ASTM E220-86(1996)e1
(Test Method)Standard Test Method for Calibration of Thermocouples By Comparison Techniques
Standard Test Method for Calibration of Thermocouples By Comparison Techniques
SCOPE
1.1 This test method covers the techniques of thermocouple calibration based upon comparisons of thermocouple indications with those of a reference thermometer, different from methods involving the use of fixed points. The precise evaluation of the electromotive force (emf)-temperature relation of a thermocouple is accomplished by determining its emf output at each of a series of measured temperatures. Calibrations are covered over temperature ranges appropriate to the individual types of thermocouples within an over-all range from about -180 to 1700°C (-290 to 2660°F).
1.2 In general, the test method is applicable to bare wire thermocouples or sheathed thermocouples. The latter may require special care to control thermal conduction losses.
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e1
Designation:E220–86(Reapproved 1996)
Standard Test Method for
Calibration of Thermocouples By Comparison Techniques
This standard is issued under the fixed designation E220; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Keywords were added editorially in November 1996.
1. Scope Monograph 150—Liquid-in-Glass Thermometry
1.1 This test method covers the techniques of thermocouple
3. Terminology
calibration based upon comparisons of thermocouple indica-
3.1 Definitions—The definitions given in Terminology
tions with those of a reference thermometer, different from
E344 shall be considered as applying to the terms used in this
methods involving the use of fixed points. The precise evalu-
method.
ationoftheelectromotiveforce(emf)-temperaturerelationofa
3.2 Definitions of Terms Specific to This Standard:
thermocoupleisaccomplishedbydeterminingitsemfoutputat
3.2.1 type of thermocouple—the type of a thermocouple is
each of a series of measured temperatures. Calibrations are
represented by a letter designation as defined in accordance
covered over temperature ranges appropriate to the individual
with Specification E230.
types of thermocouples within an over-all range from
3.2.2 reference thermometer—a thermometer whose cali-
about−180 to 1700°C (−290 to 2660°F).
bration is known within a certain specified accuracy.
1.2 In general, the test method is applicable to bare wire
thermocouples or sheathed thermocouples. The latter may
4. Summary of Test Method
require special care to control thermal conduction losses.
4.1 By this test method a thermocouple is calibrated by
comparing its indications with those of a reference thermom-
2. Referenced Documents
eter at the same temperature. The reference thermometer may
2.1 ASTM Standards:
2 be another thermocouple, a liquid-in-glass thermometer, or a
E1 Specification for ASTM Thermometers
platinumresistancethermometer,dependinguponthetempera-
E77 Test Method for Inspection and Verification of Ther-
2 ture, the degree of accuracy required, or other considerations.
mometers
4.2 Since the success of the test method depends largely
E230 Specification for Temperature-Electromotive Force
2 upontheabilitytobringthethermocoupleandthestandardized
(EMF) Tables for Standardized Thermocouples
reference thermometer to the same temperature within the
E344 Terminology Relating to Thermometry and Hydrom-
2 required limits of accuracy, considerable care must be taken in
etry
choosing the media and conditions under which the compari-
E563 Practice for Preparation and Use of Freezing Point
2 sons are made. Stirred liquid baths, uniformly heated metal
Reference Baths
blocks,tubefurnaces,anddryfluidizedbaths,usedwithproper
2.2 ANSI Standard:
techniques,arespecifiedforuseintheirrespectivetemperature
C 100.2 Direct-Current Ratio Devices: High Precision
3 ranges.
Laboratory Potentiometers
4.3 Potentiometric instruments, or high-impedance elec-
2.3 NIST Publications:
tronic instruments, must be used for the measurement of emf
Circular 590—Methods of Testing Thermocouples and
eliminating instrument loading as a significant source of error.
Thermocouple Materials
4 The details of the test method, therefore, aim to provide
Monograph 126—Platinum Resistance Thermometry
assurance that the emf measured is actually the emf output of
the thermocouple at the temperature of test and is not influ-
This method is under the jurisdiction ofASTM Committee E-20 on Tempera-
enced by emf’s arising from other sources.
ture Measurement and is the direct responsibility of Subcommittee E20.04 on
Thermocouples.
5. Significance and Use
Current edition approved March 27, 1986. Published May 1986. Originally
5.1 For users or manufacturers of thermocouples, the test
published as E 220 – 63 T. Discontinued January 1995 and reinstated as
E220–86(1996)e .
methodprovidesameansofconfirmingtheacceptabilityofthe
Annual Book of ASTM Standards, Vol 14.03.
material in the assembled state. Typically wire producers
Available from American National Standards Institute, 11 W. 42nd St., 13th
provide calibration of the individual thermocouple legs.
Floor, New York, NY 10036.
