As I’ve written about over the past two weeks, there are a number of factors that determine what the temperature of a surface will be beside the energy inherent to it. We need to understand them so that we can make a valid analysis of what is really going on.
As an example, imagine an abnormally high-resistance connection on an overhead transmission line. When you observe it with your imager, the wind is blowing and the line doesn’t look very warm at all because of the cooling effect of the convection. By the time you get back to the office, the wind speed has dropped to almost nothing and the connector has heated to the point where it melts and fails—along with your credibility as a thermographer!
We’ve talked about radiational heating and cooling and convective heating and cooling. There are several other important factors to consider as well.
All material can exist as a gas, liquid, or solid. No energy is required for it to be solid, but to be a liquid we must add energy. And to be a gas we must add more energy, specifically latent energy to break the bonds that keep the atoms in close relationship. We also know that as gases condense into liquids, and as liquids freeze into solids, this latent energy which isno longer needed, is released.
When we look at the side of a house that has been wetted by a sprinkler or a driving rain, it will appear cool (if the relative humidity is less than 100%). When molten plastic in an extruder freezes into a solid part, it will release tremendous amounts of heat. We could find many other examples of materials changing state—in either direction—that absorb or release energy in the process. When that happens the surfaces around them are either cooled or warmed.
So what is the temperature of a wet surface under the influence of evaporation? Many factors will determine this, including the amount of water, the rate of evaporation, the original temperature of the surface and the proximity to the wet area, among others. What we do know is that generally evaporative surfaces will be cooler than similar, near by dry ones.
The take-away here is to always pay attention when you are near the freezing point, dew point and evaporation point of a material. If a state change is underway, it will undoubtedly be influencing the temperatures you are seeing.
When energy is added to a material, it warms up, and when energy is removed, it cools. But a great deal depends not only on how much energy is added but also the thermal capacitance of the material. An obvious example is to image a swimming pool filled with water and a room of exactly the same volume filled with air. Add the same amount of energy to both and the room of air will be warmed to a greater temperature. The thermal capacitance of a material tells us how much energy is involved as it changes temperature. As materials of all kinds go through a temperature change there is a tendency, to one degree or another, for them to be persistent about not changing. Climb out of bed in the morning, for example, and your body print remains for quite some time!
Want to try a simple but fascinating experiment? Isolate a room in your home and let it all stabilize in temperature. The thermal image should show most of the objects in the room to be close to the same temperature. Now turn the heat up or the AC down by about 5°F (3°C). Watch the objects in the room change temperature as the room begins to stabilize at the new temperature. This could take an hour or more. The first things to change will probably be low-
mass items like drapery or a poster on the wall, while more massive items, such as wooden furniture or ceramic pots, will take longer. The point is, when you are working with your imager, are the materials you are looking at in the same sort of thermal transition? If they are, how close to being at steady state are they?
Mode of Heat Transfer
While capacitance can tell us how much energy is required for a temperature change, the way in which that energy is transferred—radiation, conduction or convection—tells us how quickly the change will occur. If we had the same room full of air and block of wood the same size as the room, it is easy to see that convection in the air is a much faster way to move heat than conduction in the block of wood.
So we need to pay attention to the modes of heat transfer involved and understand what effect they will have on the surfaces we are looking at. A good example is a large storage tank as it warms up in the morning sun. The air and liquid in the tank both change temperature as heat conducts into the fluid and convective transfer occurs. Of course the liquid will require a great deal more energy to make a unit change than does the air. If there is sludge in the tank, however, even though like the liquid it has a fairly large thermal capacitance, the heat transfer happens due to conduction. While conduction is often quite slow compared to convection, the heat only needs to move a short distance into the solid to for the change on the tank’s surface to be obvious. Again, as the thermographer we need to understand the dynamics of the situation and to see where in the thermal cycle temperatures are when we see them.
I hope you have a greater appreciation of the temperatures you see and measure now that we’ve spent the past three weeks discussing the factors that determine temperature. If you are feeling overwhelmed, just ease on into things and don’t worry. You now know you will make some mistakes but give yourself the permission and time to learn from them. The temperature values displayed in your images are only the beginning of understanding what it all means! With patience, practice and learning your analyses will get more and more accurate and that’s what it is all about!
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner