7.7
Practical advice for the thermographer
Working in a practical way with a camera, you will discover small things that make
your job easier. Here are ten of them to start with.
7.7.1
From cold to hot
You have been out with the camera at +5°C (+41°F). To continue your work, you
now have to perform the inspection indoors. If you wear glasses, you are used to
having to wipe off condensed water, or you will not be able to see anything. The same
thing happens with the camera. To measure correctly, you should wait until the
camera has become warm enough for the condensation to evaporate. This will also
allow for the internal temperature compensation system to adjust to the changed
condition.
7.7.2
Rain showers
If it starts raining you should not perform the inspection because the water will drasti-
cally change the surface temperature of the object that you are measuring. Neverthe-
less, sometimes you need to use the camera even under rain showers or splashes.
Protect your camera with a simple transparent polyethylene plastic bag. Correction
for the attenuation which is caused by the plastic bag can be made by adjusting the
object distance until the temperature reading is the same as without the plastic cover.
Some camera models have a separate
External optics transmission
entry.
7.7.3
Emissivity
You have to determine the emissivity for the material, which you are measuring.
Mostly, you will not find the value in tables. Use optical black paint, that is, Nextel
Black Velvet. Paint a small piece of the material you are working with. The emissivity
of the optical paint is normally 0.94. Remember that the object has to have a temper-
ature, which is different—usually higher—than the ambient temperature. The larger
the difference the better the accuracy in the emissivity calculation. The difference
should be at least 20°C (36°F). Remember that there are other paints that support
very high temperatures up to +800°C (+1472°F). The emissivity may, however, be
lower than that of optical black.
Sometimes you can not paint the object that you are measuring. In this case you can
use a tape. A thin tape for which you have previously determined the emissivity will
work in most cases and you can remove it afterwards without damaging the object
of your study. Pay attention to the fact that some tapes are semi-transparent and thus
are not very good for this purpose. One of the best tapes for this purpose is Scotch
electrical tape for outdoor and sub-zero conditions.
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Publ. No. 1558240 Rev. a156 – ENGLISH (EN) – March 1, 2006
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7 – Introduction to thermographic inspections of electrical installations
