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

Table 2. Temperat ure sources

Techn ical need s Fixed points Baths Metrology wells Field metrology

Accu racy ±0.001 °C to ±0.01 °C

Accu racy ±0.01 °C to ± 0.1 °C

Accu racy ±0.1 °C to ±1.0 °C

Cali brate on-site

Elim inate fluids

Get to temperature fast

Cali brate multiple sizes and shapes

Automate calibration

Use fewer tools

Learn in less than 15 minutes

Use easily

Caption to go here

Table 3. Thermometer types

Techn ical need s Thermistor PRT SPRT HTSP RT Thermocouple

Range

0 °C to 100 °C

–200 °C to 660 °C

0 °C to 961 °C

0 °C to 1450 °C

Accuracy

±0.001 to ±0.01

±0.01 to ± 0.10

±0.1 to ±2.5

• • •

• • •

•

• •

• • • •

• • • •

• • • •

• •

• • • • • •

• • • • •

• •

• •

•

wells

• • •

•

•

•

Micro-baths Dry-wells

•

Once you have chosen the
type of thermometer probe you
will use as the temperature
standard, you need to choose
a device to measure it. This
device could be a special­ized thermometer readout, or
it could be a general purpose
instrument like a digital multi­meter. Thermometer readouts
deliver the best results because
their ranges, current settings
and firmware are designed
for temperature measurement.
Selection criteria for thermome­ter readouts include the type of
thermometer and level of accu­racy required and other factors
like need for battery power,
number of inputs, and whether
data can be recorded for future
retrieval. See Table 4.

Don’t forget that if you are
calibrating temperature sensors
like RTDs and thermocouples,
you will need thermometer
readouts for them too. A multi­channel thermometer readout
can be used to measure your
temperature standard and your
thermometers under test at the
same time.

4 Fluke Calibration Temperature calibration equipment: A technicians guide

Table 4. Thermometer readouts from Fluke, Hart Scientific

Techn ical need s 1523 1524 1502A 1504 1529 1560 1595

Measure RTDs

Measure PRTs

Measure SPRTs

Measure HTSPRTS

Measure
thermistors

Measure
Thermocouples

Measure
transmitters

Measure thermal
switches

Cali brate on-site

Batter y powered

Use fewer tools

Single channel

Multi channel

Record data

Automated data
logging

Graphing

Fixed point
calibration

• • • • • • • •

• • • • • • • •

• •

• •

• • • • • •

• • • • •

• • • • • • • •

• • •

• • •

• • • • •

• • • • •

• • •

• • • •

•

914X 917X

Use two probes and the 1524 to read, log,
graph, and calibrate twice as much.

•

• •

• •

Calibrators wit h measurement capability

The 914X-P can act as the indicator for trans­mitters, thermocouples and RTDs and even a
reference PRT to improve accuracy.

5

Table 5. ITS-90 fixed point calibration equipment

Temperature Needed equip ment

–196 °C Boiling Point of Liquid Nitrogen with 7196-4 Vacuum Flask Dewar

–38.8344 °C 5900 Triple Point of Mercury (TpHg) Cell and 7341 Mercury Maintenance Bath

0.010 °C 5901-D-Q T riple Point of Water (TPH20) Cell and 7312 TPW Maintena nce Bath

29.7646 °C 5943 Melting Point of Gal lium (MPGa) Cell and 9230 Ga llium Maintenance

156.5985 °C 5904 Freezing Point of I ndiu m (FPIn) Cell and 9114 Freeze Point Furnace

231.928 °C 5905 Freezing Point of Tin (FPSn) Cell and 9114 Freeze Point Furnace

419.527 °C 5906 Freezing Point of Zinc (FPZn ) Cell and 9114 Freeze Point Furnace

660.323 °C 5907 Freezing Point of A lum inum (FPAl) Cell and 9114 Freeze Point Fur nace

961.78 °C 5908 Freezing Point of Silver (FPAg) Cell and 9115A Freeze Point Furnace

–196 °C to 961 °C 5581 Primary Standard Automated Resistance Bridge

–196 °C to 660 °C 5430-10 10-Ohm Resistance Standard and 7108 Resistor Maintenance Bath

0 °C to 961 °C 5430-1 1-Ohm Resistance Standard and 7108 Resistor Maintenance Bath

Apparatus

5313-002 10 Channel Scanner for Automated Resistance Bridge ( optional)

5313-004 Software for Automated Resistance Bridge

As mentioned earlier, fixed
point calibration is a little
different from comparison cali­bration and requires different
equipment. Table 5 shows the
kind of equipment used in a
primary standards laboratory
that performs fixed point cali­brations on SPRTs.

