What are conduction, convection, and radiation; what do they have to do with the fire service; and how do they apply to thermal imaging?
Britanniea defines the following:
- Convection is a process by which heat is transferred by movement of a heated fluid such as air or water.
- Conduction is the movement of heat or electricity through something such as metal or water.
- Radiation is the flow of atomic and subatomic particles and of waves such as those characterized by heat rays, light rays, and X-rays.
So, what do these three things have to do with the fire service? That is simple: Fire is created through a process called the fire tetrahedron, which consists of heat, fuel, oxygen, and a chemical reaction to create fire.
Materials inside a structure that have ignited cause conduction, which transfers that heat via metal piping and other metal materials through the walls and flooring to other areas. During that same fire process, convection is occurring from the point of fire origin whereas the heated air currents at the ceiling level are moving through a path of least resistance across the room and down the hallway. Radiation occurs at the same time as materials are burning, which is basically heating up other materials that can ignite.
How does this apply to thermal imaging? Your thermal imager (TI) is designed to detect a pretty simple process called infrared radiation or heat. Now think about this for a minute: You have this device, which is human-made, battery-operated, and an electronic piece of equipment that detects heat; therefore, this device must be smart! Sorry to disappoint you, but a TI is just a tool, nothing else. A TI simply receives heat through the front lens and displays an image to the rear display screen.
The smart part comes from the firefighter using the TI who understands the image interpretation being displayed and can interpret what it is he is looking at. TIs have improved vastly over the years, with image enhancements and seamless gain transitions, therefore not missing any details.
WHAT ARE WE LOOKING FOR?
Those superheated convection currents that you detect in front of you and over the top of your head as you are advancing down the hallway would be a pretty good indication that you are going in the right direction toward the fire.
As gases are heated, their primary method of cooling themselves is through contact with other gas molecules. As the heat continues to build, this collision frequency increases; however, if the heat builds at a rate at which collision frequency alone cannot dissipate it, the gas molecules begin to emit heat in rays. In other words, they radiate. It is not until a gas radiates that it can be observed by the TI. At the point that the gas radiates, it can be considered superheated.
Through the next process of that same fire, radiation is occurring, and materials, such as your personal protective equipment (PPE) ensemble, will absorb that radiated heat. Do you know what your PPE is rated for in terms of temperatures and durations? The National Fire Protection Association (NFPA) stipulates that we must replace our PPE every 10 years. Based on the size of a fire department and call volume, some fire departments’ PPE may have to be replaced more frequently because of excessive wear and tear.
As firefighters, we are wearing PPE that, if it is brand new out of the box and never laundered, will give you 17.5 seconds of protection in a flashover but unfortunately will begin to break down between 600°F and 900°F (315°C and 482°C).
The unfortunate part is that the victims trapped inside that structure fire are not wearing firefighting PPE, and their pajamas are not made from fire-retardant Nomex and are taking the full brunt of the heat. At 111°F (43°C), human skin feels hot; at 118°F (47°C), human skin can receive first-degree burns; at 131°F (55°C), human skin receives second-degree burns; and at 162°F (72°C), third-degree burns occur to human skin.
So, now we have conduction to talk about. Here is a scenario for you to think about regarding bedrooms in winter:
You arrive at a house fire in the middle of the night and are assigned to victim search. You arrive at a bedroom and scan the room with the TI. Although the smoke is thick, the image on the TI is crisp and clear. You can see everything in that room down to the hair clips lying on the carpet. What you don’t see is a victim. You move to the next room, thinking there might be a victim there. What did you miss? We use blankets to stay warm. Why? We use them because they are excellent at trapping and retaining the heat from our bodies. The more efficient they are at retaining our body heat, the warmerthey feel, but that retention efficiency is because of excellent insulative qualities. Excellent insulative qualities come from not allowing the body heat to conduct to the outside of the blanket where that heat would be lost to radiation to the atmosphere.
Now there are many variables to this equation like thicker blankets because of colder temperatures or lighter blankets because of warmer temperatures creating different insulating factors. The body mass of the victims also is a consideration— whether they give more or less of a detectable heat signature.
Although your TI can see every detail of the comforter on the bed, it may not see the victim under the comforter. In the moment, when you are moving quickly, the environment is hostile, and there are potential victims to save, your brain has a hard time realizing what you are not seeing. It only interprets what it sees. Mistakes happen; therefore, a general rule when searching in bedroom areas is that we must resort back to our conventional search methods by hand.
I am pretty good at what I do as an instructor, and I can make you disappear. Yes, and I mean like Arnold Schwarzenegger hiding from the Predator. How did he do it? He altered his heat signature. Same idea as being under the bed covers.
I cannot say this enough: practice, practice, practice. You should practice viewing the world through the TI so your brain gets better at recognizing how common objects look from a heat-based perspective. However, you should also practice realizing how things should not look. You, the smart TI operator, must train your thought process to realize that simply looking at a bed is not enough to rule out a victim. Only a hand search can do that.
Your thought process should recognize that there is a good chance that the heat movement you can see in the air on the TI is far hotter than you think it is—so no taking your glove off to find out.
No conduction, no convection, only radiation. The differences may be subtle, but the implications on interpretation and decision making are not.
MANFRED KIHN is a 19-year veteran of the fire service, having served as an ambulance officer, emergency services specialist, firefighter, captain, and fire chief. He has been a member of Bullard’s Emergency Responder team since 2005 and is the company’s fire training specialist for thermal imaging technology. He is certified through the Law Enforcement Thermographers’ Association (LETA) as a thermal imaging instructor and is a recipient of the Ontario Medal for Firefighters Bravery. If you have questions about thermal imaging, email him at Manfred_kihn@bullard.com.
