1、Designation: E220 13Standard Test Method forCalibration of Thermocouples By Comparison Techniques1This standard is issued under the fixed designation E220; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 This test method describes the principles, appa
3、ratus, 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 195 to1700 C (320 to 3100 F).1.2 In general, this test metho
4、d 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 with large-diameter thermoelements and sheathed thermocou
5、ples 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 GuideE1594.1.4 This standard may involve hazardous materials, opera-tions and equipment. This standard does not purport to
6、 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 use.2. Referenced Documents2.1 ASTM Standards:2E1 Specifica
7、tion for ASTM Liquid-in-Glass ThermometersE77 Test Method for Inspection and Verification of Ther-mometersE230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE344 Terminology Relating to Thermometry and Hydrom-etryE452 Test Method for Calibration of Refra
8、ctory Metal Ther-mocouples Using a Radiation ThermometerE563 Practice for Preparation and Use of an Ice-Point Bathas a Reference TemperatureE644 Test Methods for Testing Industrial Resistance Ther-mometersE1129/E1129M Specification for Thermocouple ConnectorsE1594 Guide for Expression of Temperature
9、E1684 Specification for Miniature Thermocouple Connec-torsE1751 Guide for Temperature Electromotive Force (EMF)Tables for Non-Letter Designated Thermocouple Combi-nations (Withdrawn 2009)3E2846 Guide for Thermocouple Verification3. Terminology3.1 DefinitionsThe definitions given in Terminology E344s
10、hall apply to this test method.3.2 Definitions of Terms Specific to This Standard:3.2.1 check standard, na measurement instrument or stan-dard whose repeated results of measurement are used todetermine the repeatability of a calibration process and toverify that the results of a calibration processe
11、s 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 reference junction compensation, nthe adjustment ofthe indication of a thermocouple such that the a
12、djustedindication is equivalent to the emf or temperature that thethermocouple would indicate if the reference junctions weremaintained at 0C.3.2.3.1 DiscussionIn most cases, the thermocouple indi-cation is adjusted by measuring the temperature of a terminalblock where the thermocouple is connected,
13、 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 of Subcommittee E20.04on Thermocouples.Current edition approved Nov. 1, 2013. Published Decemb
14、er 2013. Originallyapproved in 1963. Last previous edition approved in 2007 as E220 07A. DOI:10.1520/E0220-13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the st
15、andards Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1thermocouple reference function evaluated at the temp
16、eratureof the terminal 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 com
17、pensator, na device 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 thermo
18、meter capable of 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 relationshi
19、p between relative Seebeck emf and tem-perature.3.2.6.1 DiscussionFor common, commercially availablethermocouples, a thermocouple type is identified by a letterdesignation (types B, C, E, J, K, N, R, S, and T). The letterdesignation scheme is given in Guide E2846. The tables inE1751 and E1751 give t
20、emperature-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
21、the temperature 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-tempe
22、raturecharacteristics 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 th
23、econformance of thermocouple materials to temperature toler-ances for specifications such as the tables in Specification E230or other special specifications as required for commercial,military, or research applications.6. Interferences6.1 Since the success of this test method depends largelyupon the
24、 ability 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 fu
25、rnaces, and dry fluidized baths, properly used,are acceptable temperature comparison environments. In thecase of large diameter thermoelements and sheathedthermocouples, special attention must be given to effects ofthermal conduction.6.2 Voltage measurement instruments with sufficiently highinput im
26、pedance 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
27、 method 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.
28、The effects typically are negligibly small fornew, unused thermocouple material, but not for usedthermocouples, especially those of base-metal composition.The effects of inhomogeneity can be identified, but notaccurately quantified, by the techniques described in AppendixX4 in this test method and i
29、n section 8.2 of Guide E2846.Descriptions of the testing of used thermocouples may befound Guide E2846 and Manual MNL 12 (1).46.4 This test method presumes that the tested thermocouplesare suitable for use in air throughout the range of calibrationtemperatures. To avoid oxidation of the thermoelemen
30、ts,refractory-metal thermocouples that have not been hermeti-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 relatedproced
31、ures described in Test Method E452.7. Apparatus7.1 The 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 t
32、emperature range being covered but in all casesshall 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 s
33、ame temperature as a referencethermometer. The spatial 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 calibratio
34、n of check standards or theperiodic characterization 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 in4The boldface numbers in pa
35、renthesis refer to the list of references at the end ofthis standard.E220 132uniformity may be corrected prior to significant adverse affecton the readings. All thermocouples being calibrated and thereference thermometer must be immersed into this zone to anextent sufficient to ensure that the measu
36、ring junction tempera-ture is not significantly affected 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 Bath
37、sIn the range from 150 to 630 C(240 to 1170 F) the comparator 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
38、 (321 F), an isothermal blockof copper suspended in 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 a
39、re recommended. Aproperly constructed liquid bath will 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
40、repeatable provided that thebath liquid does not thermally decompose at high temperaturesand that the conditions of bath heating and cooling arecomparable to those that existed when the bath gradients werecharacterized. Periodic evaluation of bath gradients is neces-sary when using oil baths, since
41、oil viscosity can increasesignificantly after use at high 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 c
42、onsist ofa gas-fluidized bath of aluminum oxide or similar 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
43、 levelof powder in the bath and maintaining a uniform 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 appro
44、xi-mately 620 C (1150 F) an electrically heated tube furnacewith 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 widev
45、ariety of designs may be suitable, but it shall be demonstratedthat 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
46、thelocation of the region with the best temperature uniformity.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
47、intemperature profile slowly as the heater element ages and willrequire only infrequent remapping of the temperature profile.NOTE 1Further discussions of suitable tube furnaces are given inAppendix X1.7.2.4 Other BathsThe one essential design feature of anybath to be used with this test method is th
48、at it brings themeasuring junction of the thermocouple being calibrated to thesame temperature as the reference thermometer. Copper blocksimmersed in liquid nitrogen have been used successfully at lowtemperatures. The blocks are provided with wells for the testthermocouples and the reference thermom
49、eter. Similarly, uni-formly heated blocks have been used at high temperatures.Such baths are not excluded under this test method, 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 thermocouple
