Thermographers must have a solid understanding of how heat is transferred. We don’t necessarily need to take a college level course, but a practical understanding and being well-grounded in physics is essential. Why? Because mostly we see surfaces of things, but the heat we are most often truly interested in is internal. An abnormally warm electrical connection, like the bushing connection on the oil-filled circuit breaker (OCB) below, may be many degrees cooler on the exterior surface that we see compared to the point of high-resistance heating on the inside. We won’t necessarily be able to calculate the exact internal temperature, but we must appreciate this relationship and have a general understanding of the thermal gradients—differences between inside and out—that are involved.
Heat is transferred in solids by conduction, with the flow always being from warmer to cooler. The rate at which conduction occurs depends on three factors:
- First, on the type of material or, technically, the material property called conductivity—metals are highly conductive while “insulators” are not. We can easily refer to various conductivity values online or in an engineering handbook if we really want to understand and compare materials.
- The second factor is the thickness of the material. Obviously to reduce heat flow, we add more material (or vice versa).
- The third factor that determines the amount of heat being transferred is the temperature difference across the material—a large difference means more energy is transferred while a smaller one means less. Think about touching a frying pan when it is at room temperature—not much transfer—compared to a hot pan!
In many cases, like in the example of the electrical connector, we are seeing a surface warmed by conduction. In some cases, however, such as a wall of a building during the heating season, we are seeing a wall that is transferring its heat to the cooler outside. Thus, when conduction increases at the framing or an uninsulated area, what we see appears cooler than the insulated portions.
Clearly we need to have a temperature difference large enough to drive sufficient heat transfer through the materials we are viewing to create a
detectable thermal signature. A boiler that is not operating will not have a useful signature compared to one in operation. Similarly, a house insulated to R60 using a double-wall system may not transfer enough heat to be detectable.
For the most part, thermographers typically are comparing a “normal” signature with an “abnormal” one. However, we must always ask ourselves, “Are conditions sufficient to produce a signature we can detect if there is a problem?” This is not always an easy question to answer and sometimes we need the help of a good engineer with a table of conductivity values to help us predict what is possible.
So pick up that cup of coffee and enjoy the conductive heat flow! If the coffee is hot, the flow is into your hand. If it is iced coffee (and in many places that would be the preferred drink today!), the heat is moving from your hand into the coffee cup. Either way, you are feeling the surface while a bit deeper down is “the rest of the story” about conduction.
Next week we’ll talk about the handle on the coffee cup and, more importantly, convection, a powerful mode of transfer that affects nearly everything thermographers look at.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner