Page 10 - Chemical Technology

8 Chemical Technology • January 2013
control and instrumentation engineering
Every object with a temperature above 0 Kelvin=
-273,15°C emits infrared radiation. A thermal imager
measures the long-wave infrared radiation received
within its field of view. From this it calculates the tempera-
ture of the object measured. The calculation factors in the
emissivity of the surface of the object and the compensa-
tion of the reflected temperature (RTC), both variables that
can be set manually in the thermal imager.
Measuring object and the surrounding
environment
One needs to take into account various factors regarding
the object that is being measured as well as the environ-
ment in which it is being measured.
Important to note is that thermal images only show
temperature distribution on the surface of an object. For
this reason, one cannot look into – or even through –
objects with a thermal imager.
• Material and emissivity
The surface of each material has a specific emissivity
from which the amount of the infrared radiation emit-
ted from the material that is a) reflected and b) emitted
(radiated from the object itself) is derived.
• Colour
The colour of a material has no noticeable effect on
the long-wave infrared radiation emitted by the object
to be measured when measuring the temperature with
a thermal imager.
• Surface of the measuring object
The properties of the surface of the measuring object
play a crucial role in the measurement of temperature
with a thermal imager. The emissivity of the surface
varies according to the structure of the surface, soiling
or coating.
• Structure of the surface
Infrared radiation can bounce in holes or in-between
fins so that it appears hotter in the thermal image
than in reality. This is due to the larger surface area.
Thermography
in practice
by Natalie Liddle, unitemp, Johannesburg, South Africa
Contactless temperature measurement (in both
building efficiency and industrial maintenance)
has provided a valuable role in times of rising
energy prices and exorbitant downtime costs.
But thermography should be done properly
if accuracy in temperature measurement is
required. This article aims to equip new and ex-
isting users of thermal cameras with practical
tips and tricks that will help master the perfect
thermal image.
Wetness, snow and frost on the surface
Water, condensation and frost have relatively high emis-
sivities (approximately 0,85 < Σ < 0,96), so measure-
ment of these substances is generally unproblematic.
Bear in mind that the temperature of the measuring
object can be distorted by natural coatings of this kind.
Wetness cools the surface of the measuring object as
it evaporates and snow has good insulating properties.
Frost usually does not form a sealed surface, so the
emissivity of the frost as well as that of the surface un-
derneath it must be taken into account when measuring.
Soiling and foreign bodies on the surface
Soiling on the surface of the measuring object such as
dust, soot or lubricating oil generally increases the emis-
sivity of the surface. For this reason, measuring dirty ob-
jects is generally unproblematic. However, your thermal
imager always measures the temperature of the surface,
ie the dirt, and not the exact temperature of the surface
of the measuring object underneath.
The measuring environment
• Ambient temperature
You should also factor in the setting of the reflected
temperature (RTC) as well as the emissivity setting (Σ )
so that your thermal imager can calculate the tem-
perature of the surface of the measuring object cor-
rectly. In many measurement applications, especially
indoors, the reflected temperature corresponds to the
ambient temperature. You can measure this with an
air thermometer.
An accurate setting of the emissivity is particu-
larly important where there is a large difference in
temperature between the measuring object and the
measuring environment.
• Radiation
Every object with a temperature above absolute zero
(0 Kelvin = -273,15°C) emits infrared radiation. In par-
ticular, objects with a large difference in temperature
testo thermal camera camcorder design
This article was
first published
in 'Electricity +
Control' August
2012 and is
republished
here with kind
permission