|
|
As thermography professionals, we must be well grounded in the basics of heat transfer. If not, we’ll make mistakes in understanding, interpreting, and presenting our data. If you don’t feel 100% confident in your understanding, I urge you to move in that direction and will offer these posts as simple starting points.
 Convection happens in fluids, like this cup of tea. With a carefully adjusted image you can actually see the currents of warm and cool fluid moving! “Simple” is an important part of the process for me. Anyone doing research on heat transfer can quickly run smack into “COMPLEX!” While that approach is often necessary, let’s begin by simplifying—recognizing that the world is not always so—in order to start the foundation on which we can learn more and more “complex” later on.
Convection happens in fluids. Let’s again go back to our mug of hot coffee. When you hold your hands above the mug, heat is transferred by convection. Your hands gain energy as air, having been warmed by the mug, moves over them. How much heat is transferred is determined in large part by various circumstances,,all lumped together and called the Convective Heat Transfer Coefficient (or h). These can include velocity and direction of flow among others. Like conduction, transfer also depends on the difference in temperature between the fluid and the surfaces with which heat is being transferred (in either direction), and, if we are concerned about total energy transfer, the area over which transfer is happening.
Newton’s Law of cooling succinctly describes conductive heat transfer:
Q = h • ΔT • A
In which:
Q = total conductive heat transfer
h = Convective Heat Transfer Coefficient
ΔT = temperature difference between the fluid and the surfaces involved in the transfer
A = Area of surfaces over which transfer is taking place
As was the case with conductive transfer, remember the net transfer can occur in either direction. Stick your finger in the hot coffee and the transfer is into your body. Once the coffee has cooled to room temperature (70°F/21°C), heat is transferred from your warmer body into the coffee. There may be instances where the coffee and your finger are exactly the same temperature, in which case ΔT equals zero and no heat transfer happens!
We are all prone to simplifying how we speak about convection by saying such things as “warm air rises.” While this seems to describe reality, what is really true is that the less dense warmer air is displaced by the more dense, cooler air surrounding it. Think of a cork in water. Literally the cork is pushed upward as gravity pulls the water downward. “I thought we were going to keep this simple,” you say! Yes, but we also need to be accurate. You can read more (some of it just plain “hot air”) about the movement of fluids in a great online discussion on Home Energy Pros, a great website for building scientists.
 Here I’m holding a piece of paper just above the neck of a thermos bottle filled with hot coffee. The heat transfer from the more buoyant warm air can be heating the paper. Is the warm air rising or being “pushed” by the cold air around it? That is an interesting question to explore! Between now and next week, give some further thought to the issues related to convective heat transfer and take time to look at examples of convection with your imaging systems. We’ll come back to that mysterious “h” or Convective Heat Transfer Coefficient next week.
I also wanted to take a moment to wish all our Chinese friends—especially the Fluke China team and our mutual customers there—a Happy New Year of the Dragon.
Last week, we talked about that mug of coffee and transient heat flow. The temperature of many surfaces is constantly changing because of an imbalance in the transfer of energy with the surroundings. When things balance, reaching what we call steady-state transfer, then temperatures stabilize.
We have all heard about the three modes of heat transfer: conduction, convection and radiation. Conduction happens in solids. When you touch that coffee mug, heat is transferred by conduction between the mug and your hand. How much heat is transferred is determined by the conductivity of the materials involved, the difference in their temperature and, if we are concerned about total energy transfer, the area over which transfer is happening. Scientists have determined rates of conductivity in laboratories and published these for many common materials.
Fourier’s Law succinctly describes conductive heat transfer:
Q = k • ΔT • A
In which:
Q = total conductive heat transfer
K = conductivity
ΔT = temperature difference between the transfer surfaces
A = Area of surfaces over which transfer is taking place
When the mug is warmer, the net flow of energy is into your hand. If it is cold, your hand supplies heat to the mug. An extremely hot mug may be too hot to touch comfortably because a great deal of energy is transferred to your hand. There may be instances where the mug and your hand are exactly the same temperature, in which case ΔT equals zero and no heat transfer happens!
