Thermography – A Fast-emerging efficient technique in NDT & Predictive Maintenance

Aug. 13, 2012 - PRLog -- Thermography or Infrared Thermography is a non-destructive technique of assessing the working condition of any system, be it electrical, building, any machination etc. by means of measuring the infrared (IR) radiation or rays emitted by any test surface. The IR radiation or ‘Heat rays’ emitted gives a measure of the temperature pattern over a specified surface. The temperature distribution, so obtained, is called the ‘Thermogram’ corresponding to the scanned surface, and it is in the form of colourful images called ‘False Colour Images’. The difference in colour of a particular spot or region from another one indicates the corresponding difference in temperature. Black or purple regions indicate lower temperatures, and whiter regions specify higher temperatures. The fundamental basis of this technique is the fact that any material or substance which has a surface temperature of greater than Absolute Zero (-273 0C or 0 K) emits IR radiation, & the intensity of the radiation depends primarily on the ‘Temperature’ & the ‘Emissivity’ of the surface of the test substance. Since this technique can measure the ‘heat pattern’ of a body, it can serve as extremely useful in the detection of various anomalies in various machines, electrical systems & installations, concrete structures etc. resulting out of abnormal heat distributions or overheating. The scope of application for this technique is enormous and is ever-increasing because of emerging technologies in the IR detector & optics system. Hence, the technique of ‘Infrared Thermography’ has evolved as the most popular & efficient Non-destructive testing (NDT) method.  
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Before discussing about the benefits & applications of this versatile method, let us understand a bit about the scientific principles at work behind this technique. The first thing to understand is the ‘Infrared Radiation’ & its properties. Infrared light forms a sizeable portion of the ‘Electromagnetic Spectrum’. It extends beyond the red part of the Visible Spectrum (380 to 760 nm) at approx. 740 nm & stretches till 300 μm (300000 nm). IR radiation was discovered accidentally by a German astronomer named Sir Frederick William Herschel in 1800, while he was measuring the differences in temperature of the various fractions of the visible light created by the passage of sun rays through a glass prism. Upon measuring the temperature of the invisible radiation beyond the ‘red light’, he fathomed out the presence of IR radiation. The IR spectrum lies between the higher energy Visible spectrum & the lower energy Microwave spectrum. Although human beings cannot see IR rays but they are quite sensitive to their presence. As per a scientific study, the nerve endings in human skin respond to temperature changes as small as 0.009 0C (0.0162 0F). The origin of IR radiation lies in the heat/thermal content/energy of a system, which in turn arises out of the vibrational motion of the constituent molecules.
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The fundamental principle behind the use of the technique of ‘Infrared Imaging’ is the Planck’s law of ‘Black body radiation’, according to which every body above the temperature of absolute zero (0 K) emits IR radiation. Hence, even a cube of ice emits IR radiation, although its intensity will be quite low. The intensity of IR radiation emitted by the surface of any substance depends on the ‘Temperature’ & ‘Emissivity’ of the surface. There is a direct-proportional relationship between the temperature of a substance & its corresponding intensity of emitted IR radiation. Emissivity of a surface is defined as the ratio of thermal energy or IR radiation emitted by the test object to that emitted by a ‘black body’, both the radiations being measured at the same temperature. A ‘black body’ is a perfect absorber & radiator of IR radiation. The emissivity values of substances vary between 0 (for a perfect ‘white’ or reflecting surface like mirror) and 1 (for a perfect ‘black body’). A perfect black body is a hypothetical entity & does not exist in nature. Emissivity depends on a number of factors like temperature, wavelength, and surface characteristics of a test surface.

The IR spectrum can be roughly divided into three regions: (1) Near Infrared (0.8 μm to 3 μm), (2) Mid Infrared (3 μm to 50 μm), & (3) Far Infrared (50 μm to 1000 μm). The categorization actually depends on the type of IR detector used. The region from 8 μm to 15 μm is considered to be the most appropriate & useful region for getting optimum thermal images. This region is called the ‘Thermal Imaging’ or ‘Thermal Infrared region’.

The basic working of a Thermal imager involves capturing of the emitted IR radiation from a test surface by a number of IR detectors after being made to focus with the help of Optics. The information from the detectors is then sent to the electronic system for image processing. Finally, the generation of ‘Thermal Images’ or ‘False Colour Images’ takes place. The assignment of different visible colours to depict the temperature distribution of any surface is done on a purely hypothetical basis.
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Some of the important advantages of Thermographic testing or inspection are as follows:

• Safety, ease of operation, handling, and very less consumption of time
• Non-destructive nature of testing allows the integrity of test materials to remain intact.
• The non-contact nature of testing makes it possible to monitor those substances which are either dangerously too hot or hazardous like high voltage electric wires.
• Can be operated on test specimens during their functioning or while they are running.
• Can be operated on stationary as well as on objects in motion.
• Can detect failing components, faults in a structure or machinery, corrosion or loose connections in an electrical system at an early stage, before such abnormalities become critical & lead to shutdowns and breakdowns in industries.
• Can help prevent costly and unscheduled downtime problems, and serious accidents like an outbreak of fire etc.
• Can help with scheduling of ‘Preventive Maintenance’ programs for extending the ‘useful life’ of industrial machinery.
In contrast to other NDT techniques which determine defects at particular points of testing, thermography provides a two-dimensional image of the test surface.

Although the scope of application for this technique is quite huge, but some of the core areas (except medical application) of its use are as follows:

* Construction: Checking of insulation continuity, water ingress in roof, defective cladding Cold/warm bridging, help in energy auditing of buildings, check for flaws in concrete etc.
* Electrical: Checking of overloading, insulation breakdown, motor overheating, capacitor damage, corroded connections in electrical panels, internal fuse damage etc.
*  Mechanical: Checking of bearing wear or misalignment, steam trap, steam system leaks, corrosion in steel pipes, monitoring of heat exchanger etc.
* Surveillance: Conducting night operations, search/rescue operations, locating & identifying enemy & their hideouts etc. This application particularly concerns the military agencies.
* Electronics: Inspection of circuit boards in order to locate hot spots.
Automotive: Checking of tyres for cracks & other faults, radiators & exhausts, cooling performance of engines, window demisting performance etc

Although the advent of latest & sophisticated technologies in relation to Infrared Imaging has led to the availability of sensitive & easier-to-use Thermal imagers, but still the importance of human factor can not be ignored. The experience of a trained ‘Thermographer’ plays a crucial role in the proper generation & correct interpretation of thermal images produced by an Infrared imager. This is because a number of factors need to be taken into account for generating a proper thermal image that would correctly represent the temperature distribution of any test surface.