THERMAL IMAGING FOR THE FIRE SERVICE, PART 4: THERMAL IMAGING DEVICES
BY STEVEN P. WOODWORTH
Thermal imaging devices are similar to all other tools and equipment in the fire service in that personnel who use them must be trained frequently and effectively if the devices are to be used safely and properly.
Interpreting the image given by a thermal imaging device is comparable to interpreting the reading of a combustible gas indicator (CGI). Operating a CGI entails more than just placing the probe into a suspected flammable atmosphere, testing the atmosphere, and looking at the movement of a needle or a numerical value that appears. When the CGI gives a reading of 10, it cannot automatically be assumed that the atmosphere is 10 percent of the lower explosive limit (LEL). We know that the CGI is calibrated to a specific gas and that any readings taken for a gas other than the calibration gas must be interpreted by the operator to get the relative response. If the firefighter makes a tactical decision based on the reading without interpreting the value given, the best result would be an educated guess.
The same basic principle applies to thermal imaging devices. The device gives a visual representation of infrared energy. The firefighter operating the unit must be able to interpret the image. The difference is that the CGI reading could be interpreted through a chart or simple mathematics. The firefighter must be able to recognize the image in the view of a thermal imaging device quickly and accurately. Some images are self explanatory. A person, for example, is fairly easy to recognize once the firefighter understands how to interpret the image. Other images are not as easy to distinguish–flashover, for example. Flashovers happen rapidly, and the firefighter must be able to distinguish this phenomenon quickly. Identifying flashover images accurately and quickly makes it possible to use this information for making tactical decisions on the fireground.
THERMAL CONTRAST
The first part of the image that must be understood is the concept of thermal contrast–the difference in the temperatures of the objects being viewed. Thermal contrast makes it possible for the thermal imaging device to present an image. If there were no thermal contrast, the image would be all black or all white, which would be useless. As stated in Part 1 of this series, heat flows from objects with heat to objects without heat, and objects take in or give off heat at different rates.1 This means that all objects have different amounts of heat–hence, the thermal contrast. The thermal imaging device shows this contrast through a difference in color. Black indicates the absence (or lack of) heat, and white indicates the presence of heat. The more heat an object has, the whiter that object will appear in the image; conversely, the colder an object is, the darker it will appear in the image. The more heat that is present in an area, the more distinct the image will be. For this reason, departments that are evaluating thermal imaging devices for purchase should test them in a heated environment. An environment that has an ambient temperature (approximately 707F) would appear as a flat or nondistinct image. Thermal imaging devices (like all other equipment used in the fire service) should be evaluated under conditions similar to those under which they will be used. An environment that has ambient temperature would have little thermal contrast compared with an environment of a structure fire. A thermal imaging device would need to have a low minimum resolvable temperature difference (MRTD)2 to show the thermal contrast of objects in the room.
THERMAL INVERSION
Thermal inversion occurs when images change in color due to changes in ambient temperature. A firefighter using a thermal imaging device on the outside of a structure would see other firefighters as white, or hotter than their surroundings. On entering the structure, the image would invert. The other firefighters might now appear dark, indicating a temperature change. It is important to understand that the temperature of the surrounding atmosphere–not the individual– changed. Thermal inversion does not affect the image except to change its color. Thermal inversion can take place as a result of moving from room to room within a structure or changing the sensor`s position so that it points to the ceiling areas instead of the floor area within the fire room. Thermal inversion poses no problem for the firefighter operating the unit as long as he has been trained for it and is aware of how it may affect the image.
FIELD OF VIEW
The field of view (FoV) is the area of the thermal imaging device through which the firefighter looks. The size of this area varies with the brand of device. Although it is true that thermal imaging can give the firefighter vision on the fireground, it must be understood that that vision is really tunnel vision. The firefighter must be trained to overcome this tunnel vision. If the firefighter operating a thermal imaging device does not scan around the entire room, he could miss a victim.
DISTANCE
Distance also affects the image. The farther away an object is, the more difficult it is to make out the image. An object that is too close also will be indistinct. One popular thermal imaging device has an effective range of up to 300 feet. This can be very beneficial. I once used this type of thermal imaging device to search a confined space. The unit and a firefighter were lowered into a storm drain to search for a child reported to be lost. The distance between manhole covers was 300 feet. By lowering the thermal imaging device into the storm drain at each manhole, we were able to search the distance from manhole to manhole, eliminating the need for personnel to crawl through the pipe. The search also provided for overlap, since the pipe could be viewed from both ends. This placed the area to be searched well within the limits of the thermal imaging device.
ADDITIONAL FEATURES
Thermal imaging devices should also provide warning icons in the image. Should the battery run low, for example, the warning would let the user know that the remaining operating time is limited. Users should always carry a spare battery. Another warning icon would appear if the unit should start to overheat. The high-temperature warning would let the user know that the system could fail due to excessive heat. Often this heat is confused with the heat of the surrounding environment. Although the heat from the fire contributes to excessive heat within the unit, the vast majority of the heat is generated by electrical equipment. I personally have seen the high temperature warning activate only once. That particular unit was being used in training and had been in operation for more than two hours. Changing the battery was all that was needed for the unit to cool down to a safe operating level. During actual emergency operations, it is a good rule of thumb to change the battery each time the operator changes his SCBA cylinder.
Technology can make the firefighter`s job safer and faster. The fire service should take advantage of technology. Thermal imaging is only one way to do that. The fire service must keep in mind, though, that no matter how technological the fire service becomes, one fact remains the same: You have to be properly trained to operate the equipment. n
References
1. “The Basics of Thermal Imaging,” Fire Engin-eering, July 1996, 22,26.
2. “Thermal Imaging for the Fire Service, Part 2: The Electromagnetic Spectrum,” Fire Engineering, August 1996, 24,26.
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The thermal contrast between the firefighter and the SCBA can be seen distinctly. The firefighter`s body builds heat as he works, whereas the cylinder cools as air is released from it. (Photos courtesy of CairnsIRISw.)
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As a firefighter moves in front of a direct heat source, thermal inversion occurs. The firefighter`s image becomes darker than the heat source behind him. Note the varying shades between white and black, indicating a low MRTD of the unit.
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What distinctions can be made from this image?
STEVEN P. WOODWORTH is a firefighter with the City of Atlanta, Georgia, assigned to Squad 4. He is an adjunct instructor at the Georgia Fire Academy and a volunteer firefighter for Fayette County. He is co-author of Fighting Fires with Foam (Van Nostrand Reinhold: New York City, 1992).
