- Materials & Industrial Technology
Myths, Mysteries, and Misconceptions About Infrared Surveys
The Myths, Mysteries, and Misconceptions About Infrared Inspections
A salesman needs to make sales, no myth, mystery or misconception there. Several years ago a chief engineer said that an infrared camera salesman claimed his IR cameras were simply point and shoot; you didn’t need training, settings, or analysis. He knew better from my previous debriefings, but when I heard it directly from a salesman last year all I could think of was Casa Blanca’s Captain Renault, speaking to Rick: “I'm shocked, shocked to find that gambling is going on in here!”
What is the myth being spread by these salesmen? What is the mystery of the camera? And, what are the misconceptions of how easy the camera is to use? We will examine these questions in these four articles.
The first myth is that the cameras are merely point and shoot instruments. Yes, you can capture an image, very nice images indeed this way, however, you want an accurate analysis of a problem.
There is the mystery. Is that bright yellow spot a problem or merely the effect of electricity being run through the device? The highly sensitive camera can take good pictures, however, without correct settings for background temperature, emissivity, distance, and so forth, you simply cannot conclude anything with certainty, and this is the misconception the camera salesman has made.
If a pretty picture is what you are looking for, then point and shoot all you like, but that isn’t what you are after, you require an accurate analysis of your equipment. To achieve this you must adjust for the background radiation, measure that radiation and the emissivity and adjust each. If you mistake an item that has a low apparent temperature when in reality it has a very high temperature but very low emissivity it could result in a serious fire or critical equipment failure.
If you want accurate readings from a technically capable infrared radiometer and a trained mind to make the analysis so your productivity can continue, and to prevent potentially fatal fires, then instead of buying a camera, as I have said many times, hire a thermographer. What you need is a trained mind that can analyze the images and compare them to the equipment in the image.
Allow me to reuse an old simile: When you need a plumber, don’t go to the hardware store and by a pipe wrench, hire a plumber.
When you need a thermographic analysis of your facility, don’t buy a camera, cameras don’t perform analysis, minds do, hire a thermographer.
Following the myth and applying misconceptions can lead to one more great mystery: “What happened to our productivity?” Or even worse: “What happened to our facility?” Equipment failures periodically start fires, big and small.
I was shocked to actually hear the myth coming from the salesman’s mouth. “You are talking about comparative measurements, qualitative measurements; you don’t need to worry about emissivity.” . I had been a certified thermographer for decades and had fought that myth from other thermographers, but never from someone selling the cameras, he should have known better. Do you believe this myth? What’s the great mystery about emissivity? What was this misconception? What are the other myths, mysteries, and misconceptions in thermography?
Condition Monitoring and Diagnostics
A Camera Salesman
I was commenting back to the infrared camera salesman who had claimed that for qualitative thermography you can ignore emissivity and background radiation. I replied, “Very well, let’s assume I have a well loaded three phase circuit where I see three warm wires, and only one has, say, a ten degree rise when compared to the others. How would you classify the problem using ISO assessment criteria, which calls the ten degree rise an Advisory level issue, would you be satisfied with that assessment without adjusting your emissivity?”
“Of course, that is the correct assessment for a ten degree rise,” the camera salesman said.
“But what if the temperature of these three were approaching the maximum safe operating temperature for the specific wire? Then the ten degree rise puts the one wire over the safe operating temperature? Have you considered that? And, without correcting for emissivity, or adjusting for background radiation, perhaps all three wires were in unsafe operating conditions. A week later the wire insulation burns away and your factory burns down.
As shocked as I was to hear his first statement, I must admit he was more shocked to hear my reply, and well, he should have been. I don’t know how many people in various industries and on ships he had sold cameras to; however, he then realized his culpability and that of the manufacture he represented in telling people the cameras were simply “point and shoot.
He had forgotten to include the necessary specialized training needed to interpret the results. Emissivity is the surface condition of all solids. This determines, along with its temperature, how much of the thermal energy that reaches its surface, emits from the surface and how much remains in the mass by reflecting.
The complementary number to emissivity is reflectivity. This also is true of the surface condition and so is the same in both directions. So at any given frequency of light, and any given temperature of the solid or liquid mass, the percent of that light incident onto the surface is reflected off that surface. The angle of incidence equals the angle of reflection. Therefore, if you as a thermographer are perpendicular to an object some of your energy will reflect back into the camera.
Since these two are additive to a thermal imager, your reflection will always look warmer. Not a great problem if you recognize this reflective light you merely move out of the way. However, you have not altered the condition of the surface and made it less reflective. It is now merely reflecting something else, and that is the background radiation you will see and measure if you do not compensate for it. The question then is how do you measure background radiation? That is another story.
Perhaps hiring a thermographer is what is needed. This is no mystery at all.
I was on the USS Nimitz riding down from Seattle to San Diego when the seaman with me, tasked to assist in finding equipment to inspect, showed me a motor which was not running and asked if I could see anything with my infrared camera. While it should have been obvious that I could not, less obvious is looking at equipment operating at less than half power. What can a thermographer tell from this? Is this an efficient use of a thermographer?
