THERMAL IMAGING FOR THE FIRE SERVICE, PART 1: THE BASICS OF THERMAL IMAGING
BY STEVEN P. WOODWORTH
The American fire service has seen numerous changes over the years, especially those that have occurred through the late 1970s and the 1980s. Yet, the one factor that hinders fireground operations in all aspects is visibility. Firefighters must enter a structure with a lack of visibility caused by the smoke created by fire. Not being able to see slows down the primary search, hinders advancing hoselines, makes ventilation more difficult, and hides failing structural supports. The lack of visibility causes firefighters to become disoriented and lost within structures and affects their ability to quickly exit a structure when conditions deteriorate.
During the early `70s, the report America Burning brought forth the fire problems facing our nation. One key item addressed in the report was research and development. Even into the `80s, we did not have the ability to see effectively in a burning structure although we had the technology to put a man on the moon.
That since has changed with the technology of thermal imaging, which gives the firefighter the ability to see through smoke. Thermal imaging is rapidly becoming one of the fastest growing tools in the fire service. Little has been written on thermal imaging for firefighter use; it was mentioned in a few books such as the Fire Officer`s Handbook of Tactics by John Norman (Fire Engineering Books, 1991), a captain in the City of New York (NY) Fire Department (FDNY), and in the National Fire Protection Association`s (NFPA) Principles of Fire Protection Chemistry, second edition, by Raymond Friedman. These works explain what thermal imaging is, how to use it effectively on the fireground and as a tactical tool in other emergency incidents, and the limitations of such systems.
Thermal imaging has been available to the fire service for several years, but it has not seen widespread use within the service. As far as I know, the first fire department to use thermal imaging was FDNY. Now, the popularity of thermal imaging is increasing as technology has made the units smaller and more economically feasible. Presently, several units are being sold on the market, and their popularity is increasing.
Earlier thermal imaging devices were known as “cooled IR detectors.” The units were heavy, large, and very expensive and found very little use in the fire service. The Pyro Electric Vidicon (PEV) tube found limited use in the fire service in a handheld thermal imaging device. Microengineering has provided a third type of thermal imaging device, a focal plane array chip technology, growing in popularity within the fire service. The technology has made it possible to reduce the size of this type of thermal imaging device so that it can be mounted on a helmet, offering the advantage of keeping the firefighter`s hands free while operating.
As previously stated, smoke hinders visibility on the fireground. The most powerful lights will not effectively penetrate smoke.
Smoke is made up of two main components: fire gases/aerosols produced by the fuel`s chemical breakdown, which make smoke toxic, and carbonaceous particles, or soot, which obscure light in smoke-filled environments, thus affecting vision. The degree to which vision will be obscured by the particles is affected by the amount of soot produced by the fire and the size of the soot particles. The amount of soot produced is determined by the type of fuel (products such as methanol burn very clean with little smoke, whereas products such as diesel fuel give off large amounts of dense, black smoke), the heat of combustion, and the amount of oxygen present. A simple example of this can be demonstrated using a common hydrocarbon (acetylene) and a common oxidizer (oxygen). When acetylene is burning without added oxygen (above that of normal atmospheric oxygen–20.8 percent), the soot produced is visible. However, once pure oxygen is added, the soot is no longer visible–the combustion is now complete enough so that less soot is produced. Many of the products (such as plastics) used in the interior of structures today tend to give off great amounts of dense, black smoke. Modern structures are much more insulated than they were 20 years ago, thus holding smoke in long and compounding the problems for today`s firefighter.
When light is shone into a dark room, the light waves travel until they strike an object they cannot pass–a wall, for example. This same principle applies when light is shone into a smoke-filled environment; the particles in the smoke act as the wall. The light striking the particles cannot penetrate or pass, so the light wave “bounces” off. Fire scientists tell us that the particles vary in size from 0.1 to one micrometer. This is very close to the wavelength of visible light that would be shone from a flashlight and explains why light smoke conditions may allow some light to pass through, leaving vision blurry. Heavy smoke conditions will cause all of the light to be scattered or blocked. The light waves simply cannot penetrate the particles in the smoke. This is the reason there has never been a truly effective light for firefighting. These conditions lead to the stories we all have heard or told about zero visibility.
HEAT AND HEAT TRANSFER
While light waves do not penetrate smoke, heat does. We can attest to this fact because we can feel it. If a firefighter could see this heat in a visual picture, he/she could “see through the smoke.” This is where thermal imaging comes in. To understand how we can “see through smoke,” we must understand heat and heat transfer, since it is actually heat that we will be “seeing.”
Heat is a form of energy. It is present in all chemical reactions, including fire. The unique thing about heat, which can now be helpful to the firefighter, is that it does not stay in one place. According to the laws of heat flow, we know that heat travels from warm to cold objects and that it moves in a fire building by conduction, convection, and radiation. The movement of this heat is what allows the fire to spread. We also know that not all objects absorb or give off heat at the same rate. This may be due to their chemical makeup, the size of the object, or a combination of both.
Thermal imaging is the translation of that heat energy into a visual image. Since not all objects have the same temperature (a measure of their heat energy) and thermal imaging is a visual image of a material`s heat energy, it is conceivable that we can form a picture of an environment simply by looking at the differences in temperature in a given area. This is what thermal imaging does and how it can benefit the firefighter. When a room is on fire, the fire is giving off great amounts of heat compared with the ambient temperature of other objects in the room. This difference in temperatures is what forms the picture. As the fire grows, heat and fire gases rise to the ceiling and mushroom out. Some of that heat radiates to other objects in the room. Previously, we stated that not all objects will absorb heat energy at the same rate. Therefore, even as the fire grows, the differences in temperature are maintained. The thermal imaging device would show differences in the temperature, giving the firefighter a visual image. This would remain true until the room reaches flashover, at which time the thermal imaging device would show great amounts of heat for the entire room. The ability to more accurately predict flashover is one of the major advantages of the thermal imaging device.
Until now, the fire service has relied on lights (which had little effect) and ventilation for visibility inside burning structures. Now, we have thermal imaging. All too often, the fire service looks for a new tool or technique to solve all of its problems. Thermal imaging is not a tool designed to replace good fireground tactics such as ventilation and search lines or to reduce staffing levels. Firefighters using thermal imaging devices should still carry hand lights, use search lines, and follow safe operating practices. Thermal imaging is a tool designed to increase the effectiveness of our present operational methods. n
n 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).