Heat of the moment

5 mins read

Thermal cameras have grown in popularity as fast as prices have fallen. But, while their inspection power is immense, there are issues for the unwary, warns Brian Tinham

There is no doubting the power of thermography when it comes to inspection and maintenance. Few single-parameter techniques can rival infrared for its ability to effectively reveal such a wide range of problems – mechanical and electrical – with plant, factory equipment, even structures and entire buildings. Given thermal cameras' capabilities, few also are as apparently easy to use – particularly with recent improvements, such as 'picture in picture' (infrared and visual image alongside) and now Flir's patented MSX, which overlays visual detail on the thermal image in real time. And, to cap it all, thermography is entirely non-contact and non-intrusive – as long as panels are equipped with infrared windows. In short, thermal imaging cameras encapsulate just about everything anyone could ever want from an inspection tool. What's more, in Flir's case, target images can now be 'linked' via Bluetooth to readings from conventional instruments – such as DMMs (digital multimeters), clamp meters and moisture meters – to confirm the cause behind the detected temperature symptom. Think about switchgear: a clamp meter displaying power at equipment showing hot spots gives engineering all the information it needs. No wonder, then, that what were once expensive niche instruments aimed at the few specialist thermographers have morphed in recent years into mainstream tools. That's what happens with electronic technology when kit hits the spot. Today, versions are available for everyone – from individuals wanting a bit of fun or functionality on their iPhones, to technicians needing entry-level kit for their tool bags, to professional engineers requiring precision instruments capable of operating in exacting applications and challenging environments. Sounds irresistible? Well, probably yes – with a couple of caveats. First, choosing the right device requires some due diligence. One size does not fit all, and the sheer variety of equipment, suppliers and prices can be confusing. Entry level devices at around £1,000 may be fine, but if they're limited to, say, 80 x 60 pixels, scanning won't be as quick or precise as devices offering 320 x 240. Then there's functionality – although Flir's latest Ex entry-level cameras now include its MSX (previously only available on T-Series and above) and useful analysis tools, such as area min/max and colour alarm. Best advice is to consider the likely applications and users. If it's a general in-house troubleshooting role, such instruments are more than adequate and, with fixed focus, point-and-shoot operation, they're easy to use. But there are also now 'halfway house' devices that sit between infrared thermometers and imagers such as Fluke's latest. Its VT02 Visual IR unit is claimed to make electrical, industrial, HVAC etc thermal inspections faster than the former, but at a lower cost than the latter. The firm talks of 'hyper-thin pyroelectric technology' enabling "an array dense enough to create an infrared heat map" and "blended thermal and digital imagery". Incidentally, if you're worried about price, looking after the instrument or keeping up with the technology, you might want to consider hiring, particularly if it's likely to be occasional use only. BSRIA Instrument Solutions' general manager Alan Gilbert makes the point that a thermal camera with a reasonable resolution and picture size, such as Flir's E50, costs about £4,400 yet is available for rental from £100. And he says his engineering team is happy to visit any site and offer support and guidance. So much for the instruments: second, though, you need to consider training. Because, while some suppliers suggest little or no instruction is required, there are expensive mistakes to be made. The reason: while specialist users are necessarily well aware of thermal cameras' limitations and pitfalls, novices (and even some who may consider themselves better versed) may not be. And that can lead to production stopped while equipment is investigated unnecessarily – or, conversely, plant incorrectly given a clean bill of health – due to inexpert users misunderstanding thermal images. Hence, the stories of instruments bought and long since left languishing in cupboards, 'because they don't work'. That's why most of the major camera manufacturers and independent training organisations run courses – ranging from one to several days in length – taking attendees to clearly defined levels of competence. Flir, among others, is also now offering basic online courses. Why bother? Flir UK sales manager Andy Baker warns of the classic emissivity and reflectivity problems that still catch casual technicians unawares. "In a perfect world, all objects would emit 100% of their thermal energy," he explains. "But the world isn't perfect and different surfaces emit at different rates. So, if they want accurate and repeatable measurements, users have to be aware of emissivity." Putting this simply, energy emissions from bright, shiny surfaces can be characterised as low and variable while, for dull, matt surfaces, they are high and stable. Baker gives the example of electrical distribution bus bars, which, when the copper is new and non-oxidised, are shiny. "Using a standard emissivity setting of 0.85-0.95 is clearly going to deliver a lower temperature reading on the cross hairs than reality," he says. And while that can usually be corrected later – while generating your report, using the software provided (assuming the mistake is noticed) – it might be rather too late if the equipment was actually a fire risk or has since forced a shutdown. Worse, plant operatives will have the false sense of security that comes from believing their equipment has been inspected and passed. Seasoned users have their techniques. Some use electricians' tape. Leave it a few seconds and it attains the temperature of the surface below, so eliminating the emissivity issue. Others paint points of interest matt black. Others again use emissivity look-up tables or adjust the camera settings. The technique to use depends on the situation. And be aware that, if you're only interested in temperatures relative to a baseline survey of equipment (not absolute values), then, as long as the camera set-up is consistent, you're comparing apples and apples so emissivity won't matter. Reflectivity might matter However, reflectivity might. "If there's something shiny and hot behind you, then your thermal image might well be artificially raised by its reflection of that energy source," explains Baker. Equally, if there are cold objects behind the field of view, expect to see lower temperature images. And there are other disruptive effects, including solar loading (where thermal imaging cameras are used outside on summer days), wind chill (in the winter and in cold climates generally) and distance (for example, in paper mills, where humidity is high). "Water vapour in air can affect the transmission of energy from the object of interest to the camera. You can compensate for that by adjusting the humidity setting," suggests Baker. "In the UK we leave it at 50%, but in the Far East you might want to select 80-90%." Solar solution Thermal imaging is playing an increasingly important part in maximising energy yield from solar panels at producer Solon. R&D manager Oliver Frank says cameras are now used throughout the firm's solar panels' lifecycle from manufacture and quality assurance to field installation and maintenance. "We don't manufacture our own solar cells so strict quality controls are a necessity... And, for this, our Flir P Series camera is crucial, particularly for shunt detection," says Frank. Shunts, he explains, can reduce solar cells' energy conversion efficiency and, since the cause is generally manufacturing defects, they need to be found before lamination. Hence, Solon technicians apply a reverse bias to minimise charge flow, and then use the camera to observe the thermal pattern. "Hot spots indicate local shunts," states Frank. But Solon also uses thermal cameras for field inspection and predictive maintenance – in this case, two Flir T Series units. "We use the cameras to check solar power plant before handover to the customer and also for post installation monitoring and maintenance services," explains Solon thermographer Volker Denzle. "Before the cameras, we had to measure each string by hand, which was very time consuming, especially in large installations. And even when we found a defective string, determining the exact cell or cells was a matter of guesswork. With the cameras, we can pinpoint the problem."