1、Designation: E 220 07e1Standard Test Method forCalibration of Thermocouples By Comparison Techniques1This standard is issued under the fixed designation E 220; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revisi
2、on. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.e1NOTEEditorial changes were made throughout in July 2007.1
3、. Scope1.1 This test method describes the principles, apparatus, andprocedure for calibrating thermocouples by comparison with areference thermometer. Calibrations are covered over tempera-ture ranges appropriate to the individual types of thermo-couples within an overall range from approximately 19
4、5 to1700 C (320 to 3100 F).1.2 In general, this test method is applicable to unusedthermocouples. This test method does not apply to usedthermocouples due to their potential material inhomogeneitythe effects of which cannot be identified or quantified bystandard calibration techniques. Thermocouples
5、 with large-diameter thermoelements and sheathed thermocouples mayrequire special care to control thermal conduction losses.1.3 In this test method, all values of temperature are basedon the International Temperature Scale of 1990. See GuideE 1594.1.4 This standard may involve hazardous materials, o
6、pera-tions and equipment. This standard does not purport to addressall of the safety concerns, if any, associated with its use. It isthe responsibility of the user of this standard to establishappropriate safety and health practices and determine theapplicability of regulatory requirements prior to
7、use.2. Referenced Documents2.1 ASTM Standards:2E1 Specification for ASTM Liquid-in-Glass ThermometersE77 Test Method for Inspection and Verification of Ther-mometersE 230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE 344 Terminology Relating to Thermom
8、etry and Hydrom-etryE 452 Test Method for Calibration of Refractory MetalThermocouples Using a Radiation ThermometerE 563 Practice for Preparation and Use of an Ice-Point Bathas a Reference TemperatureE 644 Test Methods for Testing Industrial Resistance Ther-mometersE 988 Temperature-Electromotive F
9、orce (EMF) Tables forTungsten-Rhenium ThermocouplesE 1129/E 1129M Specification for Thermocouple Connec-torsE 1594 Guide for Expression of TemperatureE 1684 Specification for Miniature Thermocouple Connec-torsE 1751 Guide for Temperature Electromotive Force (EMF)Tables for Non-Letter Designated Ther
10、mocouple Combi-nations3. Terminology3.1 DefinitionsThe definitions given in TerminologyE 344 shall apply to this test method.3.2 Definitions of Terms Specific to This Standard:3.2.1 check standard, na measurement instrument orstandard whose repeated results of measurement are used todetermine the re
11、peatability of a calibration process and toverify that the results of a calibration processes are statisticallyconsistent with past results.3.2.2 isothermal block, na piece of solid material of highthermal conductivity used to promote thermal equilibriumbetween two or more thermometers.3.2.3 referen
12、ce junction compensation, nthe adjustmentof the indication of a thermocouple such that the adjustedindication is equivalent to the emf or temperature that thethermocouple would indicate if the reference junctions weremaintained at 0 C.3.2.3.1 DiscussionIn most cases, the thermocouple indi-cation is
13、adjusted by measuring the temperature of a terminalblock where the thermocouple is connected, and then adding tothe thermocouple emf an additional emf equal to the emf of the1This test method is under the jurisdiction of ASTM Committee E20 onTemperature Measurement and is the direct responsibility o
14、f Subcommittee E20.04on Thermocouples.Current edition approved May 1, 2007. Published June 2007. Originallyapproved in 1963. Last previous edition approved in 2002 as E 220 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. Fo
15、r Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.thermocouple reference function evaluated at the temperatureof the ter
16、minal block. Because the emf-temperature relation-ship of any actual thermocouple differs slightly from that of thereference function, reference junction compensation typicallyintroduces higher uncertainties compared to the use of awell-prepared ice bath.3.2.4 reference junction compensator, na devi
17、ce thatimplements reference junction compensation.3.2.5 reference thermometer, nthermometer that estab-lishes the value of temperature in a given system containingadditional temperature sensors.3.2.5.1 DiscussionIn a calibration system the referencethermometer is a calibrated thermometer capable of
18、indicatingvalues of temperature with known uncertainty. The referencethermometer provides the standard temperature for the systemat the time of test.3.2.6 thermocouple type, na standardized thermoelectricclass of thermoelement materials that, used as a pair, have anormal relationship between relativ
