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Adjusting Level & Span Manually

Last week we discussed how to adjust level and span automatically and when that feature is best used. While using AUTO can be very convenient in many situations, you’ll quickly find times when MANUAL adjustment is essential, either to getting the best image or understanding what you are seeing.

Use AUTO adjust to get “in the ballpark. The problem is, when you have a large span of temperatures—cold to hot in the case—there is little detail in any single cup.

How does MANUAL work?

Although it takes a bit of button pushing to get there, using MANUAL adjustment is not difficult and you’ll quickly learn how to do it without thinking. The current mode you are using is displayed in the upper right corner.

Many models of Fluke imagers are have a great feature designating the F1 key, when held down for several seconds, to toggle between AUTO and MANUAL modes. You can also quickly get to MANUAL by pressing the F2 key until the menu choices show MANUAL or AUTO. Either way, just press the F2 key again or until you reach the options for LEVEL or SPAN and then INCREASE or DECREASE.

I find it easiest to adjust SPAN first to what I think will be most useful.

When does MANUAL work best?

• I use the MANUAL feature when I know I’ll need a very narrow span. For example, if I want to trace down the location of heating in an electrical component, I’ll set the SPAN at a minimum and then increase LEVEL until only a small visible area is left in the image.

• If I have a good idea of what the difference temperature should be, say between insulated and uninsulated areas, I can adjust the SPAN to that difference and adjust LEVEL to the temperature of the insulated wall; the uninsulated areas will then show as warmer or colder depending on the direction of heat transfer.

• Whenever I’m looking at a situation where temperatures are changing, such as a steam trap cycling, and I want to determine exactly when they reach a certain level, I can fix both SPAN and LEVEL manually and watch for the object or process to show up.

• If there are extraneous objects in the field of view that will cause the AUTO function to adjust poorly, I can adjust manually exactly as I’d like the image to be.

When I use MANUAL mode, I can adjust SPAN to be quite narrow and LEVEL appropriate to each cup individually. The result is amazing detail in each cup.

What are the limitations of using MANUAL?

Using the MANUAL adjustment mode is not as fast as AUTO. I guarantee you will get tired of pushing buttons on some jobs! There will also be times when you’ve adjusted the image so that you can understand what you are seeing but, overall, the image will not be very good looking or even understandable to a non-thermographer. In such cases two images may well be needed, one adjusted manually showing the exact problem and another adjusted more broadly, either manually or automatically, showing the problem in the overall context.

SPAN and LEVEL adjusted appropriately for this cup.

How can I use MANUAL successfully?

All thermographers must master using MANUAL adjustments or they end up missing a great deal. There is simply no way around it. Practice on the same three cups of water I talked about last week. Interestingly, you will find that the best MANUAL adjustments are often very similar to what you achieved with AUTO when you moved in close and excluded extraneous objects. Remember, if the MANUAL adjustments are not working for you, just switch briefly back to AUTO to see what is going on or drop the image into Smartview and optimize the settings there.

SPAN and LEVEL adjusted appropriately for this cup.

Next week we’ll talk about a really great feature available on many Fluke imagers, 1-time auto adjust.

Thinking Thermally,

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

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Adjusting Level & Span Automatically

I’m all for using the AUTO adjust feature found on most thermal imagers. That said, it does not mean I can turn my brain off! Many new thermographers believe they can just put their system on AUTO and life will be good. Not true.

AUTO adjusts the span of the image to accommodate all three cups by setting the top of the SPAN at 130F and the bottom at 30F.

How does AUTO work?

The AUTO feature is brilliant. It analyzes the image and adjusts to whatever is in it. The highest and lowest radiance levels define the top and bottom of the span setting, as well as the level of mid-point.

You can easily see AUTO in action by looking at three cups of water, one hot, one cold and one at room temperature. Notice what happens when you view all three in one image. Now watch the scale change as you isolate each cup individually. I would encourage you to practice until you are clear how AUTO works and what the limitations are.

When each cup is imaged individually, AUTO changes the span setting based on which cup is in the image. For the cold cup (left) span become 29F-70F, for the room temperature cup (middle) span becomes 64F-77F and for the hot cup (right) span becomes 64F-128F. Not my warm reflection in the center cup driving the upper limit of the span up slightly.

