Chemical Engineering March 2011 - 31

Infrared
Department Editor: Scott Jenkins
ontact-based temperature sensors, such as
thermocouples and resistance temperature
detectors (RTDs), have demonstrated accurate
and cost-effective operation throughout
the chemical process industries (CPI). However,
there are many applications and settings
where they are simply not practical. In those
cases, engineers can turn to a host of noncontact
temperature measurement devices,
many of which are based on measuring
infrared (IR) radiation. IR thermometers can
routinely perform measurements in situations
where readings with contact thermometers
would be virtually impossible. Situations where
IR-based temperature measurement should be
considered include the following:
* Rapid and frequent temperature readings
are required
* Objects are moving
* Objects are within strong electromagnetic
C
Operation
Most IR thermometers operate
according to the same basic
operating principles (Figure 1).
A bandwidth ilter and optical
lens are used to focus the IR
energy emitted by an object
onto a detector, which converts
the IR radiation into an electrical
signal. After compensating
for emissivity (see below) and
ambient temperature, an analog
output is generated to provide
temperature measurement. The
analog signal can be converted
to digital when fast acquisition
rates are required.
Emissivity
* Rapid thermal changes are the norm
* Objects needing measurement are located
* The surface temperature of an object or
equipment requires measurement
* Locations are inaccessible to contact
thermometers
* Areas to be measured would be damaged
or contaminated by contact measurement
* Varying surface-temperature distributions are
present
* Objects are made from materials with low
heat capacity and low thermal conductivity
* Materials to be measured are gaseous, such
as combustion gases and flames
Within the CPI, IR thermometry is most effective
and used most frequently in semiconductor
and wafer processing, cement and
lime processing, rotating kiln shells, waste
incineration, glass processing, sintering and
heat treating, metals processing and drying
applications. While IR thermometers are generally
more expensive than contact thermometers,
they usually have longer lifetimes and
require less maintenance.
thermal radiation physics can help users apply
and operate the devices more effectively.
A general knowledge of the key aspects of
Stefan-Boltzmann law
The energy radiated by an object per unit
surface area per unit time is related to its
temperature by the Stefan-Boltzmann law, which
states that irradiance (in J/s/m2) is proportional
to the fourth power of the object's temperature
multiplied by its emissivity. A constant of proportionality
(the Stefan-Boltzmann constant) is required
for calculations. In a perfect (theoretical)
blackbody radiator, emissivity equals one.
Object
Field of view
in process chambers or behind windows
fields, such as in processes involving induction
or microwave heating
Material
Silver (polished)
Aluminum (unoxidized)
Gold (polished)
Aluminum (heavily oxidized)
Zinc (bright galvanized)
Steel (316 polished)
Soil (plowed field)
Iron (liquid)
Iron (rusted)
Water
Sand
Steel (cold rolled)
Wood (oak planed)
Brick (red, rough)
The emission of thermal radiation
is a surface phenomenon
for most materials. The term
emissivity refers to an object's
ability to emit thermal radiation.
Emissivity is defined as the ratio between the
energy emitted by an object at a given temperature
and a perfect radiator, or blackbody,
at the same temperature. Emissivity values
lie between zero and one. IR thermometers
generally have the ability to compensate for
the different emissivity values of materials.
Materials with the highest emissivity values
are the easiest to measure accurately with IR
thermometers, while those with low emissivities
are more difficult. For example, some polished,
shiny metallic surfaces, such as aluminum, are
so reflective in the infrared that accurate temperature
measurement is not always possible.
Tables listing emissivity values for various
materials have been published, and are available
for reference (Table). Some IR thermometers
allow users to change emissivity values
according to the material being measured,
while others have a pre-set emissivity value.
When using IR thermometry, it is important
to consider that materials can have different
emissivity values at different wavelengths. To
determine an emissivity value, you can heat a
material to a known temperature, then adjust
the emissivity value of the instrument until the IR
thermometer matches the known temperature.
Field of view (FOV)
The FOV is the angle of vision at which the
instrument operates and is determined by the
optics of the system. The optical system of the
IR thermometer collects the IR energy from a
circular measurement spot, and focuses the
energy on the detector. The optical resolution
of the instrument is determined by the ratio
between the distance from instrument to object
and the size of the spot being measured
Lens assembly
Filter
Aperture
Detector
S
Amplifier
and electronics
D
FIGURE 1. The optical system of an IR thermometer collects IR energy
from a circular measurement spot, and focuses it on a detector
Target greater
than spot size
Target equal
to spot size
Target smaller
than spot size
Backround
FIGURE 2. For accurate IR temperature measurement, the target area
should be greater than the instrument's FOV by a factor of about 1.5
IR Thermometer
Carbon (Lampblack)
Ice
*Provided for illustrative purposes only.
(distance-to-spot ratio). Higher ratios mean
better resolution.
Ideally, the target being measured should fill
the instrument's FOV. For the best performance,
the target object area should exceed
the FOV by a factor of about 1.5 (Figure 2).
Calibration
IR thermometers can be calibrated by aiming
at blackbody radiators that are designed
specifically for calibration and testing. By varying
the source temperature of the blackbody,
calibrators can tune the IR sensor's internal
measurement signal to known temperatures.
Selection questions to consider
When selecting an IR thermometer for a CPI
application, it is important to consider the folTemperature
Measurement
EMISSIVITY
VALUES OF COMMON MATERIALS*
Emissivity
0.01
0.02
0.02
0.20
0.23
0.28
0.38
0.43
0.65
0.67
0.76
0.80
0.91
0.93
0.95
0.98
lowing questions:
* What is the required proximity to the target?
* What is the size of object to be measured,
and will it it the instrument's FOV?
* Are there any physical obstructions between
the object to be measured and the measuring
device
* Are smoke, dust or other particulate matter
likely to be in the measurement area?
* What are the measurement control frequency
requirements?
* Does the object have a shiny surface?
* What are the output/interface requirements?
References
1. Young, A., IR Thermometry Finds CPI Niches,
Chem. Eng., Feb. 2002, pp. 56-60.
2. Omega Engineering Inc. [Internet]. c2003-
2010. Stamford (CT). [cited Feb. 2011].
Available from: http://www.omega.com/.
Recommended
Incorrect
http://www.omega.com/
Chemical Engineering March 2011

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