19、e Seebeck emf and tem-perature.3.2.6.1 DiscussionFor common, commercially availablethermocouples, a thermocouple type is identified by a letterdesignation (types B, E, J, K, N, R, S, and T). The letterdesignation scheme is given in Specification E 230. The tablesin E 988 and E 1751 give temperature-
20、EMF relationships for anumber of additional thermocouple compositions that are notidentified by a letter designation.4. Summary of Test Method4.1 Comparison calibration consists of measuring the emf ofthe thermocouple being calibrated in an isothermal mediumwhile simultaneously measuring the tempera
21、ture of the me-dium with a reference thermometer. The reference thermom-eter may be any thermometer with sufficient accuracy at thetemperature of calibration.5. Significance and Use5.1 For users or manufacturers of thermocouples, this testmethod provides a means of verifying the emf-temperaturechara
22、cteristics of the material prior to use.5.2 This test method can be used to calibrate a thermocouplefor use as a reference, or it can be used to calibrate thermo-couples representing a batch of purchased, assembled thermo-couples.5.3 This test method can be used for the verification of theconformanc
23、e of thermocouple materials to temperature toler-ances for specifications such as the tables in SpecificationE 230 or other special specifications as required for commer-cial, military, or research applications.6. Interferences6.1 Since the success of this test method depends largelyupon the ability
24、 to maintain the measuring junction of thethermocouple being calibrated and the reference thermometerat the same temperature, considerable care must be taken inchoosing the media and conditions under which the compari-sons are made. Stirred liquid baths, uniformly heated metalblocks, tube furnaces,
25、and dry fluidized baths, properly used,are acceptable temperature comparison environments. In thecase of large diameter thermoelements and sheathed thermo-couples, special attention must be given to effects of thermalconduction.6.2 Voltage measurement instruments with sufficiently highinput impedanc
26、e must be used for measuring thermocoupleemf to eliminate instrument loading as a significant source oferror. The ratio of input impedance to thermocouple loopresistance should be significantly (at least 104) greater than theratio of the measured emf to the desired emf uncertainty.6.3 The test metho
27、d relies on the assumption that testthermoelements are homogeneous. If so, their output voltage ata given measuring junction temperature is independent oftemperature variations along the length of the thermocouple.Departures from this ideal contribute to uncertainty in the useof test results. The ef
28、fects typically are negligibly small fornew, unused thermocouple material, but not for used thermo-couples, especially those of base-metal composition. Theeffects of inhomogeneity can be identified, but not accuratelyquantified, by the techniques described in Appendix X4 in thistest method. Comments
29、 on the testing of used thermocouplesmay be found in Manual MNL 12 (1).36.4 This test method presumes that the tested thermocouplesare suitable for use in air throughout the range of calibrationtemperatures. To avoid oxidation of the thermoelements,refractory-metal thermocouples that have not been h
30、ermeti-cally sealed in a sheath suitable for use in air should be testedin an inert gas environment at temperatures above approxi-mately 500 C. In this case, use of this test method isrecommended in combination with the furnaces and relatedprocedures described in Test Method E 452.7. Apparatus7.1 Th
31、e choice of apparatus used for the comparison testwill depend primarily on the temperature range to be coveredand on the desired calibration uncertainty. The apparatusrequired for the application of this test method will depend indetail upon the temperature range being covered but in all casesshall
32、be selected from the equipment described as follows:7.2 Comparison Baths and FurnacesA controlled tem-perature comparison medium (bath or furnace) shall be used inwhich the measuring junction of the thermocouple to becalibrated is brought to the same temperature as a referencethermometer. The spatia
33、l uniformity of temperature within thenominally isothermal calibration zone shall be established.Acceptable methods include measurements of the calibrationzone at the time of testing or the use of control charts thatdisplay the periodic calibration of check standards or theperiodic characterization
34、of the calibration zone. The fre-quency of such testing will depend on the inherent stability ofthe bath or furnace.The uniformity of the calibration zone shallbe remeasured sufficiently often such that any deviations inuniformity may be corrected prior to significant adverse affecton the readings.