When does AUTO work best?

• I use the AUTO feature when I have little idea of what my setting should be. AUTO will generally get me in the ballpark, hopefully at least to have a decent image and a basic understanding of what’s going on.

• I also use AUTO when I’m looking at a wide variety of new territory, whether that is a building or a manufacturing facility. Again, my intent is to have a basic image (even if the temperature range is not refined) that gets me to the next logical step.

• When I’m looking at something that is changing temperatures fairly quickly, like a radiator heating up or fluid being transferred after a valve is opened, AUTO can change settings much faster than I can manually. This allows me to see the changes and understand them.

What are the limitations of using AUTO?

If there are any extraneous hot or cold objects in the image, they will cause AUTO to adjust and accommodate these extremes. For example, if you look at a hot line connection with the clear sky behind them, the lower end of the scale will probably be set at -20F or lower and the connector will not appear hot. Or in a house, if you have an incandescent bulb and a window in the image, the insulation patterns in the wall will be lost because the span is set too wide.

How can I use AUTO successfully?

Keep extraneous hot/cold objects out of the image when possible. Use AUTO as a starting point and, as needed, switch to MANUAL (I’ll cover this next week) or drop the image into Smartview and optimize the settings there. AUTO works well as long as you use it as a tool and not simply rely on it for an answer!

Thinking Thermally,

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

What Palette to Use?

The image palettes we have available to us are remarkable. They’ve come so far since I first got into this business in 1983. We can now also easily change the palette in the software. Wow!

The Fluke AMBER palette with red and blue saturation indicators is a nearly perfect palette to use, especially in the field while working. The blue saturation shows air leakage into the ceiling in the left section of the roof. The blue rectangles are cold windows and a cold metal chimney flue

Still, many people have questions about which palette to use. My recommendations are as follows:

• While working in the field I strongly recommend using a “monochromatic” palette. Fluke has really perfected the AMBER palette, which  incorporates red and blue saturation indicators. You can’t go wrong working in this palette day in and day out! My second choice is the GRAYSCALE but it is not as good as AMBER. RED-BLUE can work well in the field, but it tends to not print well in a report.

• For reports I often use the HIGH-CONTRAST palette because it looks so darned good in print! Be aware that sometimes it can be confusing to non-thermographers—even when they are  nodding their heads saying “yes, I see…”—so make sure the imagery is clear and simple. And watch out for the green tones as they are not very intuitive.

The HIGH-CONTRAST palette can work very well in reports as people find the images very persuasive. The performance of three different windows can be seen very clearly in this winter image. The top window is single-glazed, the bottom right is double-glazed and the bottom left is a high-performance, low-E double-glazed window.

• Don’t routinely use either of the inverted palettes (AMBER or GRAYSCALE) because they are not the norm. However, every once in a while I find them very useful, especially to show small hot spots. If you use an inverted palette, clearly indicate the same to minimize the chance of confusion.

I see many new thermographers continually switch through all the available palettes. This is a waste of time. Consider my recommendation, try them for yourself, and learn which ones work best and stick with them. If you ever do have a doubt, and some scenes are challenging to portray, simply drop the image into Smartview and change it though all the options. You’ll quickly see which one works best.

Thinking Thermally,

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

Image Fusion

When Fluke IR-fusion® was first launched, I wondered whether or not it would be perceived as just another marketing gimmick, and if not, how well it would be used. Time has proven that few find it gimmicky and most use it very well.

In the field I nearly always use the Full Infrared setting (left) for IR-Fusion® rather than the Picture-in-picture setting (right) which results in a smaller version of the thermal image. The blue area is where air is leaking into the kitchen exhaust fan ductwork.

Over the years, I’ve learned a couple tricks about IR-fusion® that I wanted to pass along. First, while in the field I nearly always use the Full Infrared setting. Remember, no matter how the image is set up I can change it later in Smartview, given that I’ve saved the images in an IS2 format, to whatever I want.

I see many people use the Picture-in-picture setting while in the field, but I don’t recommend it. Why? I want to use every bit of the screen to display the vital thermal information rather than just seeing a small portion of the center of the screen.

