuffo, 1995)

 

Therefore it is arguable that the temperature differences for thermal mapping, will be greater and hence easier to see. Furthermore it is suggested, the large pore stone, is more susceptible to salt crystal decay (Theoulakis and Moropoulou, 1997).

Similarly, when the water freezes the expanding ice increases the pressure and ultimately crack up the stone or brick. The water can expand up to 9% (Lstiburek, 2010) causing substantial damage especially in tight fitting walls. It will push non frozen water through hydrostatic pressure and this liquid water freezes deeper in the material. Not only this, but the differing freezing temperatures will cause different expansion rates.

When drying out again the porosity is the important factor associated with the material (Karoglou et al., 2005). Using kinetic drying mathematical models and adding the porosity values, a forecast of the drying of the material can be achieved.

It has been seen that the physical presence of solutions can damage the substrate due to the presents of crystals. Add to this that the solutions themselves will cause chemical changes as the  solutes move into solution they are then able to react with the material which will cause changes to the structure of the stone of brick. This last point assumes that the solutes are internal and are leaching out. However pollutants in the atmosphere will be absorbed by the atmospheric moisture and precipitate onto the material causing potential damage. An example of this is the acid rain of the latter part of the 20th century. This was caused by sulphur dioxide and the nitrous oxides that were produced by the burning of fossil fuels.

Moisture and damp are also vectors for the transportation of biological material. The longer the damp is present, the more biological life will develop. The water and the chemical reactions favour biological life. It is considered that Time of Wetness (TOW) is related in some way to the damage of the stone (Camuffo, 1995) In most cases often fungal spores are the initial source of future damage. The damage can be divided into two. First is the potential damage to health as the damp walls will produce fungi and their spores in huge amounts creating health issues. Second is the physical damage to the substrate by the funguses themselves. One example is Serpula lacrymans. that can spread throughout the masonry in dark damp environments.

Dry rot on a wall (image from diydoctor.org.uk)

 

Finally, there has be a great deal of work on modelling how various building materials act under both temperature and moisture and what the materials’ properties are. Recently databases have been constructed to allow relevant information to be added to various hygrothermal models. These models are now available on basic computers (Kumaran, 2006).

 

The physics of state change

 

In water like all liquids, when moving from one state to another, the energy levels change in order to jump from phase to another. In the case for this paper the change from water as a liquid to a gas is the most pertinent. The molecules within the liquid water have an average kinetic energy and this will dictate the temperature of the fluid.  Those molecules that have a high kinetic value have the potential of escaping the liquid and therefore escape as a vapour. This reduces the average kinetic value of the remaining molecules and hence reducing the temperature.

The effects of evaporation on a wall can be considerable due to the high specific heat capacity of water. In order to achieve the break of the molecular bonding by vibration, for every gram of water evaporated, 2500J is extracted (E Grinzato, 1999) (Nave, 2005)- which is the heat of vaporisation for water.  This has a considerable effect on the substrate. Fick’s Law shows the flux of evaporating water (Ф). The law states that the movement of energy from a high concentration to an area of low concentration is proportional to the function of diffusion coefficient and distance in a single dimension

Ф = -D ΔC

Δz

(Ljungberg, 1994)

D is the diffusion coefficient

C is the Molar concentration of water

z  is the area that the migration takes place

 

This evaporative effect with its high heat of vapourisation of water is a major driver for the identification of moisture in buildings.