“Chestnuts Roasting on an Open Fire”

Although you are reading this article in December, I’m writing it in October. When I stopped by a local grocery store this morning, I couldn’t help but notice that Christmas music was already playing overhead. Christmas music in October strikes me particularly wrong. It’s not that I’m some type of Scrooge; I just think we should celebrate holidays in order: Thanksgiving first, then Christmas.

As I pondered the situation, the words of “The Christmas Song” inevitably soaked into my brain – “Chestnuts roasting on an open fire.” This may conjure warm memories of standing close to a campfire, trying to stay warm against winter’s chill, but here in Kentucky, it is not all that cold in October. In fact, the high today is supposed to be in the low 70s, so rather than mental images of staying warm, I began to conjure mental images of “seeing” warm. And since you will be reading this in December, when many of you will be fighting off winter’s chill, I decided that rather than resist, I would encourage. Go light a fire. Grab the chestnuts. Stay warm – but bring your thermal imager, as there is much to be learned!

I’m sure I don’t have to tell you how to start a fire, so I will skip that, but you will need the following:

• Firewood. You will need enough split, dried firewood to keep a hot bed of coals, but you will also want a few green pieces as well – wood that has been freshly cut or not split and dried.

• Kindling. This can be paper, small sticks or cardboard in various densities and sizes.

• A wooden box. You might have to make this one, but a wooden box configured so that you have a B, C and D wall as well as a ceiling – no floor and no A wall. The three walls should be slightly longer than the base of the fire you will create and approximately twice the height of the firewood. Thicker wood will last longer, so half-inch or three-quarter-inch plywood would be ideal. If you want to get really crazy, cut a hole in the center of the “ceiling” that represents about one-fourth of the total size of the ceiling. Cut another piece of plywood the same size as your ceiling, but with a hole in the center that represents about one-tenth of the total ceiling size and place it on top of the box so that the smaller hole is centered over the larger hole. Finally, cut a piece of plywood to cover the smaller hole. The hole-cutting is not necessary, but will extend training if you have the time.

• Your thermal imager. Make sure you are familiar with the temperature ratings associated with the colorization scheme your imager uses. If you don’t know, reference the owner’s manual, the manufacturer’s website or simply call the manufacturer and ask.

• Chestnuts. What fun would it be if you couldn’t eat while you train?

As soon as you set the fire, begin to observe with the thermal imager. Look at the difference between what you see with your eyes and what the imager sees. The two images will look very different. In the incipient stages of your campfire, the flames are likely to be large and very bright to your eyes; however, the thermal imager will view them as fairly small.

The main difference is that your eyes see light and the thermal imager does not, so while you perceive the flame to be large based on the amount of light entering your eye, the thermal imager perceives the flame as smaller as it can only assess the heat associated with the flame and cannot see the light. The secondary difference is that, in the incipient stages of a fire, lighter-weight combustibles are usually the first to pyrolize and become consumed. Lighter fuels don’t generate as much heat as heavier fuels do, but they pyrolize rapidly resulting in a large flame with relatively little heat. This is why you typically use kindling to get a campfire started.

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.