I also see many new thermographers using the Blended setting in the field. Again, I do not generally recommend this. Not only do we want a full infrared image, but we don’t want to make a mistake common to using the blended setting. Experienced thermographers know glass is opaque, but the blended setting has caused many new thermographers to assume they are seeing through a window.

Once I’ve imported the image file into Smartview, I switch the setting as appropriate. For reports, I’m a big fan of both the Picture-in-picture setting, as well as the Blended settings, because they so powerfully show the relationships between the visual and thermal worlds.

And don’t forget this important fact: when your IR image is in focus, the alignment of the thermal and visual images in IR-fusion® is perfect. No imager manufacturer other than Fluke can say that and it is worth a lot!

Take some time this week and explore how the various IR-fusion® setting work and which are best at showing the information you want. And please let us know what you think in the comments section so others can learn from your experiences!

Thinking Thermally,

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

Radiometric Temperatures

We’ve spent the past several weeks discussing heat transfer. Radiation is a special mode of transfer because that is the mode by which energy is transferred from surfaces to our imagers.

All of what we see in the thermal image is nearly always based on the amount of radiation emitted from or reflected by the surface. Most of the time, it is quite easy to distinguish strong or specular reflections because, like a mirror, they move when we move – relative to the surface.

Applying a piece of electrical tape to either a cold window glass (top) or a warm sheet of steel (bottom) with variable oxidation allows a thermographer to consistently make accurate radiometric temperature measurements. Note both materials are quite reflective. In both cases the emissivity is set for the value of the tape (0.95) with an appropriate reflected background correction value.

But we are often interested in measuring the temperatures of the objects we are viewing. When we measure radiometric temperatures, we are really only carefully quantifying the radiation that makes up the thermal image. The imagers are calibrated to correlate a temperature with a certain level of radiation intensity. If a surface emitted perfectly, we could easily know its temperature based on how much radiation it emitted. The hotter it is, the more it radiates. But no surface emits perfectly!

From opaque (non-transparent) surfaces we see a combination of emitted and reflected. Because the reflected radiation is not related to the surface temperature, we must tell the imager to disregard that portion of what it sees. We do that by correcting for emissivity (E); reflection (R) then is 1-E.  We must also correct for the temperature being reflected. With these two pieces of correction information, the imager’s processor can determine an accurate radiometric temperature.

The correction, however, is not as accurate as we’d like it to be when the surface has an emissivity of less than 0.6 (approximately) or when the reflected temperature is extremely different that the surface temperature. This is true of all imaging systems especially when they are used in the real world versus a laboratory where all variables can be controlled. Clearly the proviso about emissivity means you cannot measure temperatures of nearly all bare metals—a fact that is, unfortunately, not widely reported in our industry.

When you need a high level of accuracy, especially on low emissivity surfaces, and where safety allows, simply apply a small piece of electrician’s tape firmly to the surface. Set the emissivity to 0.95 and the background correction to the temperature of whatever would be reflected if the tape were a mirror. You should be able to achieve an accuracy of +/-2C or 2% of the measurement.

I would urge you to try some simple experiments in the quiet of your office or kitchen until you are comfortable making the corrections and achieving a high level of accuracy.

Try and it out and let us know how it goes in the comments section.

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Thinking Thermally,

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

Heat Transfer by Radiation Part 2

For young children and pets, the first look in a mirror can be very confusing! They may be asking themselves. “Is that real or…?” Most thermographers share a similar exasperation the first time they see a thermal reflection, most commonly from a bright piece of metal. Many go on to understand that reflections are not reality, but many also continue to be confused about exactly how to deal with them.

All surfaces are reflective to infrared radiation to some extent. Bright metals are the most reflective. You can feel this by moving a piece of common aluminum foil very close to your face. Suddenly your face will feel warm! Why? The foil reflects your body heat back on itself rather than having your body radiate to cooler surroundings.

Most surfaces are not particularly reflective. Human skin is among the least so, but even we—regardless of skin color—are about 2% reflective. Most painted or heavily oxidized surfaces are between 10-20% reflective.

Window glass is somewhat reflective to infrared radiation and, more importantly, highly specular or mirror-like.

