Page 11 - Chemical Technology

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Chemical Technology • January 2013
control and instrumentation engineering
from the measuring object can disrupt the infrared
measurement as a result of their own radiation. You
should avoid or deactivate sources of interference of
this kind wherever possible. By screening the sources
of interference (eg using a cardboard box), you will
reduce this negative effect on the measurement. If
the effect of the source of interference cannot be
removed, the reflected temperature does not corre-
spond to the ambient temperature. A globe thermome-
ter or Lambert radiator, for example, is recommended
for measuring the reflected radiation in conjunction
with your thermal imager.
Special features of outdoor thermography
Objects heat up in the sun as a result of absorbing sun-
light. This affects the surface temperature considerably.
It can be seen in Figure 1 that the gutter is shown colder
than the house wall on the thermal image.
Figure 1: Reflection for measurements outdoors
Weather
Clouds
A densely clouded sky offers the ideal conditions for
infrared measurements outdoors, as it screens the
measuring object from sunlight and “cold diffuse celes-
tial radiation”.
Sun
See ‘Radiation’ above
Air
Air humidity
Do not perform measurements when air humidity is con-
densing on the thermal imager. If the lens (or protection
glass) has misted over, some of the infrared radiation
hitting the thermal imager will not be received, as the
radiation fails to penetrate fully through the water onto
the lens.
Air flows
Note the speed and direction of air flows during the mea-
surement and factor these data into your analysis of the
thermal images.
Air pollution
Do not perform measurements in heavily polluted air.
Some suspended matter such as dust, soot and smoke,
for example, as well as some vapours have high emissiv-
ity and are barely transmissive. This means that they can
impair the measurement, as they emit their own infrared
radiation that is received by the thermal imager. Always
measure with the smallest possible measuring distance
for your measurement application in order to minimise the
effect of any possible suspended matter in the air.
Light
Light or illumination do not have a significant impact
on measurement with a thermal imager. You can also
take measurements in the dark, as the thermal imager
measures long-wave infrared radiation. However, some
light sources emit infrared heat radiation themselves and
can thus affect the temperature of objects in their vicin-
ity. You should therefore not measure in direct sunlight or
near a hot light bulb, for example. Cold light sources such
as LEDs or neon lights are not critical, as they convert
the majority of the energy used into visible light and not
infrared radiation.
Determining emissivity
and RTC in practical applications
To determine the emissivity of the surface of the measuring
object, you can, for example:
• Refer to the emissivity table (Figure 2). Caution: Values
in emissivity tables are only ever guideline values.
• Use a reference measurement with a contact
thermometer.
• Use a reference measurement with an thermal imager.
However, both are roughly the same temperature. The
image must therefore be interpreted. Assume that the
surface of the gutter is galvanized and has extremely low
emissivity (Σ = 0,1). Only 10% of the long-wave infrared
radiation emitted by the gutter is therefore emitted inher-
ent radiation, 90% is reflected ambient radiation. If the
sky is clear, ‘cold diffuse celestial radiation’ (~ -50°C
to -60°C) is reflected on the gutter. The thermal imager
is set to Σ = 0,95 and RTC = -55°C to ensure correct
measurement of the house wall. Due to the extremely low
emissivity and the extremely high reflectance, the gutter
is shown too cold on the thermal image. To show the
temperatures of both materials correctly on the thermal
image, you can change the emissivity of certain areas
retrospectively using analysing software.
Tips
• Always be aware of the effect of your own personal
infrared radiation (body heat).
• Change your position during the measurement in
order to identify any reflections.
• Avoid measurements close to very hot or cold objects,
or screen these.