Available from National Institute of Standards and Technology, U.S. Depart-
ment of Commerce, Washington, DC 20234.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E220
5.2 The test method provides for certifications to tempera- existingtemperaturegradientsmustbemadebeforeconfidence
turetolerancesforspecificationssuchasSpecificationE230or may be placed in such apparatus.
other special specifications as required for commercial, mili-
6.3 Reference Junction Temperatures—A controlled tem-
tary, or research applications.
peraturebathmustbeprovidedinwhichthetemperatureofthe
5.3 The test method assumes that the materials are homo-
reference junctions is maintained constant at a chosen value.A
geneous.
commonly used reference temperature is 0°C (32°F), usually
realized through use of the ice point, but other temperatures
6. Apparatus
may be used if desired. The reference junction temperature
6.1 The apparatus required for the application of this test
should be controlled to a better accuracy than that expected
methodwilldependindetailuponthetemperaturerangebeing
from the thermocouple calibration to minimize this tempera-
covered but in all cases shall be selected from the equipment
ture variation as a source of error. An acceptable method for
described below.
utilizing the ice point as a reference junction temperature is
6.2 Comparator Baths and Furnaces—A controlled com-
given in Practice E563.
parator bath or furnace shall be used in which the measuring
6.3.1 For the rapid calibration of large numbers of thermo-
junction of the thermocouple to be calibrated is brought to the
couples the reference junctions can be made at an isothermal
same temperature as a reference thermometer.
multiterminal strip, whose temperature is determined by a
6.2.1 Liquid Baths—In the range from−160 to 630°C
reference thermocouple whose reference junction is in an
(−250 to 1170°F) the comparator bath shall usually consist of
ice-point bath. This system avoids thermal loading of the ice
a well stirred, insulated liquid bath provided with controls for
bath by a large number of thermocouple wires and copper
maintaining the temperature constant. Suitable types are de-
connecting wires.
scribed in the appendix to Test Method E77. Laboratory type
6.3.2 Minimum error can be achieved only by running the
tube furnaces may be used above ambient temperature but are
thermocouple wires, without splices, from the measuring
not recommended for the most accurate work in this tempera-
junction to the reference junction. Any splice represents an
ture range.
inhomogeneity in the circuit due to the mismatch of nominally
6.2.2 Fluidized Powder Baths—In the range from−70 to
similar alloys.The magnitude of the error due to the mismatch
980°C(−100to1800°F)thecomparatorbathmayconsistofan
will depend on the temperature gradients existing.
air-fluidized bathofaluminumoxideorsimilarpowder.Such
6.4 Emf-Measuring Instruments—The choice of a specific
a bath should be monitored to ensure consistency and unifor-
instrument to use for measuring the thermocouple emf will
mity of temperature.
depend on the accuracy required of the calibration being
6.2.3 Tube Furnaces—At temperatures above approxi-
performed. Generally, the instrument can be chosen from one
mately 620°C (1150°F) an electrically heated tube furnace
of three groups of commercially available, laboratory, high-
shallusuallyconstitutethecomparatorbath.Anyoneofawide
precisiontypeswithemfrangessuitableforusewiththermom-
variety of designs may be suitable, but the furnace chosen
eters. The first two groups are manually balanced potentiom-
should have the following capabilities:
eters that are not self-contained and that require a more-or-less
6.2.3.1 Means should be provided to control the furnace at
permanentbenchsetupwithanumberofaccessories,including
aconstanttemperatureforshortlengthsoftime(approximately
a storage battery, high-sensitivity galvanometer or null detec-
10min)atanytemperatureintherangeoverwhichthefurnace
tor,andalaboratory-typestandardcell.Allinstrumentsrequire
is to be used.
periodic calibration by the National Institute of Standards and
6.2.3.2 There should be a zone of uniform temperature into
Technology or some other laboratory similarly qualified.
which the thermocouple measuring junctions may be inserted,
6.4.1 Group A Potentiometers shall be used where the
and the length of the furnace tube must be adequate to permit
highestaccuracyisrequired.Potentiometersofthisgrouphave
a depth of immersion sufficient to assure that the measuring
no slide wires, all settings being made by means of dial
junction temperature is not affected by temperature gradients
switches. All design features will be consistent with the
along the thermocouple wires.
attainmentofthehighestaccuracy.Suchinstrumentsshallhave
NOTE 1—Further discussions of suitable tube furnaces are given in
a limit of error of 0.2 µV at 1000 µV and 5 µV or better at
X1.1 and X1.2.
50 000 µV in accordance with ANSI C100.2.
