Page 14 - Chemical Technology

12 Chemical Technology • January 2013
The ideal measuring conditions are:
• Stable weather conditions
• Cloudy sky before and during the measurement
(for measurements outdoors)
• No direct sunlight before and during the measurement
• No precipitation
• Surface of measuring object dry and clear of thermal
sources of interference (eg no foliage or chips on the
surface)
• No wind or draught
• No sources of interference in the measuring environ-
ment or transmission path
• The surface of the measuring object has high emissivity
that is known exactly
For building thermography, a difference of at least
15°C between the inside and outside temperature is
recommended.
The perfect thermal image
When taking a thermal image, you should pay attention
to two things in particular:
• Choosing the right subject area
• Focusing the thermal image correctly on the area
relevant to the measurement
As with a normal digital picture, you cannot change
either the subject area or the focus of the image once
the thermal image has been saved.To obtain a perfect
thermal image, you can make the following changes in
your thermal imager and in the analysing software:
Figure 4: Adjusting the temperature scale
control and instrumentation engineering
• Change the emissivity and the reflected temperature
compensation (RTC) setting. This can also be done
point-by-point or in sections with professional analys-
ing software (see Figure 3).
• Choose an appropriate colour palette (eg iron,
rainbow, etc.). Depending on the colour palette,
you will get a high-contrast, easy to interpret
thermal image.
• Adjust the temperature scale manually. This is how
you can improve the temperature grading or colour
grading of your thermal image (see Figure 4).
Conclusion
Observe the following tips for taking the thermal image:
• Factor in, prevent or screen all sources of interference.
• The surface of the measuring object should be clear
of optical and thermal sources of interference. Where
possible, remove covers and objects causing interfer-
ence from the environment.
• Change your position when taking the measurement
in order to identify any reflections.
• Your measuring spot should never be bigger than your
measuring object.
• Keep the measuring distance as small as possible.
• Use a lens appropriate to your measurement task.
• For exact measurement of details, it is recommended
you use a stand.
• The design of your measuring object should be
known in order to be able to correctly identify
thermal features.
• Use a thermal imager with a built-in digital camera
so that you can use real pictures for analysis at a
later date.
• Note all ambient conditions and measure and document
these where necessary for the subsequent analysis of
the thermal images.
Bibliography
[1] Auer, D. 2010. Pocket Guide Thermography. Germany.
testo, AG.
Acknowledgement
The author would like to thank testo, AG for research
supplied.
Figure 3: Change of emissivity by area for exact temperature analysis