The seaman asked if we needed power to equipment we inspected. Others have asked how much power we need to analyze equipment. I have needed to inspect very important equipment which could not be fully powered, at lease while I was present, and seen a small rise in temperature. I explained to the Chief Engineer why this was classed, not as the software wanted to classify it, rather as a high priority repair, but why?
People seem to think the level of power running through a circuit is unimportant in thermographic analysis as if the camera had some mysterious power? However, the same people are familiar enough with Ohm’s Law to remember that P=I(IxR), or the power is equal to the current squared times the resistance and ought to conclude that if the power is at 50% the heat generated from a problem is four times that if the same system at 25% and 1/4 that if it were at full power. It is called the square law. Basic math tells you if the system is running at 25% then the heat from a problem will not be less than one sixteenth that when run at full power.
You also need to know the specific heat of the components and the path the heat travels to reach the IR Radiometer (“IR Camera”).
Look at the casing of a motor with an overheated bearing and imagine how far the heat has traveled and though what to reach you. Heat travels in three dimensions, some of that travels through a poor conductor, the grease surrounding it, some conducts through metal, much of it goes elsewhere.
Now consider a large box circuit breaker where the hot contact radiates heat at a specific emittance into an air pocket. In that some energy heats the air and some energy is absorbed by the Bakelite casing, conducted to its surface and then emitted again, quite the convoluted path.
Not to belabor the point, but even if you are looking directly at the hot contact, what is its conductivity and emissivity at the specific temperatures as it heats up according to Ohm’s Law? If you have direct sight of an overheated copper contactor, Ohm’s Law is likely all you need to consider, if not, then you need to think about these other factors.
I recommend using ISO assessment criteria, and the difference between, say, a connector under 25% load at an 8C rise in temperature (an ISO “Advisory” issue) to running the equipment at full power according to Ohm moves this assessment to a problem requiring immediate attention.
Most thermographers insist there be at least 50% power for five minutes before an inspection. This allows for improper resistance (i.e. problems) to generate heat and that heat to build, then conduct, convect and emit to the imaging device so the thermographer can see the issue at hand and correctly analyze the equipment.
Without this, you cannot achieve a correct reading or analysis.
Some untrained people, even some trained thermographers believe the myth that you can use the same assessment criteria to rate the severity of a problem both on direct and on indirect images, that is, both where you are actually seeing the affected component, or where it is masked by some other device, configuration, or insulation. I have seen this done many times where mysteriously, no compensation is made for indirect measurements and yet, a significant number of major issues are being assessed too low for the true problem risking critical equipment failures. What should be done in these cases? One more reason you ought to hire experts, thermographic analysis is not do it yourself.
Assessment criteria for thermographers are there for a reason, no mystery there, however, you need a to determine which assessment criterion to apply in each case. This takes knowledge and experience. It is a myth that, once assessment criteria are adapted, they are directly applicable to all items being inspected.
The subject here is the difference between direct and indirect measurements and why you should not use the same assessment criteria to both, rather, you need to consider how the thermal energy arrives at your imaging device, what is the path it took to get to your camera?
In a direct measurement there is little or no distance or thermal insulation between the IR camera and the subject being imaged. In short, you are looking at something that is the actual problem. Let’s look at an overheated contact and, let’s say it appears to be 15C above another connection, ISO assessment criteria (ISO 18434) states this as an Intermediate Problem.
The next problem looks identical, the same temperature but you notice the side of the circuit breaker is hot, so this is an internal contact some distance from the connection which appears hottest. What criteria should you use? Is this a serious issue or not? Remember this is an indirect measurement where there is considerable distance and thermal insulation, from an air gap and then a plastic box before the thermal energy emits to the infrared imaging device.
Similar things should be treated similarly. However, these are dissimilar issues even though the temperatures seen by the camera are the same. You must treat them differently and use your to understand what a reasonable assessment appliclation would be.The correct assessment is critical to understanding how soon the problem needs to be addressed. An intermediate problem can be addressed at the next scheduled maintenance. However, in the case presented, an overheated internal contact, the temperature may be 50C above the apparent temperature, or, say, 65C, a critical problem that needs immediate attention.
If a similar problem arose in a bus duct the internal temperature may be 265C, already causing serious internal oxidation of components and well on its way to causing an explosion or fire. Lastly, and more commonly, the casing around a motor indicates a similar temperature on the bearing. Once again, the heat is being generated internally by bearings which are desiccating the lubricant and poorly transferring heat though the grease to the metal, then through the painted surface. That same 20C rise in temperature can mean bearings that are 70C, and, once more, demands immediate attention before the motor bearings fail and loss of functionality of that equipment is experienced, the motor needs replacing, or worse.
Understanding direct and indirect measurement and adjusting assessment criteria is critical to proper assessment in this important nondestructive testing method. Without this understanding, you have little hope of reducing faults and equipment failures.