How to Guarantee Your Failure as an Infrared Thermographer چگونه عیوب حرارتی شناسایی شده توسط دوربین ترموگرافی را به عنوان یک ترموگرافر حرفه ای تضمین کنیم ؟ Ronald Lucier ABSTRACT The usefulness of thermal imaging in Predictive/Diagnostic Maintenance (PDM) has been demonstrated and documented for over twenty five years. Practitioners in the trade generally fall into two categories – heroes or goats! This paper highlights ten common activities that if not properly addressed can lead a thermographer down the path to failure. By recognizing and avoiding these pitfalls the paths to success will be much clearer. فعاالن تااری در این زمینه االاا به دو. سال می باشد52 کاربرد ترموگرافی در تعمیرات پیشگیرانه یا پیش بینانه دارای سابقه ای بالغ بر بیش از عنوان مهم در بازرسی ترموگرافی می پردازد که در صورت عدم01 این مقاله به برجسته سازی. دسته قهرمان و بازنده تقسیم بندی می شوند با رعایت این اصول بنیادی درصد موفقیت و تشخیص دقیق عیوب افزایش می یابد. توجه به آنها باعث بروز اشتااه در تشخیص اپراتور می گردد . Keywords: Quantitative thermography, qualitative thermography, corrective action, risk, image subtraction, IRNDT تست غیر مخرب ترموگرافی، تحلیل تصاویر مادون قرمز، خطر، اقدامات اصالحی، ترموگرافی کیفی، ترموگرافی کمی: کلمات کلیدی THE DISTANCE BETWEEN SUCCESS AND FAILURE CAN BE A FEW MICRONS The successful track record of infrared thermography as a predictive maintenance tool is well established. What isn’t so well established are programs and efforts that have not been successful. Over nearly twenty years of experience I have seen both the successes and failures. The failures seem to have many things in common. In fact there are ten common attributes. Identifying and avoiding these ten attributes may not guarantee your success. Ignoring or not recognizing some of these attributes however will probably guarantee your failure. TEN CRITICAL PITFALLS 1. Over Emphasizing Temperature Measurement Ever since the introduction of radiometric systems there has been a tendency to rely on temperature measurements as an indicator of component operability. NETA1, and others have generated “equipment severity ratings” based on temperature. Therefore, through training and subsequent research, infrared thermographers have become accustomed to providing temperature measurements in their reports. There should be several key questions you ask yourself about temperature measurements: ! What is the relative accuracy of the measurement? ! Could I defend this measurement in court, against an Expert Witness? ! Does the temperature measurement provide meaningful information? www.deltasanat.com 021-77515138-9 It is important to note that Infrared Cameras do not measure temperature. Radiant energy is measured (Watts) and from that value and your inputs temperature is calculated. Your “measurement” accuracy is directly related to the accuracy of the camera calibration and the accuracy of your inputs for emissivity, background temperature and any applicable environmental parameters. This also assumes you have the skill to acquire an image in focus and within the spot size ratio of your system. Potentially there could be liability for your measurement. Trial lawyers search wide and search deep for anything that could help or hurt their client’s case. If you report a temperature, particularly for a client other than your employer, be sure of your results and express the uncertainties in your measurement. A larger issue for “temperature measurement” is whether that measurement is meaningful or not. The two thermograms below show a battery and cables. There is a 9 ºF difference between the good battery stud and the one overheating. The absolute temperature of the bad stud is 119 ºF, +/- 3ºF. The melt point for Lead is 620 ºF. Therefore under most common severity guidelines this would be classified as a MINOR problem. The important information regarding this component is not necessarily the temperature but the electric current (load) running through it. The stud is hot due to high resistance. As this is a DC application, the power dissipated at the connection can be calculated using the equation P=I2R. At the time of measurement the battery was being trickle charged at 60 ma (0.06 Amps). The battery was rated for 200 amps. Ignoring the fact that the resistance and heat www.deltasanat.com 021-77515138-9 transfer will increase with temperature, a simple ratio of the power dissipated, at a load of 200 Amps the connection would dissipate about 11 Million times the power it is in the image. Obviously, this would cause a failure. Thermograms of battery and cables Lesson Learned The important point here is that this is one example where the actual temperature measurement or temperature rise measurement is nearly meaningless unless the current is also noted. Reporting just the temperature and acting based on that temperature could have resulted in a battery fire. The load (current) is crucial in electrical system analyses. 2. Ignoring Temperature Measurement OK, so there could be big problems with temperature measurements. Maybe it is best to do everything qualitative? Let’s look at one example. 108.2°C The thermal image of a breaker with a hot conductor to the right is a classic example of where qualitative thermography can lead to errors. As the conductor temperature is highest at the connection point, the assumption of a loose/corroded connection may be a good one. Therefore the typical PDM report may state that the corrective action is to clean and tighten the connection. In fact this may solve the connection problem but is the connection problem the whole story? In this case a secondary problem arises from the fact that the conductor temperature is 115 ºC, at or above the temperature limit on the cable insulation. This would result in the corrective action of cleaning/tightening in identifying the more serious issue – the insulation has degraded to the point where new conductor must be pulled to correct the problem. In this real life example the insulation was hard and broke off when the “connection” problem was to be fixed. A quick repair turned into an major effort and considerable equipment down time. Lesson Learned Quantitative thermography is essential when equipment temperature limits are approached. There are many other examples where quantitative measurements are required such as diagnosing steam traps, safety relief valves or determining corrective action priority. The important point here is BALANCE – look at what you are looking at, how important it is and based on that knowledge determine whether qualitative or quantitative thermography is necessary. This is not as obvious as some may think. When you are unsure, get help. Get the entire PDM team including engineering, operations, management and maintenance involved. A wrong decision by a group is “a learning experience.” A wrong decision by an individual is a “bad call.” 3. Crying Wolf! It should be obvious that extreme care must be taken when determining corrective action priority. It should also be apparent that temperature alone cannot always be the final decision maker. The danger is that unnecessary repairs or www.deltasanat.com 021-77515138-9 100 80 60 40 25.8°C www.deltasanat.com 021-77515138-9 shutdowns will be asked for or that no action is called for and the component fails. Either way, the infrared thermographer becomes the goat and most companies sacrifice goats! ITC is aware of nearly a dozen corrective action guidelines, almost all relating to temperature. None of these are universally applied to mechanical and electrical equipment. In fact as demonstrated above a temperature based corrective action guideline can easily yield undesired results. A concept that can be universally applied to all corrective action guidelines is to assess the “Risk of Failure” for an individual component. The definition of risk is: RISK = (probability of failure) x (consequences) The problem here is that every equation requires finite numbers to plug in to that equation. Unfortunately neither the probabilities nor the consequences are easily quantified. Probability of failure In mathematical terms the entire range of probability ranges between zero (will not happen) and one (will happen). To quantify this number for every application would require a statistical analysis of hundreds of thousands of failures under known conditions. However, using your own temperature data, prior equipment history and perhaps intuition, the probability of failure can be broken into LOW, MEDIUM and HIGH categories. Likewise the consequences. What you end up with can be placed in a matrix. PROBABILITY OF FAILURE Based on: Temperature Temperature Rise Past History Industry Experience Intuition CONSEQUENCES Based on: Equipment Damage Fire Potential Personnel Injury System Shutdown Availability of Parts Scheduling Lesson Learned PROBABILITY OF FAILURE C LOW O N LOW Q MEDIUM U HIGH NORMAL ACCELERATED REPAIR NEXT REPAIR REPAIR EQUIPMENT SCHEDULE OUTAGE ACCELERATED REPAIR NEXT REPAIR AS REPAIR EQUIPMENT SOON AS SCHEDULE OUTAGE POSSIBLE REPAIR NEXT REPAIR AS REMOVE FROM EQUIPMENT SOON AS SERVICE OUTAGE POSSIBLE ASAP S E MEDIUM E N C E HIGH S In a perfect world action criteria would be based on hard science rather than a subjective determination. However, that is not the case, but by utilizing your own experience, knowledge and intuition, a better method of corrective action priority should yield better results. www.deltasanat.com 021-77515138-9 www.deltasanat.com 021-77515138-9 4. Misidentification of Components The many hats that infrared thermographers have to wear includes the hat that says “EXPERT”. Whether you like it or not you are considered the expert in whatever you are looking at with your IR camera. Whether you really are or not may be discovered by reviewing your reports. Choose from the two below. Which thermographer do you think needs some education? 49.4° C 49.4° C 48 48 46 46 44 44 42 42 40 40 38.6° C 38.6° C IR Text Comment Value IR Text Comment Value Equipment Black Box Equipment Breaker for P-38 Pump Item Wire on bottom Item Load side conductor, B Phase Fault Wicked Hot Fault Lug is 7 ºC hotter than adjacent lugs Recommendation Fix It Recommendation Inspect for damage, clean, tighten Re-inspect after repair It is very tough for a non electrical person to identify electrical components. There is no excuse for not correctly identifying components yet this is done every day. Sources for the correct identification of components include plant personnel such as mechanics, electricians, technicians as well as engineers, management, equipment operation manuals and the industry groups on the internet. Lesson Learned You should be able to produce a professional looking report