Glass and still water are two common surfaces that are very thermally curious. While they are only 20% and 5% reflective, respectively, because they are very smooth they can look mirror-like or specular. When polished smooth, any materials  like stone, tile, wood or even glossy paint, will appear specular even though they many not be highly reflective.

There are two easy ways to understand how reflective a surface is. First, while looking through your imager, simply move back and forth. If what you see changes appreciably, the surface is probably fairly reflective or specular. Next, heat the surface 10-20F above ambient temperature and firmly apply a piece of electrician’s tape to it. If the tape shows up clearly, the material is probably fairly reflective. Try this!

For all materials not transparent to infrared radiation—and fortunately that means most materials—reflection and absorption have an inverse relationship. Human skin  is both 2% reflective and 98% absorbing. Bright aluminum is approximately 95% reflective and only 5% absorbing.

Over the course of the next week, have some fun exploring which materials are reflective and which are not so reflective. See what you can learn. We’ll come back next week and discuss the issue of emissivity, in particular how understanding it is essential to making an accurate radiometric temperature measurement.

Thinking Thermally,

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

Heat Transfer by Radiation

For the past several weeks, we’ve been reviewing heat transfer. Thermographers must understand the basics if they are to successfully interpret their images. Over the next two weeks, we’ll wrap up the review with a discussion of radiation.

Electromagnetic radiation is not only a powerful mode of heat transfer, it is also the way energy moves from a surface to the detector of our imaging systems. We already know a great deal about many forms of radiation. Electromagnetic radiation travels at the speed of light, in a straight line and can most easily be defined—even if not completely accurately—as a “wave.”

Radiation is defined by its wavelength. We define various “bands” of wavelengths, such as ultraviolet, visible light, and long-wave infrared, based mainly on how they interact with materials. While each band or form differs in significant ways, some also share commonalities. Some materials are transparent to certain forms, like light passing through a glass window. Look in a mirror and you are seeing electromagnetic energy being reflected. Sit outside on a sunny day or next to a fire and your skin absorbs radiant energy making you feel warm.

My wife, sitting by the fireplace, enjoys the fact that infrared radiation is strongly absorbed by human skin.

When infrared radiation interacts with materials, interesting things also happen. Thin-film plastic, regardless of color, is quite transparent. Most bare metals, even if somewhat oxidized, are fairly reflective. And nearly all non-metallic surfaces absorb infrared radiation efficiently—no matter what their color.

For thermographers, the sun is typically the most critical source of radiation affecting our work. Sunlight is composed of a full spectrum of radiant energy rather than just light or infrared so it can behave uniquely. Outside, its energy is quickly absorbed causing surfaces to heat various degrees (no pun intended). We need to pay attention to the color of the surface because absorption of full-spectrum sunshine, unlike infrared radiation, does vary with color. A brown ceramic insulator in a group of gray polymer insulators will often be warmer in the sun. The dark trim on a light-colored house will be warmer. You can have fun on a sunny day seeing dark-colored letters heat up enough to be able to read a sign in a thermal image. On a cloudy day the words are typically invisible!

A “Think Thermally” t-shirt differentially absorbs the sun and the lettering heats up enough to be seen clearly in a thermal image. What would this look like in the shade?

Next week, we’ll review how reflection of radiation affects our work. It is one of the most important aspects of thermography we need to pay attention to, especially if we are making quantitative assessments and measuring temperatures.

Thinking Thermally,

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

More About “h”

As we continue the discussion about convective heat transfer, it is useful to define two types of convection, natural and forced. When quantities of fluids are moved in either way, we also use the term “bulk energy transfer” because it is really the movement of the fluid itself, and the energy inherent in it, that results in the transfer.

Using a blower door forces convection. The shape and location of the various cracks and holes in the thermal envelope can result in dramatic thermal patterns.

Natural convection is powered by the difference in temperature, and thus the density, of the fluids involved. As fluids are warmed, they become less dense, and, at the same time, are also displaced by more dense, cooler fluids. As a result, warmer fluids are pushed upward as the cooler fluids sink. The greater the temperature difference, the greater the convective movement, and the more energy transferred. A summer thunderstorm is a classic example of natural convection.

Forced convection involves a pump or fan or some other means of artificially moving the fluid. Forcing convection in this way typically results in a great deal of energy being transferred. Think of how quickly the air temperature can change when a sudden windstorm springs up.

