Tips on Installing IR windows
Tips on Installing Infrared Windows
An infrared window or viewing port is a permanent fitting and consideration must be given to several issues to ensure that you get the best results possible. One of the most important areas of consideration which is overlooked is the actual targets that you wish to view using your IR window. Electrical cabinets, etc are full of different materials of varying emissivity, they can range from .95 to 0.15 and these values can change with age and temperature.
The emissivity of an object is the ratio of radiant energy emitted by that object divided by the radiant energy which a blackbody would emit at that same temperature. If the emittance is the same at all wavelengths, the object is called a gray body. Some industrial materials change their emissivity with temperature and sometimes with other variables also. Emissivity always equals absorption and it also equals 1 minus the sum of reflectance and transmittance (E = A= 1 - T - R).
The images below show an non-energised electrical Bus Bar assembly with apparent temperature differences, this assembly has never been used and all components are at exactly the same temperature. These differences are caused by differing material emissivity, reflected background temperatures, etc.
The graph below shows, as with transmission, how calculated temperatures can be adversely affected if you get them wrong and, as with transmission, if your emissivity is too high the temperature is too low! Therefore it is imperative that the thermographer knows the emissivity of the target components within the panel. A good method used by thermographers is to standardise the emissivity of all your targets in the panel. we cover or coat all targets with a material of a known emissivity, i.e. electrical tape, bar-b-q paint, stickers, etc. This removes the errors that can be caused by entering the wrong emissivity number into the camera and improves the quality of the data gathered using IR windows.
Remember that this data must be obtained during installation as it will be impossible to place emissivity markers on live components. If you have installed your IR windows into live cabinets, ensure that you record the material emissivity of the targets you are inspecting (See List) and install emissivity markers at the earliest opportunity.
The graph shows how calculated temperature readings change when you vary the emissivity rates. The emissivity rates were changed from 0.99 to 0.50 using the same image. This gave a difference of 12.2 °C
Using IR Windows
An important thing to remember when using IR windows is to identify the window with a unique number; this will be invaluable especially when you have multiple windows on electrical panels, etc. It is also advisable to identify the type and wavelength of the crystal.
The most essential data to record is the transmission rate of the crystal and also as previously stated the emissivity of the component or components that you are measuring through the IR window, the most effective way of using IR windows is to, where possible, prepare all components that are inspected so as they have the same emissivity with electrical tape, paint etc, thus all components being inspected will have the same transmission rate and emissivity readings, consequently the results gathered will be more accurate.
It should be noted that there may be multiple targets through the IR window, these need to be recorded on the ID label, the most common method of locating the targets required is by using the clock face method, i.e. Bus Bar connections at 4 O’clock, etc. This data together with the transmission and emissivity can all be recorded on ID labels.
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