Mitigating Inherent Effects

There are several factors that affect the production and subsequent proper interpretation of a thermal image. These factors include the target's emissivity, reflectance, distance from the imager, temperature, background temperature, ambient temperature, orientation, target size, and the transmittance of the intervening atmosphere. In addition, the image, as presented on the imager, is not temperature but radiosity. Imagers measure the radiant energy emitted by the target plus the radiant energy reflected from and transmitted through the target. The sum of these radiant energies is the commonly accepted definition of radiosity.

There are practical considerations that will simplify the following discussions of the inherent effects. In general, the transmittance (energy transmitted through the targets) can be ignored in most, if not all, cases for targets in a power plant. Transmittance is an important factor in industries where the temperature of a thin film of plastic or other infrared opaque targets are being observed. Also, with the exception of absolute temperature measurements being required, the transmittance through the atmosphere can be ignored as well. The major exception would be in cases where long distances were involved in a humid atmosphere (that is, hydrogen igniters or spray nozzles in containment).

4.1.1 Emissivity and Reflectivity

A review of the references in the bibliography (Appendix E) will show that no one subject is discussed more than emissivity. The effective emissivity of a target clearly must be known in order to measure its absolute temperature. This is discussed in detail in Appendix A. Table 4-1 provides some values of emissivity for common objects. Aluminum, the most commonly used

background will reflect their energy off these surfaces. These mirrors do have surface thermal patterns. It is difficult to measure them, however, because of the low emitted energy and the natural ability to reflect thermal energy as well as light. In general, if a target is acting as a visible mirror, it is acting as an infrared mirror as well. An exception to this rule is the germanium lenses used on the thermal imager. These lenses transmit more than 90% of the energy in the infrared spectrum but have light-reflecting coatings that reflect more than 90% of the energy in the visible spectrum.

Table 4-1

Normal Emissivity Values of Common Materials

Material Emissivity Aluminum

Highly polished plate 98.3% pure 0.039

Polished plate 0.040

Rough plate 0.55

Chromium 0.080 Copper

Commercial, emeried, polished, with no pits remaining 0.030 Commercial, scraped, shiny but not mirror-like 0.072

Polished 0.023

Iron and steel

Cast iron, polished 0.21 Wrought iron, highly polished 0.28 Cast iron, newly turned 0.435 Oxidized surfaces

Iron plate, pickled then rusted red 0.612

Completely rusted 0.685

Rolled sheet steel 0.657 Steel oxidized at 110° 0.79 Cast plate, smooth 0.80 Cast plate, rough 0.82

Given that shiny objects have surface thermal patterns that are hard to image, there are several techniques that improve the ability to establish a satisfactory image. The most common way to obtain a useful thermal image from a shiny or low-emissivity surface is to add a coating to it that has a higher emissivity. (This is not practical and is not recommended for an energized electrical surface.) There are three common non-permanent materials that have been used to improve emissivity. These are:

• Foot powder

• Dye check developer

• Electricians’ tape 4.1.2 Foot Powder

Foot powder is sprayed on a target to create a uniform layer that reduces the reflections. After the powder has reached thermal equilibrium with the surface, the temperature measurements can be made. The emissivity of foot powder has been estimated to be 0.96. Before any coating is applied, however, the chemical composition of the coating should be determined to avoid any negative effects from its application.

4.1.3 Dye Check Developer

(Caution: Ensure that all manufacturer precautions are followed prior to use of any developer.

For example, Magnaflux Zyglo^® developers, such as ZP-9E and ZP-9F, might produce chlorine gas or become flammable when they come in contact with moderately heated surfaces.)

An alternative to foot powder is liquid dye penetrant developer. It has an estimated emissivity of 0.97 and might already have been formulated to conform to QA requirements for sulfur and halogen purity. Application of it is identical to the foot powder. Given the temperature of the target, it might take several minutes for the developer to reach thermal equilibrium as its propellant cools the target's surface. The best way to use this in an actual survey would be to apply it to all targets to be surveyed before commencing the actual survey. This will ensure that all target surfaces will have reached thermal equilibrium.

The target in Figures 4-1 and 4-2 is a shiny metal can of dye check developer with the label removed. There are no hot objects in or near the can. With the imager's emissivity set at 1.0, an analysis of the temperature distribution over the can yielded a temperature range of 74.6°F to 67.9°F (6.7°F ∆T). The reason for the variation is reflection of the cold window plus geometric considerations in measuring a curved surface. Setting the emissivity at 0.10, a more realistic figure for a shiny surface, yielded a maximum temperature of 66.2°F and a minimum

temperature of 23.9°F (42.3°F ∆T). The room ambient temperature was 68°F. Without changing anything, the can surface was coated with developer and allowed to achieve thermal equilibrium.

The first effect noted was the observation of the level of the developer in the can (Figure 4-3).

visible on the thermogram. The emissivity was reset to 0.97 and the can was allowed to achieve thermal equilibrium with the room. After 10 minutes, the maximum temperature observed was 70.4°F and the minimum temperature was 69.5°F (0.9°F ∆T), close to room ambient of 68°F.

(The 0.9°F temperature spread is normal because the dye check developer might not have uniformly coated the surface.) Clearly, the developer served its intended function of improving the surface emissivity and, therefore, the results.

Figure 4-1

Emissivity Improvement by Coating—Setup

Figure 4-2

Thermogram of an Uncoated Shiny Metal Container

Figure 4-3

Container Has Been Coated to Improve Emissivity—Thermogram Now Reveals Fluid Level

4.1.4 Electricians' Tape

Another alternative that improves the surface emissivity is the use of electricians' tape (it has an estimated emissivity of 0.95). This method is easy to use and apply but can present problems if the glue on the tape contains chlorine or other chemicals that can attack the target surface.