with nice visual and infrared pictures. You should also correctly identify the components you inspected. If you don’t know what it is called, find out! 5. Not Having an Open Mind I have been promoting the concept of IRBWA (Infrared By Walking Around) for many years. What this means is that while you have a thermal imager in your hands, take time to look at as much as you can. We live in a thermal world and there are all sorts of interesting things out there, some of them important! Much of what you see may not have much relevance. Some of it will. I was with an electrician in a manufacturing plant not too long ago when he took the IRBWA concept to heart. He located and reported a bad bearing on a conveyor belt roller. A simple task to many of us but to this electrician it opened up a whole new world to him. Some of the images you obtain with IRBWA can be very interesting. Some even useful! There is a downside to not practicing IRBWA. Everyday that you use an Infrared Camera you should be increasing your own knowledge base. A narrow or limited knowledge base limits your experience and may limit your future in infrared thermography. Lesson Learned Unless your work rules prevent it, keep the camera running! You never know what,s lurking out there. www.deltasanat.com 021-77515138-9 www.deltasanat.com 021-77515138-9 6. Not Understanding the Science It has already been noted that Infrared Cameras do not measure temperature. There are organizations that know this but make temperature measurements with a fixed value of emissivity. If your program is set up to fix everything that is found, regardless of temperature or temperature rise, this is ok. If your program is set up to react based on a temperature or temperature rise you should know the science and get the temperatures as accurately as possible. The value that the infrared camera uses to calculate the temperature is the radiated energy from the target surface. Once that is known, a simple Physics model based on the Stefan-Boltzman Law ( W = !"T 4) is used to determine the temperature. Before the calculation occurs, several corrections are made. Detected Radiance minus measured internal camera equivalent temperature (due to electronics, case, lens) Plus Energy lost through the lens (transmittance of the optical materials) Plus Energy lost through the atmosphere (distance, humidity, atmosphere temperature) Minus (1-emissivity)(Tbackground temperature) this is Tamb in our systems and is the reflected temperature Finally divide by the emissivity. This is the value used in the calculation above to calculate temperature The net result of this is that if you are expecting a correct measurement you must put in correct values for emissivity and Tamb. The distance, humidity and atmosphere temperature have a larger contribution in shortwave (3 to 5 µm) than longwave (8 – 12 µm) systems. Putting the wrong values in every time will yield the wrong measurement out every time! Lesson Learned Understand the science and you will be able to defend your measurements or your decision to not make a measurement. 7. Not Understanding IR Limitations Beyond misidentification and errors in emissivity, the most common problem we see in infrared reports is being too far away to make a measurement. The infrared camera utilizes the detected radiance values over a certain number of pixels to calculate the temperature. It is the job of the camera operator to make sure the target is physically or optically close enough to make this measurement. In other words, know your Mfov (measurement field of view) and Spot Size Ratio (1/ Mfov ) An additional concern is low emissive targets. Whenever possible, particularly when electrical equipment is deenergized, apply a high emissive material such as opaque electrical tape to the equipment. This will give you a highly emissive target (the tape) at a known emissivity from which to make your measurements. Lesson Learned There is a huge abundance of technical information provided by IRTECH, ITC and others. This material is worthless if it stays on a shelf and collects dust. Periodically review any relevant technical information and specifically review your camera operating manual (or read it for the first time…). 8. Not Understanding IR Capabilities One of the least utilized capabilities of Infrared Cameras is the ability to analyze objects on a time dependent basis. This can be accomplished by recording live events (starting a component, filling a tank, mechanically loading a structure, etc) or taking a series of pictures and applying the techniques of image subtraction. Analyzing live or recorded events allows you to plot temperatures as fast as the infrared camera can store them. This can be as fast as 900 frames per second with the FLIR SC-3000, up to 30 frames per second with the SC-2000 series (includes 695 with the digital board) and a frame grabber, up to 5 frames per second through the PCMCIA card on a lap top computer or up to as many seconds, minutes, hours as you need. Geometric registration (fixed camera viewpoint) is essential. www.deltasanat.com 021-77515138-9 www.deltasanat.com 021-77515138-9 The speed that a process needs depends largely on the temperature difference, the amount of heat flowing through the object and a heat transfer property called “thermal diffusivity”, which is basically how fast heat conducts