As thermographers we have to watch for the effects of both natural and forced convection. Inside a building, as anfor example, you may see a difference in temperature between the floor and the ceiling of 3-5°F due to natural circulation patterns. Outside the wind can quickly change the temperature, typically by cooling, of the side of a building or a connector in a substation. If you fail to take these changes into account, you will not be able to accurately and fully understand the nature of what you are seeing.

Other variables (aside from velocity, which we talked about last week) must be considered as well. Geometric shape and orientation to the fluid flow can both have a large influence in the patterns we see.

I often notice this around windows. This is because windows are typically either a good deal warmer or cooler than the walls, and because they also project in or out geometrically. In the winter, I can watch cool air falling across the inside of a window to chill the floor below and in the summer the effect of air being warmed by the window can be seen on the ceiling above.

Of course architects recognize this fact and often put HVAC ducts near windows to mask over these effects of convection. Look at any active heating or cooling system and you will see the affects of disruption to the moving air by the surrounding surfaces.

A blower door forces convective airflow through small cracks and holes in the envelope and their shape and location determine what the resulting thermal patterns will look like. You’ll see similar patterns around active machinery where airflow from both from the machine itself as well as the HVAC system, what is termed “spurious convection,” can result in interesting thermal signatures.

A boiled lobster has a good understanding of convective heat transfer!

The consequences of not understanding convection or not paying attention to its effects can mean making mistakes in your understanding of what you are seeing. Take the time to learn the basics and then be careful as you apply that knowledge in the field. Most of what we need to know is firmly rooted in a common sense understanding so we can all be successful if we just apply ourselves.

Next week, we’ll talk about heat transfer by electromagnetic radiation. This is important to us both because of the effect on temperatures around us, but also because our imagers detect this kind of radiation.

Thinking Thermally,

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

“h” or the Convective Heat Transfer Coefficient

When heat transfer occurs in fluids—defined simply as non-solids—the rate and total transfer are governed by several factors, two of which are easily known: temperature difference and area. More challenging to define precisely is “h” or the Convective Heat Transfer Coefficient. This all-important variable is the amalgamation of a number of influences on heat transfer in fluids. Let’s look at the most important of them and how that might affect what we see with our imaging systems:

• Velocity of the fluid

• Orientation to the flow

• Geometric shape

• Surface condition

• Viscosity

All of these factors are important to consider, whether you are a thermographer working on buildings, industrial equipment or both. For most of our work, the fluid in question is the air surrounding us and the surfaces we are inspecting. The transfer of heat between the air and the surfaces can be considerable and can also result in patterns that can be confusing if we are not paying close attention to convection.

This remarkable image, taken by one of my students, show changes in wall temperature caused by air currents moving around and through fiberglass batt insulation in the walls of a home. Interestingly, the blower door was not being used to depressurize the home at the time!

Of the five major influences, velocity typically has the greatest affect. Whether we work out-of-doors, where we must pay attention to the wind, or indoors where air currents can have a dramatic influence, the velocity of the fluid has a powerful affect on “h” and, as a result, on total convective heat transfer. The relationship is direct: increase velocity and you increase convective heat transfer.

A simple experiment demonstrates this. Put your hand in the air and notice what it feels like. Now wave it back and forth several times. Notice you can literally feel the difference in heat transfer—cooling in this case—caused by the increased velocity of the air movement. You have, in essence, increased “h” simply by waving your hand.

If you look at the upwind and downwind sides of any building or any piece of industrial equipment influenced by air currents and you’ll see differences in surface temperature—both warmer and cooler—related to velocity.

I have found it important to measure the velocity of air movements so that I better understand their influence. This can be done very easily using any of the simple “personal weather stations” now readily available in the marketplace. Kestrel in particular, makes several fine, reasonably priced products that work well.

A personal weather station, like this Kestrel brand model, provides a simple means of quantifying both wind and air currents.

Convection is both a large and an important topic so next week we’ll continue the discussion about “h” and other issues related to convection. In the meantime, please keep your thermal eyes open and watch for thermal images showing the influences of this powerful mode of heat transfer!

Thinking Thermally,

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