1、Designation: E2730 10Standard Practice forCalibration and Use of Thermocouple Reference JunctionProbes in Evaluation of Electronic Reference JunctionCompensation Circuits1This standard is issued under the fixed designation E2730; the number immediately following the designation indicates the year of
2、original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers methods of calibration and use ofthermoco
3、uple reference junction probes (cold junction com-pensation probes) in the evaluation of electronic referencejunction compensation circuits. Their use with instruments thatmeasure only voltage is also covered.1.2 The values stated in SI units are to be regarded as thestandard. The values given in pa
4、rentheses are for informationonly.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limita
5、tions prior to use.2. Referenced Documents2.1 ASTM Standards:2E220 Test Method for Calibration of Thermocouples ByComparison TechniquesE230 Specification and Temperature-Electromotive Force(EMF) Tables for Standardized ThermocouplesE344 Terminology Relating to Thermometry and Hydrom-etryE563 Practic
6、e for Preparation and Use of an Ice-Point Bathas a Reference TemperatureE1129/E1129M Specification for Thermocouple Connec-torsE1684 Specification for Miniature Thermocouple Connec-torsE1750 Guide for Use of Water Triple Point CellsE2623 Practice for Reporting Thermometer Calibrations2.2 Other Refer
7、ences:NIST Monograph 175 Temperature-Electromotive ForceReference Functions and Tables for the Letter-DesignatedThermocouple Types Based on the ITS-90ASTM MNL12 Manual On The Use Of Thermocouples InTemperature Measurement3. Terminology3.1 DefinitionsThe definitions given in TerminologyE344 shall app
8、ly to this guide.3.2 Definitions of Terms Specific to This Standard:3.2.1 correction, nan offset value added to the result of ameasurement to obtain a correct result.NOTE 1This definition is from Test Method E220.3.2.2 reference junction compensation, nthe electricalcorrection of the indication of a
9、 thermocouple such that thecorrected indication is equivalent to the emf or temperature theinstrument would indicate if the reference junctions werephysically maintained at 0 C.3.2.3 reference junction probe, na probe constructed fromthermocouple materials and high purity copper wire for thepurpose
10、of serving as the reference junction for a thermocoupleassembly. Reference junction probes may be constructed aspart of the measuring probe or they can be manufacturedseparately and later attached to thermocouple sensors via plugsor other connection types.3.2.4 UUT, nUnit Under Test.4. Summary of Gu
11、ide4.1 Calibration of a Reference Junction Probe (RJP) con-sists of establishing the emf error in the RJP relative to theapplicable thermocouple reference function by placing the1This practice is under the jurisdiction ofASTM Committee E20 on TemperatureMeasurement and is the direct responsibility o
12、f Subcommittee E20.04 on Thermo-couples.Current edition approved Nov. 1, 2010. Published January 2011. DOI:10.1520/E273010.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, re
13、fer 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.measuring junction and the reference junctions at knowntemperatures while measuring the voltage with a digital volt-meter (
14、DVM) or potentiometer. Three methods are describedfor establishing the two known temperatures and thus thetemperature difference. For the temperature measurement,many devices such as Standard Platinum Resistance Thermom-eters (SPRTs), Platinum Resistance Thermometers (PRTs),thermistors, or thermocou
15、ples, and a variety of readout instru-ments are suitable, depending on the required accuracy. Themeasured voltage at the copper leads indicates the emf asso-ciated with the temperature difference of the references. Erroris determined by comparing the observed emf to the calculatedemf for the known t
16、emperature difference. The emf error isthen applied as a correction. The corrected emf can then beconverted to temperature.NOTE 2In particular, cold work should be avoided in the sections ofcopper and thermocouple wire that pass from the top of the ice bath toambient temperature continuing on to the
17、 terminal connection. Carefuldesign of the RJP, with supporting sleeve and strain relief, can minimizecold work in these sections.4.2 Use of the calibrated RJP consists of applying thecorrections obtained during calibration appropriately for themode of use. Three modes of use with corresponding appl