 All these containers are filled with hot coffee of the same temperature so ΔT is the same in all cases. They exhibit different surface temperatures based on difference in conductive heat transfer. From left to right, compare the plain paper cup to a ceramic mug to an insulated cup to a vacuum bottle. Note the thermal reflections on the counter in front of the two on the left and the reflections off the surfaces of the two on the right. We commonly change the conductive properties to control heat transfer. If we want to keep coffee warm (or our beer cold!) for a longer time period, we can put it in an insulated—less conductive—container. We can keep our fingers comfortable by adding a material that is less conductive, like a paper sleeve.
Insulating our home reduces conductive transfer and as a result, keeps us warmer in the winter and cooler in the summer. We know that in extreme climates, either hot or cold, we need more insulation if we are to effectively control heat loss and gain. Also buildings with a greater surface area will lose/gain more energy, so just minimizing area is another viable way to control total heat transfer. Typically, we talk about resistance to heat transfer, or R-value, when describing insulation. Stated simply R = 1/k.
Thermographers must fully understand conductive heat transfer, one of the main reasons it is integral to the Level I curriculum. You can learn more on your own as well by getting out your imager, filling a cup of coffee (or two) and having some fun exploring this important topic. Next week we’ll review convective heat transfer.
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
Last week we talked about that mug of coffee and the notion of thermal capacitance. Thermal persistence is another way to think of this concept.
To one degree or another things are slow to change temperature. This is due to both the way heat is transferred (convection is rapid while conduction tends to be slower)and the amount of heat needed to cause that change. An obvious example is that a great deal of energy is involved when water changes temperature compared to air changing temperature.
 If we drink the hot coffee quickly, the mug remains warm.
|
 Over time the mug begins to cool as its heat transfers to the surrounding air
|
 As more time passes the mug cools even more. Eventually it will reach a “steady-state” temperature in relationship to the ambient surroundings.
|
The coffee mug was slow to heat up. We could see that in the three time lapse images below. The heat from the very hot coffee transferred into the cooler mug. In time, the coffee cooled enough that it is drinkable and the mug warmed enough to feel quite pleasant. These relationships are the first half of a cycle of transient heat transfer.
When we drink the coffee completely, the second half of the cycle begins. The air in the mug quickly cools toward the ambient air temperature. The mug now begins transferring heat to the air in the mug and around it. Notice how long the mug remains warm compared to our hands. The heat in the ceramic is slow to move (by conduction) and there is quite a bit of it to move.
We need to be careful whenever we look at materials we are working with. Whether it is the wall of a house or an overhead line, we need tounderstand whether or not they are in a transient state, and if they are warming up or cooling down. If the sun has begun to shine on a south-facing wall, for example, when will that heat show up on the inside? If the load has recently decreased on a breaker, how long will it be before the circuit cools to a new steady-state temperature?
Failing to take these simple relationships into account means we will make mistakes in our analysis. These could simply be a valuable learning experience or they could be quite costly. In either case it is important to pay attention to both thermal capacitance and transient heat flow.
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
“I’ve learned nearly all I know about thermography from others and have, honestly, barely scratched the surface, so I resolve to learn more from my professional colleagues.”
I was fortunate to visit the home of Sir William Herschel in the Fall of 2010. Here I am with my friend, Professor Francis Ring who is also Chair of the Herschel Society. When I wanted to learn more about Herschel (http://en.wikipedia.org/wiki/William_Herschel) before seeing his home, I turned to Wikipedia among other resources.
One of the places I regularly go to for details or more information about a topic is Wikipedia. When Wiki first began in 2001, I was one of many who felt it would never amount to anything useful. After all, I was a child who grew up spending hours reading Encyclopedia Britannica on any given rainy Saturday. Early Wiki could never match that amazing ten foot long, A to Z resource!
Wiki has come so far. While I still keep my healthy skepticism engaged when doing any online research, I’m also amazed at what a powerful and practical resource it has become. It is the fifth most widely used website in the world serving nearly one-half billion people each month. And, hurrah, there are no advertisements on Wiki! What a relief in an otherwise overly commercialized online world.
Wiki is funded solely by donations and if you’ve used it lately, you surely have noticed they are asking for contributions right now. I remember those big old encyclopedias were not cheap. My mom and dad bought that Britannica on “time payments” that totaled several hundred dollars—in an age when a new car cost less than three thousand dollars. Knowledge has value.
Because I now routinely rely on Wiki as a source of information, I decided to become a regular financial supporter. It was easy and felt great to just send them something each month through PayPal.
I know many readers of this blog are also big users of Wiki and that many of you also already support it financially. If this is not something you do yet, I’d ask that you consider doing so. I know many of you also contribute to Wiki by writing, and I offer my sincere thanks to you. Making written contributions to Wiki is something I’ve not yet done, but hope to do soon. When I read the section on Infrared Thermography, I see it is in desperate need of being updated and expanded by industry experts.
 Even Frosty the Snowman has an entry in Wikipedia (http://en.wikipedia.org/wiki/Frosty_the_Snowman). Frosty says “Please make a contribution to this great learning resource.” So if you are into making New Year’s resolutions, please consider chipping in to Wikipedia. It is a resource we all seem to rely on more and more and one that is worthwhile supporting. Even Fluke has a reference! With less than 100 staff members, it is an amazing organization that deserves our support. Thanks.
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
As we inevitably move into winter here in Vermont, talk often turns to the weather, and in particular, the temperature. We all live with some unrealistic illusions about temperature. For instance, people can be tricked into thinking they are comfortable simply by adjusting a thermometer to show a slightly higher temperature, and in fact they should. We think nothing of walking around in light clothing on a 55°F day in January, but bundle up in winter coats when the July thermometer shows the same temperature!
Part of the issue is comfort. We tend to filter information about temperature, which is not only quite subjective, but also based on much more than just air temperature. Ambient air temperature is what we typically are referencing when we ask “what is the temperature?” Comfort is dramatically affected by drafts, radiation to nearby warm or cold surfaces, our individual physiology, humidity, and even our beliefs. No wonder it is hard to correlate temperature with comfort!
 Human beings, despite our amazingly complex thermal systems, usually know whether or not we are comfortable. However, we don’t, always know the temperature and we rarely understand how the two—temperature and comfort—are related. When we then add thermal imaging into the mix, life gets even more complicated. We are under the illusion that we can simply point this magical imaging device at some target, possible correct for emissivity and background, and display the “correct” temperature.
The truth is a bit more complicated. If we work within the limitations of physics and the tool we are using, we should be able to arrive at the most accurate measurement possible with a certain degree of precision.
Is that “exact?” Wikipedia has an excellent introductory discussion of what is considered a highly complex topic. I recommend it to all readers. We can make accurate measurements even if they may not be exact. Even if we can understand the difference among ourselves, can we explain it if needed, to our customers?
We must be able to both understand what we can and cannot do and be able to communicate the truth clearly to those with whom we work. And even when we understand the limits of what we can do, we must then ask ourselves “what does it mean?” We’ll continue that discussion another time!
I’m certain these and similar issues will become more commonly discussed and argued as more and more people use thermal imagers. Unfortunately, I suspect in many cases both the tools and the tool users will not be well-suited to the task as hand. The New Year would be a great time to promise ourselves that we’ll learn more about what our imaging systems can and cannot do and how we can get the best results possible.
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
One year ago I made some predictions about where I thought the market might be headed (click here to read full post). It is useful, I think, to see how today’s reality compares with yesterday’s dreams.
• A $1,000 imager
We are close, but thankfully not yet there. Sales of lower-cost systems continue to grow. This has filled many useful voids, especially in the buildings and maintenance markets, but it has also led to more and more people using the technology without knowing what they are doing.
• Radiometric video recording in the imager
Several manufacturers have this capability in systems priced under $10k. While Fluke isn’t in that group yet, I am confident we will see one in this New Year and that it will be exceptional. I predict having video once again (we used to, but it was expensive) will open up new worlds of application.
• New and better training
I still dream of “new and better training,” but maybe I don’t really know what that looks like. After all I grew up on filmstrips and video training! Certainly training and good information is easier to get. Two examples of this are more on the internet and shorter, more focused classes—but there is also more bad training than ever and way too many examples of information on the web that is just inaccurate.
