We know the direction of heat transfer is from hotter to cooler, but sometimes the temperatures we measure can be misleading. Particularly, when the process of transfer is still in transition, the temperatures will not have peaked yet. Also when a surface is being strongly influenced by outside forces—such as evaporation, sun, wind or clear sky cooling—the temperatures we measure will probably not represent the true nature of transfer through or from the materials involved. If we don’t recognize these influences or understand their affects, we can get into trouble fast!
We encounter this sort of dilemma in buildings during both the summer and the winter. For example, when solar loading puts a wall into a transient condition, temperatures and thermal patterns often reverse themselves. Long ago, I learned this lesson rather painfully when I told a contractor that the north wall of building was insulated while the south wall was not.
I had assumed the all the walls were in steady state, but they were not. It turns out that during any time of the year, as the sun heats the exterior, the direction of transfer can fairly quickly change move toward the interior. When this happens, an uninsulated cavity will heat rapidly, but surprisingly in most situations that will be followed by the insulated cavity warming next. Last to warm will be the framing, a fact that at first seems counterintuitive because it is much more conductive than the insulated cavity.
The explanation, of course, is that the thermal capacitance of the framing is much greater than the insulation, i.e. we have to wait for more heat to be transferred before the same temperature is reached. So depending on when we show up to inspect the wall, it may appear insulated or uninsulated. Even more insidious is the fact that in some instances the wall will be so uniform in temperature as to appear to have no framing!
What can you do to avoid trouble?
I find it useful to watch temperatures. If they are changing, it can give me an indication that transfer is still transient or that unseen influences are at work. A “cooling curve” can be a great way to portray and see these sorts of things.
Try this simple demonstration. Pour a cup of coffee into a thick mug and, using your imager, every 60 seconds measure two different temperatures—the surface of the mug and the surface of the water. When you plot a cooling curve graph of both temperatures over time, you’ll see some interesting thermal relationships.
The mug will increase in temperature until it reaches steady state, at which point it will begin to cool to the temperature of the ambient surroundings. The hot water begins cooling immediately and will continue to do so until it too reaches the temperature of the surroundings. But the interplay of the two curves with each other is both fascinating and educational and can help us learn how to better understand temperature and heat transfer in our real work. If you have a wall in your house or office that faces the sun, you can learn even more by watching it (from the interior) as it goes through a complete thermal cycle!
Temperatures can be either meaningful or misleading. If you understand heat transfer conditions and direction of flow, temperature is your friend. If you just blithely measure temperature and think it means something, you’ll likely end up in hot water, no pun intended. Next week we’ll talk more about transient heat flow, especially in buildings, and the particularly disturbing trend among some in the industry who believe they can calculate R-values from thermal images.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner