Page 12 - Chemical Technology

10 Chemical Technology • January 2013
Determining the emissivity by means of a
reference measurement
• Method using a contact thermometer
First, measure the temperature of the surface of
the measuring object with a contact thermometer.
Now measure the temperature of the surface of the
measuring object with the thermal imager, with a
preset emissivity of one. The difference between the
temperature values measured by the contact ther-
mometer and the thermal imager are the result of the
emissivity being set too high. By gradually lowering
the emissivity setting, you can change the mea-
sured temperature until it corresponds to the value
obtained in the contact measurement. The emissivity
then set corresponds to the emissivity of the surface
of the measuring object.
• Method with the thermal imager
First, stick a piece of (heat-resistant) emissivity adhe-
sive tape to your measuring object. After waiting a short
time, you can measure the temperature of the surface
of the measuring object in the taped-off area using your
thermal imager with a set emissivity for the adhesive
tape. This temperature is your reference temperature.
Now regulate the emissivity setting until the thermal
imager measures the same temperature in the area
that is not taped as the reference temperature just
measured. The emissivity now set is the emissivity of
the surface of the measuring object.
As an alternative to the emissivity adhesive tape, you
can also:
• Coat the measuring object with a coating or paint with
a known emissivity.
• Coat the measuring object with a thick layer (> 0,13 mm)
of heat-resistant oil (Σ ≈ 0,82).
• Coat the measuring object with a thick layer of soot
(Σ ≈ 0.95).
Determining the temperature of the
reflected radiation
Once you have eradicated all the possible sources of
interference that could affect your measurement, the tem-
perature of the reflected infrared radiation is the same as
the ambient temperature. You can measure the ambient
temperature with an air thermometer and enter the RTC in
your thermal imager on the basis of this. However, if sourc-
es of radiation are present in the measuring environment,
you should determine the temperature of the reflected
radiation to ensure an accurate measurement result.
Measurement of reflected temperature
using an (improvized) Lambert radiator
A Lambert radiator is an object that reflects incident
radiation with the optimum diffusion, in other words
equally strongly in all directions. You can measure the
temperature of the reflected radiation on a Lambert ra-
diator using the thermal imager. A piece of aluminium foil
crumpled and then unfolded again is a suitable substi-
tute for a Lambert radiator for this purpose. The foil has
Material (material temperature) Emissivity
Aluminium, bright rolled (170˚C) 0,04
Aluminium, not oxidised (25˚C) 0,02
Aluminium, not oxidised (100˚C) 0,03
Aluminium, heavily oxidised (93˚C) 0,2
Aluminium, highly polished (100˚C) 0,09
Cotton (20˚C) 0,77
Concrete (25˚C) 0,93
Lead, rough (40˚C) 0,43
Lead, oxidised (40˚C) 0,43
Lead grey oxidised (40˚C) 0,28
Chrome (40˚C) 0,08
Chrome, polished (150˚C) 0,06
Ice, smooth (0˚C) 0,97
Iron, emery ground (24˚C) 0,24
Iron with casting skin (100˚C) 0,8
Iron with rolling skin (20˚C) 0,77
Gypsum (20˚C) 0,9
Glass (90˚C) 0,94
Granite (20˚C) 0,45
Rubber, hard (23˚C) 0,94
Rubber, soft, grey (23˚C) 0,89
Cast iron, oxidised (200˚C) 0,64
Wood (70˚C) 0,94
Cork (20˚C) 0,7
Radiator, black, anodised (50˚C) 0,98
Copper, slightly tarnished (20˚C) 0,04
Copper, oxidised (130˚C) 0,76
Copper, polished (40˚C) 0,03
Copper, rolled (40˚C) 0,64
Plastics: PE, PP, PVC, (20˚C) 0,94
Paint, blue on aluminium foil (40˚C) 0,78
Paint, black, matt (80˚C) 0,97
Paint, yellow, 2 coats on aluminium foil (40˚C) 0,79
Paint, white (90˚C) 0,95
Marble, white (40˚C) 0,95
Brickwork (40˚C) 0,93
Brass, oxidised (200˚C) 0,61
Oil paints (all colours) (90˚C) 0,92 to 0,96
Paper (20˚C) 0,97
Porcelain (20˚C) 0,92
Sandstone (40˚C) 0,67
Steel, heat-treated surface (200˚C) 0,52
Steel, oxidised (200˚C) 0,79
Steel, cold-rolled (93˚C) 0,75 to 0,85
Clay, burnt (70˚C) 0,91
Transformer paint (70˚C) 0,94
Brick, mortar, plaster (20˚C) 0,93
Zinc, oxidised 0,1
control and instrumentation engineering
Figure 2: Emissivity table