Fluke 1523, 1524 Application Note
Temperature
calibration equipment:
A technician’s guide
Have you ever been brought a new thermometer to calibrate, and asked yourself,
“What am I going to need to calibrate this?” This guide is intended to help you work
out the kind of equipment you need for your particular calibration situation. Of course,
there are many considerations—including accuracy, temperature range, automation
requirements and budget. This guide covers the main points, but you’ll probably want
to speak with an experienced Fluke application specialist before you make a nal
decision; as always, we stand ready to help keep you up and running.
Getting started
Most often, thermometer type
determines the kind of equipment needed. Some of the most
common items that need to be
calibrated are listed in Table 1.
The equipment types shown in
the Needed equipment column
should not be considered
definitive. For example, the
same equipment used to calibrate an RTD or an SPRT could
also be used to calibrate a PRT,
but this is a good guide to
what you would most likely
use. In addition, choice of
equipment may depend on
where the calibration is
performed. For example, you
might use a calibration bath
and temperature standard to
calibrate an RTD in a laboratory, but a portable calibrator
would be more appropriate for
on-site calibrations. More about
the equipment needed will be
explained below.
Calibration method
No matter what your temperature calibration application,
you’re going to need a temperature source to heat or cool
your thermometers to a known
temperature. During the calibration, the thermometers are
Application Note
placed into a heat transfer
medium in the temperature
source. The heat transfer
medium might be a stirred fluid,
a metal block, or a fixed point
cell. The heat transfer medium
maintains a constant and
uniform temperature environment that allows the reading
of the thermometer under test
to be compared to a more
accurate known temperature.
The known temperature
value is going to come from
one of two places:
Naturally occurring
•
phenomena, such as
the triple point of water
(0.010 °C)
A temperature measure-
•
ment, made by a
temperature standard
These two different methods
of getting the more accurate
known temperature lead to two
distinct methods of calibration:
comparison calibration and
fixed point calibration.
Comparison calibration is the
most common type.
Table 1. Common thermometers that need to be calibrated
Workload Where calibrating? Needed equipment
Dial thermometer On-site A portable temperature calibrator
Liquid in glass Laboratory Cali bration bath, f luid level adapter, magnifier, carousel, and
RTD On-site A portable temperature calibrator and (optional) temperature
Laboratory Cali bration bath and temperature standard
PRT On-site A portable temperature calibrator and (optional) temperature
Laboratory Cali bration bath and temperature standard
Thermocouple On-site A portable temperature calibrator and (optional) temperature
Laboratory Cali bration furnace and temperature standa rd
Thermistor Laboratory Calibration bath and temperature standard
SPRT Laboratory Fixed point cells, maintenance furnaces /baths, a standard
Infrared
thermometer
On-site Radiometrically calibrated infrared calibrator
Laboratory Radiometrically calibrated infrared calibrator, or an infrared
temperature standard
standard
standard
standard
resistor, and a resistance bridge
calibrator (plate) and a reference radiometer, or an infrared
blackbody (cav ity)
Comparison
calibration
For contact thermometer
comparison calibrations, you
will need:
A temperature source to heat
•
or cool the thermometer(s)
under test
A temperature standard to
•
provide the accurate known
temperature that is compared
with the thermometer under
test
(Optional) Measuring devices
•
to read the temperature standard and/or thermometer(s)
under test
As the name implies, during
a comparison calibration, a
thermometer under test is
compared to a more accurately calibrated temperature
standard, while both are maintained at the same constant
temperature in the temperature
source. Typically the standard
is four times more accurate
than the thermometer under
test. Any thermometer can be
calibrated by comparison, and
comparison calibrations can
take place either in a laboratory or on-site.
For non-contact thermometer
comparisons you will need:
A radiance source to gener-
•
ate the known radiance
observed by the infrared
thermometer
A radiometric temperature
•
standard to provide the
accurate known temperature
that is compared with the
thermometer under test
The radiance source can be
either a painted surface or a
blackbody cavity. Good blackbody cavities have a well
known emissivity value (i.e.
0.95 ±0.001). The key performance indicator of a radiance
source is its spectral emissivity. The spectral emissivity
depends on wavelength, the
geometry of the surface, the
finish of the surface, and the
types of plate material and
paint used. The emissivity of a
painted surface is different for
each wavelength; therefore, its
radiance is only known if it is
measured over the same wavelengths used by the infrared
thermometers being calibrated.
For example, measurements of
surface by a radiometer over
the band of wavelengths from
2 Fluke Calibration Temperature calibration equipment: A technicians guide
8 to 14 microns will be good
for calibrating thermometers of
the same bandwidth (8 to 14
microns).
Surfaces used to calibrate
infrared thermometers should
be calibrated radiometrically
over the correct bandwidth, or
else a radiometric temperature
standard (radiometer) with the
correct bandwidth needs to
be compared with the thermometers under test during
calibration. For example, the
Fluke, Hart Scientific 4181
Precision Infrared Calibrator is
calibrated radiometrically from
8 to 14 microns and does not
require a separate radiometric
temperature standard over that
bandwidth. Alternatively, the
Hart 9132 Infrared Calibrator is
not radiometrically calibrated
and does require a separate radiometer for infrared
traceability.
Fixed point
calibration
For the most accurate thermometers under test, the only
sufficiently accurate temperature standard is a primary
standard. Fixed point cells are
the primary standards used
in temperature calibration. In
a primary standards laboratory, SPRTs are placed in fixed
point cells and given ITS-90
calibrations. The ITS-90 is
the international temperature
scale used by the International
System of Units (SI) to define
Kelvin and Celsius temperature
values for the world.
Fixed point cells rely on the
intrinsic properties of nature
to provide a very precisely
known temperature. Extremely
pure substances (i.e. tin, zinc
or water) under the right
conditions of temperature
and pressure become very
precise and reliable temperature standards. Thermometers
are calibrated by placing
them inside the cell so that
the thermometer and the
fixed point cell are resting at
the same temperature. This
means a fixed point cell is
both a temperature source
and a temperature standard.
Fixed point cells are the most
accurate type of temperature
calibration equipment but they
are also the most difficult to
use and are found mainly in
primary standards laboratories.
For fixed point calibrations,
you will need:
A fixed point cell (tempera-
•
ture source)
A device to maintain the
•
temperature surrounding the
cell (i.e. bath or furnace)
A resistance bridge to
•
measure the SPRT being
calibrated
Choosing a temperature
source
When choosing a temperature source, you often need to
choose the best compromise
between accuracy and some
other technical requirement.
Table 2 compares various
types of temperature sources
against some common technical requirements.
Choosing a calibrated
thermometer
(temperature standard)
For comparison calibrations,
you need to choose a calibrated thermometer for your
temperature standard. There
are several types to choose
from. Your choice depends
on your temperature range
and the required accuracy of
your measurements. Table 3
provides a guide for finding the
right type of thermometer for
your application. Other conditions that should be considered
are degree of ruggedness,
and needed probe dimensions
such as length and diameter.
A general rule for resistance
thermometers such as PRTs,
SPRTs, and HTSPRTs is that the
more rugged the instrument,
the less accurate it becomes.
3
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