- Buyer's Guide
Infrared thermography, typically referred to simply as IR, has been around for a number of decades. It is a staple of condition monitoring (CM), often the entry point for newly minted reliability programs, and is even widely known outside of the CM and condition-based maintenance (CBM) world. However, like any tool in your toolbox, it has its limitations. Understanding those limitations is the key to avoiding the most common traps associated with the application of IR. There are a number of common traps, but in the interest of time, and the generally reduced attention span of most folks these days, we are going to focus on the top five.
Before doing that, let’s talk about the history of infrared thermography. Outside of reliability, most people associate IR with military and police applications. Sadly, for those of us who utilize IR in our professional lives, the military and police capabilities are usually exaggerated in movies and television. There are still otherwise informed people who believe that IR can see through walls and such.
Interestingly enough, it is the U.S. military that is credited with getting the ball rolling with applications for infrared thermography detection. In the late 1950s and early ’60s, sea, surface and airborne detection systems that utilized IR technology were first developed and deployed. In the late ’60s, the first commercially available IR systems were in use. They were not at all portable and quite expensive. Innovation led to changes in the size and types of components used in detection systems, with handheld imagers emerging in the ’70s and ’80s.
The biggest change came in the mid-1990s with the development of the focal plane array (FPA) IR detector. Instead of using a detector that had to be cooled, either by the use of liquid gases or onboard compressors, FPAs could be uncooled and were a fraction of the size of the detectors used in the earliest handheld imaging systems. Their cost was significantly less as well. Today’s modern IR imagers are orders of magnitude more powerful in their detection and display capabilities, and cost pennies compared to the first commercially available systems.
Infrared thermography imagers detect a portion of the electromagnetic spectrum that is invisible to the human eye. The portion of the electromagnetic spectrum that is generally considered to comprise the infrared radiation spectrum is energy with wavelengths from 0.75 microns to 300 microns. To refresh your memory, a micron is one-millionth of a meter. The average human hair is approximately 100 microns in thickness, for comparison’s sake.
IR imagers are passive instruments. They detect energy that is leaving surfaces. Any surface at a temperature above what is known as absolute zero (minus 273.15 degrees C) emits IR radiation. The higher the temperature of the surface, the greater the amount of energy that is emitted. Since no surfaces are perfect emitters (in terms of IR radiation), all surfaces will also reflect IR radiation that comes into contact with them. The infrared thermography imager does not know the difference between what is emitted naturally by a surface in relation to its temperature and what is being reflected from a surface. IR imagers detect what is known in physics as total thermal radiosity.
Imager ease of use and a relatively low cost are why infrared thermography is utilized as widely as it is in reliability and condition monitoring. The mystique surrounding the capabilities of imagers is what leads to some of the more common traps associated with their use. It is important to have at least a rudimentary understanding of how imagers work to avoid falling into these traps. As previously mentioned, there are plenty of traps associated with the application of infrared thermography, but this paper will cover the top five.
There are limitations to what an infrared thermography imager can detect (see) and quantify (measure). The ability to see and measure spots of a particular size is driven by a combination of imager resolution and optics. Applying telephoto lenses can help see and measure spots at greater than optimum distances, but many imagers are not equipped for their use.
It is important to understand the resolution of the imager you are using to ensure it is suitable for the task being attempted. The resolution for any imager on the market today can be found in its specification sheet. The most common resolution values are 320x240 and 160x120. Some imagers have resolutions as high as 640x480. These numbers represent the physical number of elements found on the FPA detector. Higher numbers equal higher resolution. An imager with a detector density of 640x480 has 307,200 individual detector elements on its focal plane array. The higher the resolution, the greater distance an imager can see and measure spots.
Conditions such as wind, precipitation, sunlight and air temperature variances can all play a part in the attempt to measure temperature. Obviously, inspections must occur on a schedule. As reliability professionals, we cannot control the weather, and even the meteorologists are not all that great at predicting it. The best we can do is take note of it and make sure we account for it in our reporting.
However, extreme conditions such as high wind must be avoided. A wind current of 15 miles per hour can reduce the delta between two surfaces by as much as half. Inspecting a system on a day with high wind could result in missing an anomaly that would be visible if the wind currents were lower.
Also, direct sunlight can heat surfaces above their normal operating temperature. Think about touching a dark painted car in the sunlight. Outdoor surfaces are best inspected when they are not in direct sunlight. That is not to say they must be inspected in the dark of night. Early in the morning when the sun is not directly over the surface being inspected is often a good time.
Another factor that plays a role is ambient temperature. The lower the ambient air temperature, the more quickly a surface will cool. Therefore, if comparing items, make sure they are measured at the same approximate air temperature.
One of the more popular descriptions used for thermographic inspections is “looking for hot spots.” Well, that is true for sure, but occasionally the problem is not hot. In electrical systems, current generates heat. There is normal heat and abnormal heat. An electrical system in use will be inherently warm, with some components, depending on their mode of operation, being downright hot. In mechanical systems, friction and compression generate heat. Again, there is normal heat and abnormal heat.
When inspecting electrical power systems, items like power factor correction capacitors should be warm. If you see a cold one, there is no current flow through it; it might be the bad actor. When inspecting steam traps, one that is cold might not be cycling. Sometimes the cold item is the one causing trouble.
One of the more common misconceptions about infrared thermography imagers is that they are extremely accurate at measuring temperature. Unfortunately, this is just not true. Surface conditions such as the emissivity of the surface, ambient conditions like the aforementioned sunlight and wind, and the resolution of the imager used will all have an impact on the ability to be precise with temperature measurements.
The best advantage to be had with the application of infrared thermography is that imagers give a visual representation of surface temperature. When inspecting components within any system, a very simple rule applies. You want to focus on the difference in the thermal pattern, not so much the apparent temperature. Think Sesame Street. There was a segment of the show where three kids would be shown doing one thing, like bouncing a ball, and a fourth kid would be doing something different, like jumping rope. They would even play a song, “One of these things is not like the other …”
The same is true with infrared thermography inspections. When comparing like components, the one that looks different is probably the one causing trouble. Often, the temperature is of secondary concern. The visual representation gives you all the information you need.
IR is an easy and relatively inexpensive technology to employ. Unfortunately, many CM programs start and end there. As remarkable as it is, infrared thermography cannot find every failure mode at its earliest stages.
Other technologies, like vibration analysis, motor circuit analysis and lubricant analysis, also yield outstanding results. Your best advantage is in combining technologies. Applying reliability processes like criticality analysis and failure modes analysis to determine how to inspect and with which technologies will help you find failure modes in their earliest stages, which is what reliability is all about. IR can be used to validate the findings of other technologies and vice versa. It also helps to communicate, not just within the CM group but across the facility. If you do not champion your own program, certainly no one else will.
Now that you are aware of the common traps of infrared thermography, it is time to get to work.