through a material. All of this forms the basis for Infrared Non-Destructive Testing (IRNDT). In the IRNDT case the basic principle is to flow a uniform amount of heat into a surface and see what happens to it. If it appears uniform out of that surface then the material is uniform inside. If not, a non-uniformity may exist due to a variety of reasons (de-lamination, voids, inclusions, etc). Image subtraction is the process where two thermograms, geometrically registered, are obtained and one is subtracted from the other. This process results in a “difference thermogram”, showing how the individual pixels changed in temperature. This is commonly used to analyze changes on the surfaces of semiconductors and/or printed circuit cards. Start of sequence End of sequence 23.4dF 23 13 22 11 21 9 7 S28 20 34 45 23 1 12 5 19 S1 18 18.0dF Difference Thermogram Surface Plot of Difference The images above are from the warm side of a simple Peltier Cooler. The first image is one second after the application of power. The second image is ten seconds later. Obviously the amount of heat removed from the opposite surface is evident. The third image is the difference thermogram – Image one minus Image two. What was not apparent from the first image is that there are several relatively hot pixels. In fact a Peltier Cooler should be expected to be fairly uniform in temperature. This one was not. Image subtraction provided the answer! The difference thermogram and the Excel surface plot show the non-uniformity that is hard to see in the end of sequence image. What is also apparent is that this technique subtracted out the background effects – only the Peltier cooler is visible. Lesson Learned There are other capabilities, particularly spectral filtering, that are under utilized. Keep and Open Mind about what your camera can do! 9. Over Reaching An infrared camera has more uses in industry and science than probably any other device yet invented. Unfortunately its proper operation is bounded by the Laws of Physics. The Infrared Thermographer must balance www.deltasanat.com 021-77515138-9 www.deltasanat.com 021-77515138-9 his or her enthusiasm (and perhaps the need to generate an invoice) by these Laws. Don’t over reach your (or the camera’s) capabilities. One example of “over reaching” is the attempt to locate a ghost. The Learning Channel and other television shows have hunted for ghosts with psychics, acoustic instruments, ultrasonic instruments and infrared cameras. The results have been very conclusive – infrared cameras do not work for this application. Again, it is those darn Laws of Physics. Infrared cameras detect energy emitted from solid surfaces. By definition a ghost has no solid surface so how could an infrared camera detect them? One television show indicated that a ghost must have been present as a change in room air was detected. Next time you are in an air conditioned hotel room, look for the cold plume of air coming out of the air conditioner. Or that draft in your house. You see the walls, ceiling and floor thermal patterns but not the air as the solid surfaces are very small and very far apart (definition of a gas). Perhaps if we could utilize a spectro-radiometer and determine the spectral characteristics of the specter (snicker) we could design a filter…. Better yet, stick to looking for poor insulation in houses. Lesson Learned There are ample uses for infrared cameras beyond routine PDM applications. 10. Being Complacent All right, you have many years of experience, have been to many training courses and wonderful conferences like InfraMation and you regularly interface with your peers. You must “Know It All”. Think again. Ask the most experienced person you meet at this conference if they “Know It All”. They will answer NO. Infrared Thermography is a science with many, many applications. It is nearly impossible to know and be competent in all aspects of this science. Even if you stick with one discipline, do not be complacent. There is still much to be learned about electrical, mechanical and roofing applications. If there wasn’t there would be no point in attending InfraMation! Lesson Learned Never stop learning and never take what you see for granted. Catastrophic equipment failures can and will occur despite your best efforts to identify them beforehand. SUMMARY This paper describes ten things that can lead you down the path to failure. Obviously we do not want anyone to fail but to rather be an enormous success. While there may be more than these ten paths to failure there are only three paths to success: People Hire the best, train with the best and interface with the best. Remember, attending a training course or two is not the end of your education but only the beginning. Resources Know what resources are available to you and utilize those resources. Keep in touch with your infrared camera manufacturer, trainer, customers, co-workers and frequently review any available technical documentation. Technology Acquire the best technology you can and make sure you understand how to operate it! REFERENCES 1. ANSI/MTS-2001, “Maintenance Testing Specifications for Electric Power Distribution Systems”, NETA InterNational Electrical Testing Association, (2001) www.deltasanat.com 021-77515138-9 www.deltasanat.com 021-77515138-9
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