There is at least one fixed

point cell that is used in
almost all temperature cali­bration laboratories around
the world. The triple point of
water cell is the backbone of
the temperature calibration
laboratory, even if comparisons
are the only type of calibra­tion performed. The triple point
of water is not just required
for calibrating SPRTs, it’s also
required for maintaining their
accuracy after calibration.
So if you own an SPRT, you
should also own a triple point
of water cell. Even secondary
reference PRTs used as calibra­tion temperature standards are
also checked regularly at the
triple point of water to ensure

their continued reliability. Mini
triple point of water cells are
available to make this more
convenient and economical.

A few examples

To reinforce this discussion,
a few examples arranged in a
good, better, and best format
are summarized in table 6.
A good solution for perform­ing calibrations on-site is a
dry-well. If you are new to
on-site temperature calibration,
a handheld dry-well like the
9100S or 9102S is a good place
to start. You can learn to use
these in less than 15 minutes,
they can be taken just about
anywhere you need to calibrate
something, and they are very
reasonably priced.

A better solution would be a
field dry-well with an exter­nal reference thermometer for
improved accuracy. A 1523 is
a great choice for the readout
because it graphs temperature
in real-time, letting you know
exactly when it’s time to take a
reading, and it measures PRTs,
thermistors and thermocouples.

6 Fluke Calibration Temperature calibration equipment: A technicians guide

Table 6. Examples of calibration solutions

Good Better Best

Accu racy 1.0 °C to 0.1 °C 0.1 °C to 0.01 °C 0.01 °C to .001 °C

Workload Dial ther mometers RTDs/PRTs PRTs

Thermocouples Liquid in glass SPRTs

Typical environment Mostly on-site Lab or on-site Lab

Entry-level solution Handheld dry-well:

Premium solution

9100S or 9102S

Handheld readout: 1523
or 1524

PRT : 5616 5699 SPRT 5901D-Q TPW cell

Field dry-well: 9103 or
9141

Field Metrology Well:
914X-P
5626 reference PRT

1560 with 256X Module 7012 TPW Maintenance

70XX or 60XX bath 9114 fixed point furnace

9210 mini Tpw
Maintenance Apparatus
5901B mini TPW cel l
9260 Mini Furnace
594X Fixed Point Cells
7196 LN2 comparator

Bath

590X fixed point cells

7341 TPHg bath

5900A TpHg cell

7196 LN2 Comparator

5681 SPRT

5581 Resistance bridge

More accurate thermometers
like RTDs and LIGs (liquid in
glass thermometers) require
more accurate equipment to
calibrate them. The equipment
used to calibrate RTDs and LIGs
often bridge the gap between
on-site and laboratory calibra­tion work and could be used
in either location. For example,
914X Field Metrology Wells
can be carried on-site in one
hand and are great for speed
and versatility. 917X Metrology
Wells are portable and yet are
sufficiently accurate for labora­tory calibration work. On the
other hand, certain calibration
baths, are sufficiently compact
to, be taken on-site (i.e. 6102
or 7102 Micro-Baths), but most
calibration baths are used
exclusively in the laboratory,
because of their size.

For best results, SPRTs and
better PRTs can be calibrated
by fixed point. Entry-level fixed
point calibration is usually
done with mini-fixed point
cells for high-end PRTs. The
maintenance apparatus for
these are significantly less
expensive than maintenance
apparatus for large cells, and
the smaller dimensions make
it easier to calibrate PRTs than
SPRTs (i.e. 9 in to 15 in) and
would not be long enough for
calibration in the large fixed
point cells. Fixed point calibra­tions do not require a reference
thermometer, unless a compari­son at the normal boiling point
of liquid nitrogen (–197 °C) is
substituted for the triple point
of Argon (–189.3442 °C).

7

Conclusion

If you have a new type of thermometer to calibrate, hopefully you
now have a better idea of what you are going to need to calibrate
it. If you have SPRTs, you are going to need the equipment for
fixed point calibration, and if you are doing comparison calibra­tions of other types of thermometers, your choice of equipment
may hinge largely on where you have to go to calibrate them. If
it’s in the laboratory, you’ll probably use baths and SPRTs with
thermometer readouts, and if you’re calibrating on-site you’ll
be using a calibrator like a Field Metrology Well or Micro-Bath.
Whichever way you decide to go, don’t forget to talk to the
experts; we’ll help you get started and keep you up and running.

8 Fluke Calibration Temperature calibration equipment: A technicians guide

Fluke Calibration.

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Fluke Calibration

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PO Box 1186, 5602 BD
Eindhoven, The Netherlands

For more information call:

In the U.S.A. [phone] or
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Web access: http://www.fluke.com

©2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 8/2010 3592831A A-EN-N

Modification of this document is not permitted without
written permission from Fluke Corporation.

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