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Cold and Getting Colder (at least north of the equator)

We’ve turned the calendar to a new month and year—Happy New Year readers! We’ve also turned the corner of the Winter Solstice, at least here in the Northern Hemisphere, and are headed to longer days.

We may be getting more daylight, but it will be about six more weeks until the average temperature begins to increase. Why is that? The energy exchange of the planet with our surroundings is still not in balance. In this period of thermal transition more heat is still leaving the Earth than is being gained so the end result is our temperatures north of the Equator, on average, continue to drop.

A heavy ceramic mug, before being filled with hot coffee, is in a steady-state relationship with its surroundings, gaining and losing heat at rates that keep it at an unchanging temperature. Note: Hot spots are reflections)

Almost as soon as hot coffee is poured into the mug, it begins to warm up. The surface is still not as warm as the coffee.

In a short time the surface of the mug has come to quasi steady-state temperature again with the new, warmer conditions. Note the surface of the mug is still cooler than the coffee.

It is similar to what happens when we fill a heavy, cold mug with very hot coffee. We can easily hold the mug, at least for a minute or two, but most of us have also discovered the coffee remains too hot to drink. After a short time enough heat is transferred from the hot coffee into the cooler mug—heat always flows from warmer to cooler unless work is being done to reverse the flow—to make the mug too hot to hold. Like the Earth, it takes a bit of time for the mug to heat up. This is because of its high thermal capacitance as well as the relatively slow rate of heat transfer through the mug to our hand.

We can learn a great deal from these examples that can be usefully applied to our day-to-day work. When we look at a surface through our imaging system—whether a fused disconnect, an insulated wall or a large outdoor storage tank—we need to ask ourselves “is this surface stable or still in a thermal transition?” Often we see things as a snapshot in time. What if the surface is still in the process of warming up (or cooling down)? How will such changes, unseen by us and perhaps unanticipated, affect our analysis?

More on capacitance next week, part of a short series on IR Basics I thought would be a good way to begin the New Year.

Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

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