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As the fire develops from consuming lighter combustibles and begins to consume heavier fuels such as the firewood, you should see things begin to change again. Depending on your imager’s colorization schemes, you may see color enter the image as an indication of approximate temperature ranges. You should also see the sizes of the visible flame and thermal flame begin to equalize. This is the more fully developed stage (since we have not contained the fire, we don’t experience a flashover).
As the fire assumes a steady state, observe how the colorization on your thermal imager reacts as well as the temperature sensing. Most thermal imagers in the fire service today have some type of temperature measurement capability. This commonly consists of some indicator (a dot, plus sign, box) at the center of the image and a readout (bar graph or numeric) on the right of the image. The readout is indicating the approximate temperature of the indicator at center. Place the indicator on the fuel. What do you see on the readout? Now, look through the flames horizontally, so that there is no fuel in the background. Now what do you see on the readout?
Occasionally look away from the fire and then back again. Did the image “freeze” for a moment? All microbolo-meter thermal imagers sold into the fire service today consist of two or more gain states. Although the technical issues are beyond this article, every time the imager shifts gains (from high to low or low to high) it requires a recalibration by the imager. This results in a very brief, almost momentary, freezing of the image. This is normal, but you should be aware of it.
As the fire dies down and enters a decay state, observe again the difference between your eyes and the thermal imager. At this point, your eyes may perceive no flame at all – just smoldering coals. On the thermal imager, however, you may see what appears indistinguishable from flames. Again, the thermal imager is showing you the heat generated by the current fire state.
Now that you have seen the difference between what you experience with your eyes and what the imager experiences, stoke the coals back to a free-burning state and watch what happens when various fuels are introduced. What is the heat behavior when split and dried firewood is introduced vs. green wood? Whole logs vs. sticks? Paper tightly wadded vs. same paper in sheet form? Cardboard vs. paper? How does the imager react and “see” these events? What implications do these reactions have in terms of fire behavior?
Last, with the fire properly stoked, carefully lower your three-sided box over the fire. Watch inside the box, through the absent A-wall. What happens? How does this simulated room react to the fire? Where does the heat collect? Where does it flow? How does the imager’s color scheme react to the room? Where does colorization begin and how does it progress?
If you took the time to cut the holes mentioned earlier, remove the top piece of plywood exposing the smaller hole and observe the change in heat conditions and heat flow, what changes do you see? Replace the piece of plywood for a moment and then remove both the small piece of plywood and the piece that has the smaller hole in, leaving only the piece with the larger hole. Now what changes do you see in the box? What are the differences between the simulated room ventilated with a small hole vs. a large hole?
Finally, once all of this has been completed, grab the chestnuts and roast away! You can discuss what you observed while filling your belly with warm goodness and celebrate the season.