6.2.4 Other Baths—The one essential design feature of any
6.4.2 Group B Potentiometers will normally be sufficiently
bath to be used with this method is that it brings the
accurate for most work. Such potentiometers may contain a
thermocouple being calibrated to the same temperature as the
slide wire, but all design features shall be directed toward high
reference thermometer. Copper blocks immersed in liquid
accuracy. Instruments of this class shall have limits of error of
oxygen or some other refrigerant have been used successfully.
1 µV at 1000 µV and 12 µV at 50 000 µV.
The blocks are provided with wells for the test thermocouples
6.4.3 Group C Instruments include electronic digital volt-
and the reference thermometer. Similarly, uniformly heated
meters and analog-to-digital converters of potentiometric or
blockshavebeenusedathightemperatures.Suchbathsarenot
other high-impedance design. Instruments of this class have
excluded under this test method, but careful explorations of
limits of error similar to those in 6.4.1 and 6.4.2. These
instruments permit fast readings of a large number of thermo-
couples. Such fast readings demand less temperature stability
Callahan, J. T., “Heat Transfer Characteristics in Air Fluidized Solids up to
900°F,” ASME Paper 70WA/Temp 3, Journal of Basic Engineering, 1971. of the bath with time.
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E220
6.5 Connecting WireAssembly—Connecting wires from the eter may be used from−180°C (−300°F), or lower, to 400°C
reference junction to the potentiometer are of insulated copper (750°F), or even higher with special types. Generally, the
and should be run in a grounded conduit or braided cable if accuracy of these thermometers is less below−60°C, where
they are subject to electrical pickup. organic thermometric fluids are used, and above 400°C where
6.5.1 Selector switches may be used to switch between dimensional changes in the bulb glass may be relatively rapid,
different thermocouples being calibrated and the standard requiring frequent calibration. The uncertainties of different
thermocouple. Such switches should be of rugged construction types of liquid-in-glass thermometers are given in X2.3.
and designed so that both connecting wires are switched when Specifications forASTM thermometers are given in Specifica-
switching from one thermocouple to the next, leaving thermo- tion E1.
couples not in use entirely disconnected from the potentiom- 7.4 Types R and S Thermocouples (Platinum-Rhodium/
eter. The switches should be constructed with copper contacts, Platinum)—Theplatinum-10%rhodium/platinum(TypeS),or
connections, and terminals and must be located in the copper the platinum-13% rhodium/platinum thermocouple (Type R)
portionofthecircuittopreservetheall-coppercircuitfromthe of 24-gage (0.51-mm) wire is recommended as the reference
reference junction to the potentiometer. Precautions should be thermometerfortemperaturesfrom630°C(1170°F)to 1200°C
takentoprotecttheswitchesfromtemperaturefluctuationsdue (2190°F). Their use may also be extended down to room
to air currents or radiation from hot sources. temperature.Accuraciesattainablewithcarefulusearegivenin
6.5.2 Terminalblocksmaybeusedintheconnectingcircuit, Table 1. Group A and B potentiometers (6.4.1 and 6.4.2) and
if convenient, but should be provided with copper binding Group C instruments (6.4.3) can be used with these thermo-
posts and should be protected against the development of couples.
temperature gradients in the blocks. 7.5 Type B Thermocouples (Platinum-Rhodium/Rhodium-
6.6 Thermocouple Insulation and Protection Tubes—Two- Platinum)—The platinum-30 % rhodium/platinum6 %
hole ceramic tubing may be used to support and electrically rhodium (Type B) thermocouple, formed from 24-gage (0.51-
insulate the immersed portion of the two bare conductors of a mm) or larger size wire, is recommended as the reference
thermocouple.Onlysuitableceramicshouldbeused,chosenof thermometer for temperatures above 1200°C (2190°F). The
a material which will not contaminate the thermocouple and uncertainties of temperature measurements with this type of
which will provide the necessary electrical insulation at the thermometer are given in Table 1. Group A and B potentiom-
highest temperature of the calibration. To avoid unnecessary eters (6.4.1 and 6.4.2) and Group C instruments (6.4.3) are
massandtominimizeaxialheatconductionintheregionofthe suitable for use with this type of thermocouple.
measuring junction, the tubing should be relatively thin walled 7.6 Type T Thermocouples (Copper-Constantan)—This
and should have bore diameters that provide a loose fit for the type of thermocouple may serve as a useful reference ther-
thermocouple wires without binding. During the test, the mometer in the range of−180 to 370°C (−300 to 700°F) in
thermocouples may be inserted in a protection tube which some instances, although its accuracy is, in general, limited by
should be resistant to thermal shock, noncontaminating to the the stability of the wire at temperatures above approximately
thermocouple materials, and gastight. 200°C (400°F), and by the accuracy of the emf measurements
6.6.1 Sheathedthermocouplesmaybetestedwithoutfurther and the inhomogeneity of the wire below 200°C. Twenty-four
protection or support in liquid or
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