News from TEST and around the industry

The rise of Thermal Imaging, but does the camera lie?

By March 31, 2015 News No Comments

Thermography

As the popularity of thermal imaging increases it is fast becoming an important diagnostic process in many of today’s industries enabling the experienced/trained/competent user to spot potential problems invisible to the naked eye.

Thermal Imaging provides a safe, clean, visual way of assessing the temperature and condition of live electrical equipment. It is the most effective way to identify and predict high resistance joints, poor connections and overloaded circuits before reaching critical failure and fire. Such problems are virtually impossible to identify by any other means.

Companies who take advantage of the predictive capabilities of regular surveys often go on to see the benefits of reduced insurance premiums, and a growing number of insurers are making thermographic surveys compulsory with regard to fire prevention.

Diversity of Application

One of the greatest benefits of thermal imaging development is the diverse range of uses to which the technology can now be applied. When we also consider that costs of thermal cameras have fallen year on year due to manufacturing economies while technical features have continued to multiply, it’s easy to see why they are more popular than ever before.

The challenge

These developments however present new challenges, as lower costs and increased accessibility brings with it the opportunity for abuse, which poses a big problem for safety.

As we will see, Thermography is much more than simply purchasing a camera, pointing and shooting then relying on software to do the work. One of the major pitfalls is specialist technology in the hands of someone without the relevant training and experience to set up, apply and interpret the information correctly as failure to carry out proper diagnostics can hide potentially dangerous risks.

Does the camera lie?

So, does the cameras lie? And if so, just how serious can incorrect diagnosis of a thermal image be?

To answer this question, we must first understand some of the principles around thermography and in this article we are going to consider how these cameras are used in an electrical application.

What is Thermal Imaging?

Discovered in 1800 by astronomer William Herschel, infrared light is light which is not visible because its wavelength is too long to be detected by the naked eye. Quite simply, thermography is used to both ‘see’ and ‘measure’ an objects thermal energy emissions.

Thermal imaging is the use of technology which detects infrared radiation when transmitted through an infrared detector, and in turn displays a visual image on a screen at the back of the device. This image allows the user to detect the amount of heat present in the image.

To the untrained eye, it would be very easy to be drawn to the hotter parts of a thermal image and immediately assume there is a problem, but this is not always the case. There are many contributing factors which must be considered when analysing a thermal image and many things that can lead to errors. We are now going to consider the two most common mistakes in the use of thermal cameras for electrical inspections.

Misinterpretation of Reflections

With visible light we see reflections all the time, mirrors, glass, polished surfaces will all reflect their surroundings. Similarly, reflections of heat are also present in the infrared spectrum.

When a thermal camera is pointed at polished surfaces, such as copper, steel and aluminium, only a very small percentage of what the camera is seeing is radiated from the actual metal itself, the vast majority is in fact reflected from other surrounding objects impacting on the target.

To illustrate this, the following example shows a piece of copper heated up on a hotplate. Here we can see a piece of black electrical tape has been placed over the copper to give a true temperature reading and is used as a reference point:

thermo1

Two spot tools have been placed onto the thermal image, Sp1 is on the black tape, reading 76.7°C giving us a true reading, whereas Sp2, which is on the copper plate, reads only 26.2°C. The surface of the copper plate is reflecting, so what the camera is actually reading here is the reflected radiation from the objects in front of the copper. To the camera, the copper appears cooler than it actually is.

Considering how many times you would come across metal contacts in an electrical environment, there is enormous opportunity for error when carrying out diagnostics. Failure to understand this can result in potentially dangerous and catastrophic problems being missed entirely and declared as safe in a report.

Emissivity & Reflected Apparent Temperature

So why are temperature readings so important, after all surely you can spot the problems simply by looking for the hottest parts on the image? Firstly, it is important to understand that an image will often look the same regardless of the settings used – this can be seen in the illustration below. Secondly, when it comes to electrical applications, the use of correct temperature readings is paramount as this information will enable the engineer to correctly classify the severity of a fault and take the appropriate actions or recommendations.

We can already see the importance of training, experience and competence of the thermographic engineer when understanding your environment however, of equal importance is applying the correct settings to the camera and the two which have the greatest impact are called Emissivity and Reflected Apparent Temperature.

Emissivity

Emissivity is the capacity of an object to emit, or give off, heat. Objects with low emissivity are not suited to infrared thermography, as is shown in our previous example. Low emissivity objects will reflect more so obtaining a true temperature reading can be a challenging for the engineer.

Reflected Apparent Temperature

Reflected apparent temperature is the culmination of radiation from all objects impacting on the target. When operating outside this will even include the sky. This setting will vary according to the environment and will impact on any temperature readings the camera displays.

Below we have two examples which illustrate the impact of these two settings.

thermo2

Fig 1 Values

  • Reflected Temperature = 20.0°C
  • Spot Temperature = 55.7°C

Fig 2 Values

  • Reflected Temperature = 76.0°C
  • Spot Temperature = 52.9°C

thermo3

Fig 1 Values

  • Emissivity = 0.95
  • Spot Temperature = 55.7°C

Fig 2 Values

  • Emissivity = 0.50
  • Spot Temperature = 29.3°C

Note that within each example above, the colouration of the image is exactly the same, only the settings have changed. Whilst we have used some fairly dramatic examples, it does illustrate just how easy it is for a thermal camera to lie when it is not used correctly!

Again considering the example above, incorrect emissivity settings can lead to the user reporting the temperature as being much lower than it actually is. The impact of this is dangerous faults which are missed, or signed off as within acceptable limits. In this scenario, if the fault is not rectified it will deteriorate over time, potentially leading to catastrophic failure or fire.

As we can see, it is vital to understand how to analyse thermal patterns in order to determine types of faults and the impact that may have on other equipment and infrastructure. Accuracy is the cornerstone of thermographic assessments and thermal imaging is a highly specialist service which must be undertaken by competent engineers with the appropriate skills, experience and expertise. Thermographers should be ITC certified and follow guidance set in International Standards such as those shown below.

thermo-books

The benefits

Carried out correctly the speed and non-destructive nature of thermography has major benefits as a full and comprehensive programme can be undertaken without hindering the day to day operations of property and business. The visual nature of the results is one of thermal imaging’s main advantages. As opposed to presenting clients with an exhaustive list of complex data, a thermal image provides a compelling breakdown of issues by literally showing the most pressing concern in an easy to understand colourful image.

At Test all of our Thermographers are ITC certified and fully qualified electrical engineers with extensive inspection, design and installation experience. This combination of specialist disciplines adds significantly richer diagnostic value to the inspection process, giving you the assurance that your thermography is safe in the hands of the experts.