 More and more building experts, like these two from Vermont’s Weatherization Assistance Program, continue to successfully expand the use of lower-cost imaging systems. • High-resolution applications development
Namely medical and natural science, have continued to develop even if more slowly than I’d hoped. These sorts of applications also tend to take time to become visible to the general public.
• RESNET standards for thermography
The most disappointing prediction I made was about the new RESNET standards for thermography. They have, unfortunately and probably predictably, gotten mired the organizational bureaucracy. Certainly this fact has done little to slow the use of thermal imaging among buildings professionals even if some of the work is not always of the quality it should be. We’ve especially seen this in the Weatherization Assistance Program that helps low-income people improve the energy efficiency of their homes.
 Image quality in 2011 continued to improve dramatically even while prices dropped. A small piece of missing insulation in this cathedral ceiling demonstrates the remarkable power of the technology to help us locate even small problems in buildings. Like stock market predictions, you should always read the fine print and hedge your bets. That said, thermal imaging is still one of the best bets I know of. Even if I was not 100% on target with some of the details, the market for thermal imagers has never been stronger. If you have not jumped in yet, the next month or anytime in 2012 is a good time to do it. The tax advantages of an investment in an imager are excellent (consult your accountant please!). The imagers are better than ever and the value you get for you money can’t be beat!
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
The Thanksgiving feast was delicious and the leftovers were too. I’m certain my calorie intake over the past week is quite out of balance with my use of energy.
 A wooden kitchen match, burned entirely, gives off approximately on British Thermal Unit (Btu) of energy. One Calorie is the energy equivalent of burning four kitchen matches. Normally, I do pretty well at balancing the energy I take in while eating with the energy I use for all my activities. We often think only of exercise as “burning calories,” butwe use energy 24/7 while breathing, eating, walking and sleeping. I tend to not keep track of calories or my weight, though I know many who do, however I do keep a rough estimate of my caloric balance. I use measures such as do my pants feel too tight or have I enjoyed a long walk today? Even by my rough calculations, I know I took in more energy over these holidays than I’ve expended!
As we’ve discussed here in the past, we can measure heat in various units including a British Thermal Unit (Btu) or a calorie (c). A Btu is the amount of energy required to raise a pound of water by one degree Fahrenheit. A wooden kitchen match, when burned entirely, is roughly one Btu. If we were able to add all that heat to a 16-oz bottle of water, we could expect the temperature to increase by 1°F.
The much more commonly used term calorie is defined as the energy needed to raise the temperature of a gram of water one degree Celsius. One calorie is equal to 0.004 Btu, so it is quite a small measure of energy. The kind of measure I have in mind when thinking about my Thanksgiving meal is one Calorie (C) or 1000 calories (c). So one Calorie is equal to roughly four kitchen matches.
With these equivalencies in mind, I have a much better idea of why my energy intake is out of balance with energy expenditure. The ice cream alone that I added to my apple pie was 200°C or 800 kitchen matches worth of energy. Wow! When I do work up a sweat, the heat I’m giving off—some of those 800 kitchen matches—is obvious, but clearly it will take some exercise to burn up all that energy. In the meantime, my body stores the excess as fat, supposedly for later use.
Just as we can look up the caloric value for the foods we eat, we can also find detailed charts showing us how much energy we expend in various activities. In the end the goal is to balance the two. If you want to learn more about caloric balance in all kinds of animals, especially humans, I recommend you read Bernd Heinrich’s Why We Run. With that in mind, I think I’ll take a long walk to think about what we’ll discuss next week!
Thinking Thermally,
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner
Many of the same principles of a thermal inspection in a home can be applied to a commercial property inspection. In this video, Chip Wade shows you how you can take your business to the next level by offering commercial property inspections. He spots a problem in a warehouse that is being converted to an office building that could have cost thousands of dollars down the road.
Chip then sums up the benefits of investing in a thermal imaging camera from Fluke. As he states, an imager can pay for itself with just ONE commercial building inspection. Fluke also makes it easy to get started, offering in-the-box training, and many hands on training and certification courses. Learn more about how thermal imaging offers a clear, competitive advantage by clicking here.
Click HERE to watch the final episode. Thanks for tuning in!
|
|
Twitter
Facebook
LinkedIn