18、ica-tion equations are described.NOTE 3Homogeneity is assumed in both the thermocouple andcopper wires. Care should be taken to minimize the stress induced overtime and during use on both sets of wire. Cold work in particular shouldbe avoided in the sections of the copper and thermocouple wire from
19、adistance 5 cm (2 in.) below the top of the ice/water mixture to a distance5 cm (2 in.) above the top of the ice-point bath.NOTE 4Proper operation of the measuring instruments is not de-scribed in this guide. To ensure correct results, the operator mustunderstand and apply proper technique in the us
20、e of all measuringinstruments involved.5. Significance and Use5.1 Many electronic instruments that are designed to beused with thermocouples use some method of reference junc-tion compensation. In many industrial applications it may beimpractical to use a physical ice bath as a temperaturereference
21、in a thermocouple circuit. The instrument musttherefore be able to measure the temperature at the point ofelectrical connection of the thermocouple and either add orsubtract voltage to give a corrected equivalent of what thatthermocouple would indicate had there physically been 0 Creference junction
22、s present in the circuit. There are two typesof instruments that generally apply these techniques: electronicthermometer readouts that use thermocouples as the sensor,and calibrators designed to calibrate these digital thermometerreadouts. Additionally, the probe and circuit described in thisguide c
23、an be used with a voltmeter to emulate a thermometeror a voltage source to calibrate temperature-indicating instru-mentation. In all cases the probe must be calibrated if trace-ability or an uncertainty analysis, or both, is required.6. Reagents6.1 Laboratory or commercially produced distilled water
24、 isrequired to create an accurate ice bath. Clean tap water can beused in cases where high accuracy is not a requirement; in thiscase the temperature of the bath should be measured directly.Chlorine, fluorine, and other chemicals such as dissolved saltsin the water or ice will depress the ice point
25、and the amount canbe significant in some measurements. See Practice E563 forfurther guidance.7. Procedure7.1 Calibration of RJP (Methods A, B, or C).7.1.1 Reference Point Temperature Source:7.1.1.1 Method A: Ice Bath MethodPrepare the referencepoint temperature source using an ice-point bath per Pra
26、cticeE563. Refer to Fig. 1.NOTE 5Be careful to closely follow the guidelines in Practice E563for establishing and maintaining an Ice Point Reference as significant errorcan occur over time without proper maintenance.7.1.1.2 Method B: Triple Point of Water (TPW) CellMethodPrepare the reference point
27、temperature source usinga triple point of water cell per Guide E1750. Refer to Fig. 2.NOTE 6Be careful to closely follow the guidelines in Guide E1750 forestablishing and maintaining a TPW cell as significant error can occurover time without proper maintenance.7.1.1.3 Method C: Variable Temperature
28、Source MethodPrepare the reference point temperature source using a variabletemperature source (calibration bath or dryblock calibrator) setto 0.0 C and verify using a reference thermometer. Refer toFig. 3.7.1.2 Prepare the room temperature source using a variabletemperature source (calibration bath
29、 or dry-block calibrator)set to 25 C and verify using a reference thermometer. Thetemperature of 25 C is nominal; in actual testing the tempera-ture of the bath should be set as close as possible to the ambientroom temperature. Throughout this procedure 25 C will beused to designate the ambient temp
30、erature. There are manycases where the terminal ends may be at a temperature higherthan ambient temperature. Connections inside an instrument orcontrol box can reach temperatures of 40 C or higher. The RJPcan be calibrated at multiple temperatures and the resultingRJP correction can be modeled as a
31、first or second orderpolynomial correction versus RJP temperature.7.1.3 Weld, solder, or braze the thermocouple wire ends ofthe RJP together to create a thermocouple measuring junctionand then insert it into a protective sheath. Twisting or crimpingthe wires is acceptable if a reliable electrical co
32、nnection can beachieved. The measuring junction should be electrically iso-lated from the sheath. All fluxes or chemicals that may havebeen used should be thoroughly removed.7.1.4 Place the measuring junction and protective sheath inthe temperature source that has been stabilized at 25 Cnominal. Pla
33、ce the reference junction probe in the referencepoint temperature source. Both the sheath and probe should besufficiently immersed to make stem conduction error negli-gible. WarningThe individual positive and negative connec-tions must be electrically isolated from each other and thesheath regardles