35、All thermocouples being calibrated and thereference thermometer must be immersed into this zone to an3The boldface numbers in parenthesis refer to the list of references at the end ofthis standard.E22007e12extent sufficient to ensure that the measuring junction tempera-ture is not significantly affe
36、cted by heat conduction along thethermocouple and reference thermometer assemblies. To avoidcontaminating the thermoelements and insulation of un-sheathed thermocouples, direct contact with calibration bathfluids should be avoided.7.2.1 Liquid BathsIn the range from 150 to 630 C(240 to 1170 F) the c
37、omparator bath shall usually consist ofa well stirred liquid bath provided with controls for maintain-ing a constant and uniform temperature. Suitable types aredescribed in the appendix to Test Method E77. At the liquidnitrogen boiling point, 196 C (321 F), an isothermal blockof copper suspended in
38、an open dewar of liquid nitrogen canprovide a very effective single-point liquid bath. In the rangebetween 196 C (321 F) and 150 C (240 F), the bathconstruction is relatively complex, and commercial systemsthat rely on liquid nitrogen for cooling are recommended. Aproperly constructed liquid bath wi
39、ll have temperature gradi-ents that are small relative to either fluidized powder baths ortube furnaces. A disadvantage of liquid baths is the relativelysmall operating range of any one bath fluid. The temperaturegradients in a liquid bath will be repeatable provided that thebath liquid does not the
40、rmally decompose at high temperaturesand that the conditions of bath heating and cooling arecomparable to those that existed when the bath gradients werechracterized. Periodic evaluation of bath gradients is necessarywhen using oil baths, since oil visosity can increase signifi-cantly after use at h
41、igh temperatures. Baths with multipleheaters require a monitoring system that enables the user toreadily determine that all heaters are operational.7.2.2 Fluidized Powder BathsIn the range from 70 to980 C (100 to 1800 F) the comparator bath may consist ofa gas-fluidized bath of aluminum oxide or sim
42、ilar powder.Temperature equalizing blocks are almost always necessarywithin fluidized baths to minimize spatial and temporal tem-perature variations. The repeatability of thermal gradientswithin such a block depends on maintaining a constant fill levelof powder in the bath and maintaining a uniform
43、gas flowthrough the powder. The thermal gradients of a fluidizedpowder bath shall be verified by including either a secondreference thermometer or a check-standard thermocouple ineach comparison test.7.2.3 Tube FurnacesAt temperatures above approxi-mately 620 C (1150 F) an electrically heated tube f
44、urnacewith a suitable nominally isothermal zone will usually be used.Laboratory type tube furnaces may be used at any temperatureprovided that the increased uncertainty due to their spatialtemperature variance is accounted for. Any one of a widevariety of designs may be suitable, but it shall be dem
45、onstratedthat the furnace chosen can maintain a temperature stability of61 C over a period of 10 min at any temperature in the rangeover which the furnace is to be used. The axial temperatureprofile of a tube furnace shall be mapped to determine thelocation of the region with the best temperature un
46、iformity.Furnaces with multiple heaters require a monitoring systemthat enables the user to visually determine that all heaters areoperational and will require periodic remeasurement of theaxial temperature profile. Single-zone furnaces may vary intemperature profile slowly as the heater element age
47、s and willrequire only infrequent remapping of the temperature profile.7.2.4 Other BathsThe one essential design feature of anybath to be used with this test method is that it brings themeasuring junction of the thermocouple being calibrated to thesame temperature as the reference thermometer. Coppe
48、r blocksimmersed in liquid nitrogen have been used successfully at lowtemperatures. The blocks are provided with wells for the testthermocouples and the reference thermometer. Similarly, uni-formly heated blocks have been used at high temperatures.Such baths are not excluded under this test method,
49、but carefulexplorations of existing temperature gradients must be madebefore confidence may be placed in such an apparatus.7.2.5 Isothermal BlocksThe use of an isothermal blockcan substantially reduce the temperature differences betweenthe reference thermometer and the test thermocouples. Such ablock should be manufactured from a material of high thermalconductivity that will not contaminate the thermocouples undertest. High thermal conductivity reduces the spatial temperaturevariations in the block, resulting in better thermal equilibri
