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Non-Contact Temperature Measurement
A Technical Reference Series Brought to You by OMEGA
VOLUME
1
TRANSACTIONS
Volume 1
05
7
NON-CONTACT TEMPERATURE MEASUREMENT
IR Thermometer Calibration
Why Calibrate?
Blackbody Cavities
Tungsten Filament Lamps
IR Thermometer Calibration
ecause of normal variations
in the properties of materials
used to construct radiation
temperature sensors, new
instruments must be individually calibrated in order to achieve even
moderate levels of accuracy. Initial
calibration is likely to be performed
by the sensor manufacturer, but peri-
B
Working much like a hot plate, this infrared calibration source uses a high emissivity, specially textured surface to provide a convenient temperature
reference.
odic recalibration—in-house or by a
third-party laboratory or the original
manufacturer—is necessary if any but
the most qualitative measurements
are expected.
The ongoing accuracy of a noncontact temperature sensor will
depend on the means by which the
calibration is performed, how frequently it is recalibrated, as well as
the drift rate of the overall system.
Ensuring the absolute accuracy of
non-contact temperature measurement devices is more difficult than
with most
direct contacting
devices, such as thermocouples and
resistance temperature detectors
(RTDs). Limiting the absolute accuracy to 1% is difficult; even in the
most sophisticated set-ups, better
TRANSACTIONS
than 0.1% accuracy is seldom
achieved. This arises, in part, from
the difficulty in accurately determining the emissivity of real bodies.
Repeatability or reproducibility is,
however, more readily achievable
than absolute accuracy, so don’t
pay more if consistency will do.
If absolute accuracy is a concern,
then traceability to standards such as
those maintained by the National
Institute of Standards & Technology
(NIST) will also be important.
Traceability, through working to secondary to primary standards is central to the quality standards compliance such as those defined by the
ISO 9000 quality standard.
Why Calibrate?
There are generally three methods
of calibrating industrial radiation
thermometers. One method is to
use a commercial blackbody simulator, an isothermally heated cavity
with a relatively small aperture
through which the radiation thermometer is sighted (Figure 7-1). As
explained in the earlier chapter on
“Theoretical Development,” this
type of configuration approaches
blackbody performance and its
emissivity approaches unity. A standard thermocouple or resistance
temperature detector (RTD) inside
the cavity is used as the temperature reference. At higher temperatures, calibrated tungsten filament
lamps are commonly used as references. A final alternative is to used a
reference pyrometer whose calibration is known to be accurate, adjusting the output of the instrument
being calibrated until it matches.
In any case, the radiation source
must completely fill the instrument’s
field of view in order to check the
Back Thermocouple
Front Thermocouple
Refractory
Sphere
Controller
Control Thermocouple
Figure 7-1: A Spherical Blackbody Cavity
Volume 1
53
IR Thermometer Calibration
7
calibration output. If the field of
view is not filled, the thermometer
will read low. In some instruments,
calibration against a blackbody reference standard may be internal—a
chopper is used to alternate
between exposing the detector to
the blackbody source and the surface of interest. Effectively, this provides continuous recalibration and
helps to eliminate errors due to drift.
Blackbody Cavities
Because calibration of a non-contact
temperature sensor requires a source
of blackbody radiation with a precise
means of controlling and measuring
the temperature of the source, the
interior surface of a heated cavity
constitutes a convenient form, since
the intensity of radiation from it is
essentially independent of the material and its surface condition.
