Infrared thermography, or thermal imaging, can be used as a means of detecting radiated energy/heat from objects. Thermography can be a convenient method for detecting excessive heat build-up, which may be attributed to conditions of increased mechanical wear, electrical faults, steam leaks, to name a few.
There are two primary devices used in industries for thermography: spot radiometers and infrared cameras. Both contain sensors that detect radiating energy that is linked to heat. It should be noted that infrared thermography does not directly measure temperature; instead, temperature is calculated by software using inputs from the user.
Spot radiometers measure the sum of energy over a small area and display a temperature readout. The area over which the reading is taken depends on the distance between the radiometer and the target; the further away you are from the target, the larger the area, and vice versa. Often, a laser is used to aid users in targeting the spot where the measurement will be made.
Infrared cameras generate a thermographic image by detecting heat. A visual image is shown on the camera, showing the relative temperature differences between hot and cold regions using a colour scale. This scale can be customized using user inputs./
Spot radiometers are useful for taking targeted readings whereas infrared cameras give a visual frame of reference that shows relative temperature differences. Neither of these instruments should be relied upon to give accurate temperature readings, since this will depend on user inputs and other external factors.
Infrared cameras can only measure radiated energy from the first 0.001” of a surface. This means that they will not be able to see through most solid objects, and will only give readings near the surface.
The emissivity of a surface can affect readings. Emissivity refers to the energy emitted by an object compared to a ‘black body’ (a perfect emitter) that is at the same temperature. Since thermal imaging takes readings using emitted energy, the emissivity value can play a role in the accuracy of readings.
Reflectivity of a surface can also impact readings with an infrared camera. For instance, a reflective surface such as a mirror will give high thermal readings on a sunny day, although its actual surface temperature may be very low. This can give very misleading results if you are unaware of how thermal imaging works.
The transmission properties of an object can also give variable readings. Surfaces that transmit a high amount of energy from other heat sources can give larger than expected readings.
Wind or air movement conditions can also affect accuracy due to convection. On a windy day, the air will be deflecting radiated energy away from a surface, which will give lower than expected measurements.
Some general rules for getting reliable measurements when using thermal imaging include: • Emissivity of the surface should be above 0.6
• There should be no wind or air movement, or the conditions should be known
• The material properties and other nearby sources of heat should be known
Thermal imaging is a useful technology that can be used for detecting some of the following conditions:
• Overheating bearings
• Cooling issues
• Electrical winding faults
• Blockages in steam systems
• Electrical arcing
Infrared thermography equipment comes in a wide range of options. Equipment can be found in all price ranges and it can be a good method for machine monitoring in industrial plants. The technology is primarily useful in giving relative temperature scales as opposed to absolute readings, and it can be an effective tool if used properly.