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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 warmer they feel, but that retention efficiency is due to 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. This lack of outward radiation would mean that, although your thermal imager can see every detail of the comforter on the bed, it cannot 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.
What about convection? I am sure many of you have seen convected air currents on your thermal imager. Next time you are at a training fire, watch the ceiling in the burn room. You can definitely see heat being convected across the ceiling. Once again, you are seeing radiated heat and the difference here is just as critical as the search mentioned above.
Convection occurs when air is heated. As the air is heated, the molecules start moving around and bumping into each other. Every molecular connection transfers heat from one molecule to the next and also creates separation. This transfer and separation makes the air less dense, so it rises. As it rises, a low-pressure area is created and draws in denser, cooler air.
We all naturally know this, and the process is quite intuitive – so intuitive that we almost expect to see it. If we expect to see it, and then look at a thermal imager and see what we expect, it sets off no alarm bells at all – but it should. 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 thermal imager, and at the point that the gas radiates, it can be considered superheated.
So, back to that house you were searching. During your search, you see currents of heat moving along the ceiling with your thermal imager, and it seems normal. Your brain mistakes it for what you would expect to see. Heat moves away from the fire and toward a vent point. Your brain would expect smoke and heat to be moving along the ceiling, so the thermal image looks normal; however, if your thermal imager can see the heat moving along the ceiling, you are not seeing the convected air currents. You are most likely observing the radiated heat from a column of superheated gas, and the implications are very different.
Train your brain
Practice, practice, practice. You should practice viewing the world through the thermal imager so that your brain gets better at recognizing how common objects look from a heat-based perspective; however, you should also practice in order to realize how things should not look.
Train your brain to realize that simply looking at a bed is not enough to rule out a victim. Only a hand search can do that. Train your brain to recognize that there is a good chance that the heat movement you can see in the air on the thermal imager is likely far hotter than you would think it is. No conduction, no convection; only radiation. The differences may be subtle, but the implications on interpretation and decision-making are not.
BRAD HARVEY is the Thermal Imaging Product Manager at Bullard. He is a veteran of public safety as a firefighter, police officer and paramedic and is certified through the Law Enforcement Thermographers’ Association (LETA) as a thermal imaging instructor. Harvey has worked as a high-angle rescue instructor and is a certified rescue technician and fire instructor. If you have questions about thermal imaging, you may email him at firstname.lastname@example.org.