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Heat Transfer, Temperature and R-value

Hopefully many of you tried my suggestion last week of filling a mug with hot coffee or tea and observing it with your imager as it went through a thermal cycle. If not, please do so as there are many valuable lessons to be learned. Also useful is distilling the temperature changes for the mug and the hot liquid and watching their interplay as they teach us about capacitance, heat flow and temperature.

Understanding R-Value
There is a growing interest in our industry to be able to use the surface temperature of buildings as a way to calculate conduction, or more accurately resistance to conductive heat transfer (R-value). With this information in hand, they hope to plot the energy cost for conditioning a building. This is a great idea that, is not as simple as some would have us believe, unfortunately.

Certainly this sort of thing is often done in a laboratory where all the variables are known and controlled. In fact, it isbasically the way scientists determine the standard conductivity values and R-values for most materials and products we use every day, everything from insulation to frying pans! The process is a simple one in which a stable, known temperature difference is created across the material and the flow of energy is measured. Again, I emphasize that all conditions are carefully controlled and all variables are known and monitored.

Possibility vs. Probability
In the real world, however, especially when “home energy audits” are being done from a moving vehicle or aircraft, the task is much more difficult, to be honest, simply impossible. As a good friend of mine says, “possibility does not imply probability.”

While it would be wonderful if accurate R-value calculations actually could be determined from thermal images in this way, they cannot. My fear is that these attempts will become more widely used or, more likely, misused. Even more troubling than people just experimenting with these processes is the fact that some would like to patent software to automatically make the calculation of these energy transfers. I fear we will all be “tarred with the same brush” and discredited by their inaccurate results.

Conclusion
I don’t want to be a naysayer. I freely admit there is much I still have to learn. If anything, I’m more aware of that fact now than I was nearly 30 years ago when I first used an imager. But in that time, I’ve also seen too many enthusiasts claim they can do things I’ve found to be impossible, limited either by the tools we had/have or the physics of the situations. “Correlation does not always imply causality,” as my friend also says.

Thermal image of a cup of coffee cooling down

A great deal can be learned from carefully observing the simple things that surround us in life, such as a cup of coffee cooling down!

So take an hour and sit down with your cup of coffee or tea and your imager and think about what amazing things we can do and what is not so easy.

I’m not sure where we will journey to next week in this blog. Where would you like me to take us?

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

6 comments to Heat Transfer, Temperature and R-value

  • This is just the situation we have in Ukraine now. Everybody wants to know R-value to calculate heat losses, it does not matter, with thermal imager or with heat flow meter. But it is very, extremely difficult to explain, that actually it is very difficult to measure it with good tolerance, and it is better to find “bad places” and correct them instead.
    There is one standard, yet from USSR times, issued in 1985, describing using thermal imaging for determination of heat transfer properties of building envelope. But the procedure described there requires at least several days for measurements, with use of many thermocouples and heat flow meters, where thermal imager is used only to determine characteristic points to place thermocouples and just check critical places, like corners, windows, doors… Even though this standard exists, it is completely senseless from practical point of view! And I even do not know anybody, who would try and agree to make the complete set of measurements, espessially in view, that the final result is not garranteed anyway :)

  • I feel your pain, my friend!

  • Michael Stuart

    Perhaps one day, someone will discover a reliable, easy, and repeatable way to do this. (I know that many brilliant people have been thinking about it.) For now, however, we must do our best to explain what a truly difficult task it is to those who may not understand the limitations of infrared technology, or some of the complexities of thermodynamics when it comes to materials science. The pain is felt by many.

  • I think, here is not a problem of IR technology limitations only. Too many variables out of control.

  • Vaughn Gonzalez

    There are just so many lessons over a cup of coffee.

  • I’m about to buy a FLIR i7, but I don’t expect to be able to estimate heat flow with it without also investing in some heat flux sensors (Omega HFS-3 as an example). Even then, a lot of assumptions are needed. One of my pet peeves is lack of good data on heat loss thru concrete floors and gravel beds. I expect I’ll have to actually build test structures and measure their performance. I’ll probably even put together a guarded hot plate to get verifiable readings on these materials. I agree … while thermography is great, some applications of them require a lot of wishful thinking.

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