Successful infrared thermography
Published: 05 April, 2016
Some suggest it can take years to become an expert in infrared (IR) imaging. However, to get there a bit quicker, FLIR has identified 10 critical (and common) thermography pitfalls that are well worth avoiding in order to ensure the capture of successful thermograms: over-emphasising temperature measurements; ignoring temperature measurements; not assessing the risk; misidentifying parts; not being open-minded; not understanding the science; not understanding the limitations of IR; not understanding the capabilities of IR; over-reaching; and complacency. PWE reports.
Over-emphasising temperature measurements
Qualitative thermography has a strong role to play in IR imaging success, such as knowing whether it’s the temperature or the thermal pattern that’s most important. FLIR explains it has encountered many examples of this over the years. A notable case involved a large, 45 kg battery that despite only being under the influence of an 85 mA trickle charge, was showing high heat. The thermographer was trying to measure the temperature, but in reality, there was no need as it was plain to see that the internal resistance was exceptionally high. In fact, the battery was almost a short circuit – if the breaker had closed it would have failed catastrophically.
An experienced thermographer knows when to spend time measuring temperature, and when not. This was a clear, near-critical failure that demanded immediate attention without the need for temperature measurements.
Ignoring temperature measurements
While some thermographers want to set emissivity to 1.0 and do everything qualitatively, this isn’t advisable. FLIR explains it knows of many instances where this tactic proved to be a failure. In one conspicuous example, a crimp on a VFD (variable frequency drive) was simply reported as ‘hot’ following an IR thermogram. However, had the user of the IR camera taken the time to measure the temperature each day against a pre-defined baseline, he or she would have seen that it was increasing.
Unfortunately, in this instance, the crimp failed before the manufacturer could replace it, incurring unnecessary downtime and cost. Put simply, part of the idea behind a baseline survey is to establish conditions ‘today’ that can be compared with past and future. Remember, temperatures are better comparisons than a subjective interpretation of a colour palette.
Not assessing the risk
Any electrical connection showing heat can be deemed a failure; the question is, what type of failure: minor, intermediate or critical? The vital factor to consider here is risk, which can be defined as the probability to failure multiplied by the consequences.
The probability of component failure can be affected by: the operating temperature of the part and its environment; temperature rises; past operation history; individual experience of the thermographer; and the intuition of experienced repair people. In a similar vein, the consequences of failure can be based on everything from equipment damage, fire potential and personal injury, through to system shutdown, availability of parts and scheduling.
In short, a low probability of failure with low consequences will mean little more than a normal repair, while a high probability of failure with high consequences will demand immediate removal from service, with every other scenario between.
All professional thermographers need to make sure that their reports are checked sufficiently so that all components have been identified correctly.
Thermographers are often expected to take IR images of many different machines, systems and structures. No one can be an expert in them all, so it’s easy to make mistakes. In the event of uncertainty, always ask.
Not being open-minded
There’s little doubt that taking IR images requires an open mind. Any thermographer who flashes their gaze across an image without proper care and attention will simply think there is nothing amiss and move on.
This can be a dangerous strategy. There is an amazing world of thermal things to find in every IR image. The advice is always take the time to look, and keep an open mind.
Not understanding the science
According to FLIR, there are thousands of thermographers who know precious little about the science behind IR images – they simply take pictures. However, few would want to end up in court against an expert witness should something go wrong. A thermographer of this nature would be unable to defend their temperature measurements sufficiently.
By way of example, how many know that there are three sources of energy that reach the temperature detector: the camera’s internal and external optics (IR window); the atmosphere (distance, air temperature, humidity); and the target (emissivity and reflectivity)? All these must be considered to provide accurate temperature measurements.
Fewer still may be aware that emissivity is not always the most important property, such as when imaging furnace tubes where the target surface is lower than the T-reflective environment (due to T-reflective radiation). Put simply, understanding the science means the application becomes easy.
Not understanding the limitations of IR
An IR camera can do many things but it cannot, for example, see through walls. In further examples, a bolometer detector used in many IR cameras cannot image a fast-moving object, while an InSb detector cannot easily image an object with a large temperature range.
Ultimately, it’s important to define the measurement and imaging mission before selecting the best camera to use.
Not understanding the capabilities of IR
Some IR cameras can target below 4 µm in size, at over 2000 frames per second. At the other end of the scale, certain IR cameras offer the capability to view low Earth-orbiting satellites.
These are exciting times for the IR industry, where cameras have become far more affordable and software capability has soared. As a result, the list of applications seems to grow constantly, particularly in industry and science. However…
IR cameras have been put to some questionable uses over the years. They have even been known to make appearances on so-called ‘ghost hunter’ television programmes.
Here, the user of the IR camera will see the colour of the room change, which they claim proves the presence of a ghost. The reality is of course somewhat different. Any experienced thermographer will be aware that this is simply the effect of bolometer-type camera drift witnessed frequently at near-ambient temperatures.
According to FLIR, no one should consider themselves a complete expert in IR imaging. Put simply, thermographers looking to guarantee their success and not their failure, must avoid becoming complacent, and instead become eager to learn more.
For further information please visit: www.flir.com