In order for a blackbody cavity to
work appropriately, the cavity must
be isothermal; its emissivity must be
known or sufficiently close to unity;
and the standard reference thermo-
A handheld IR thermometer is calibrated against a commercial blackbody source—the internal cavity
is designed to closely approach a blackbody’s unity emissivity.
couple must be the same temperature as the cavity. Essentially, the
blackbody calibration reference consists of a heated enclosure with a
small aperture through which the
interior surface can be viewed (Figure
7-1). In general, the larger the enclosure relative to the aperture, the
more nearly unity emissivity is
1.0
0.95
0.9
0.8
Effective Emissivity, ε
0.8
0.7
0.6
0.5
0.4
0.3
φ
Cavity Surface Emissivity
0.2
0.1
0.0
0
20
40
60
80
100
120
Aperture Angle, φ (Deg)
Figure 7-2: Effective Emissivity of Spherical Cavities
54
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160
180
approached (Figure 7-2). Although
the spherical cavity is the most commonly referenced shape, carefully
proportioned cone- or wedgeshaped cavities also can approach
unity emissivity.
In order to provide isothermal surroundings for the cavity, the following materials commonly are used:
• Stirred water bath for 30-100°C (86212°F) temperature ranges;
• Aluminum core for 50-400°C (122752°F) temperature ranges; and
• Stainless steel core for 350-1000°C
(662-1832°F) temperature ranges.
And while blackbody cavities
have their appeal, they also have
some disadvantages. Some portable,
battery-operated units can be used
at low temperatures (less than
100°C), but blackbody cavities are,
for the most part, relatively cumbersome and expensive. They also
can take a long time to reach thermal equilibrium (30 minutes or
more), slowing the calibration procedure significantly if a series of
measurements is to be made.
TRANSACTIONS
7
Filament
Pointer
Nickel
Support
Glass/
Ceramic Base
Lamps, however, must be calibrated in turn against a blackbody standard; the user typically is provided
with the relationship between electric current to the filament and its
temperature. Emissivity varies with
temperature and with wavelength,
but material is well understood
enough to convert apparent temperatures to actual.
Just as a blackbody cavity includes
a NIST-traceable reference thermocouple, instrument calibration against
a ribbon lamp also can be traced to
NIST standards. In a primary calibration, done mostly by NIST itself, fila-
IR Thermometer Calibration
ment current is used to balance standard lamp brightness against the
goldpoint temperature in a blackbody furnace, in accordance with the
ITS-90. Typical uncertainties range
from ±4°C at the gold point to ± 40°C
at 4000°C.
In secondary standard calibration,
the output of a primary pyrometer,
i.e., one calibrated at NIST, is compared with the output of a secondary pyrometer when sighted
alternately on a tungsten strip lamp.
Many systematic errors cancel out in
this procedure and make it more
practical for routine calibration. T
References and Further Reading
Figure 7-3: Typical Tungsten Lamp Filament
Tungsten Filaments
As a working alternative to blackbody cavities, tungsten ribbon lamps,
or tungsten strip lamps, are commonly used as standard sources
(Figure 7-3). Tungsten strip lamps are
highly reproducible sources of radiant energy and can be accurately calibrated in the 800°C to 2300°C range.
They yield instantaneous and accurate adjustment and can be used at
higher temperatures than those readily obtainable with most cavities.
TRANSACTIONS
• Handbook of Temperature Measurement & Control, Omega Press, 1997.
• New Horizons in Temperature Measurement & Control, Omega Press, 1996.
• Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, and
N. H. Baker, Omega Press, 1975.
• The Detection and Measurement of Infrared Radiation, R.A. Smith, F. E.
Jones, and R. P. Chasmar, Oxford at Clarendon Press, 1968.
• Handbook of Temperature Measurement & Control, Omega Engineering
Co., 1997.
• Infrared Thermography (Microwave Technology, Vol 5), G. Gaussorgues
and S. Chomet (translator), Chapman & Hall, 1994.
• Instrument Engineers’ Handbook, Third Edition, B. Liptak, Chilton Book
Co. (CRC Press), 1995.
• Process/Industrial Instruments and Controls Handbook, 4th ed., Douglas
M. Considine, McGraw-Hill, 1993.
• Theory and Practice of Radiation Thermometry, David P. DeWitt and
Gene D. Nutter, John Wiley & Sons, 1988.
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