34、s of the type connection or temperature stabi-lizing method used.7.1.5 Connect the copper leads of the RJP to the voltmeter.E2730 1027.1.6 Allow the setup to stabilize as indicated on thevoltmeter; the stability required depends upon the level ofuncertainty required.7.1.7 Measure the temperature of
35、the 25 C source (TMJ)and the reference point temperature source (TRJ) if usingmethod C.7.1.8 The RJP error in microvolts is given by Eq 1.ERJP error5 Eobserved*TRJTMJSABT!dT (1)where:ERJP error= RJP error, in V,Eobserved= voltage indication, in V,TMJ= ambient source temperature, in C, as mea-sured b
36、y the reference thermometer,TRJ= reference junction temperature, in C (as-sumed to be 0.000 C in Method A, 0.010 Cin Method B, and measured by referencethermometer in Method C), andSAB(T) = Seebeck coefficient at temperature T, in V/C.NOTE 7Use the correct value for the SAB based on the actualtemper
37、ature of the reference point temperature source. Values given arebased on the ice Melting Point (MP) (0.000 C).NOTE 8The values given are taken from NIST Monograph 175.7.1.9 The RJP error in microvolts is algebraically approxi-mated using Eq 2 for method A.ERJP error5 Eobserved Eexpected(2)where:ERJ
38、P error= RJP error, in V,Eobserved= voltmeter indication, in V, andEexpected= thermocouple voltage, in V, at the ambientsource temperature, as computed by the ref-erence function or interpolated from the ther-mocouple table.7.1.10 The RJP error in microvolts is algebraically approxi-mated using Eq 3
39、 for method B.ERJP error5 Eobserved Eexpected1 0.010 C 3 SAB0C! (3)where:ERJP error= RJP error, in V,Eobserved= voltmeter indication, in V,Eexpected= thermocouple voltage, in V, at the ambientsource temperature, as computed by thereference function or interpolated from thethermocouple table, andSAB(
40、0 C) = Seebeck coefficient at 0 C, in V/C (Ref.Table 1).7.1.11 The RJP error in microvolts is algebraically approxi-mated using Eq 4 for method C.ERJP error5 Eobserved Eexpected1 SAB0C! 3 TRJ(4)FIG. 1 Schematic Diagram of Calibration Method AE2730 103where:ERJP error= RJP error, in V,Eobserved= volt
41、meter indication, in V,Eexpected= thermocouple voltage, in V, at the ambientsource temperature, as computed by thereference function or interpolated from thethermocouple table,TRJ= reference junction temperature, in C, asmeasured by the reference thermometer, andSAB(0 C) = Seebeck coefficient at 0 C
42、, in V/C (Ref.Table 1).7.1.12 Corrections are identical to errors in magnitude butof opposite sign. Calculate the voltage correction in V usingEq 5.ERJP correction5 ERJP error(5)where:ERJP correction= RJP correction, in V, andERJP error= RJP error, in V.7.1.13 Calculate the temperature correction in
43、 C using Eq6.TRJP correction5 ERJP error!SAB0C!(6)where:TRJP correction= RJP correction, in C,ERJP error= RJP error, in V, andSAB(0 C) = Seebeck coefficient at temperature 0 C,in V/C (Ref. Table 1).WarningEq 6 may provide incorrect results for thermo-couples having significantly different values of
44、SABat 0 C and25 C7.2 Use of the RJP (Modes 1, 2, or 3).NOTE 9The following instructions apply to the use of an ice bath,TPW cell, or variable temperature source for the reference point tempera-ture. Thus, the equations are shown in the generalized form. When usingthe equations, the value for t1must
45、be the assumed values for the ice bathor TPW cell, or the actual measured temperature of the variable tempera-ture source, as applicable.7.2.1 Mode 1Use of the RJP as a reference junction in athermocouple circuit. Refer to Fig. 4.7.2.1.1 Prepare the reference point temperature source.7.2.1.2 Connect
46、 the thermocouple end of the RJP to thethermocouple to be measured using an approved thermocoupleconnector.7.2.1.3 Connect the copper wire end of the RJP to themeasuring instrument, observing polarity.7.2.1.4 Place the RJP probe into the reference point tem-perature source.7.2.1.5 Place the thermoco
47、uple measuring junction in thelocation to be measured.FIG. 2 Schematic Diagram of Calibration Method BE2730 1047.2.1.6 Allow sufficient time for the indications to stabilize.7.2.1.7 The thermocouple voltage corresponding to thelocation being measured is determined by Eq 7. WarningSee 8.2.2 regarding
48、 the potential for additional error using thismethod.EMJ5 Eobserved1 ERJP correction1 SAB0C! 3 TRJ(7)where:EMJ= corrected thermocouple voltage, in V,Eobserved= voltmeter indication, in V,ERJP correction= reference junction probe correction, inV,SAB(0 C) = Seebeck coefficient at 0 C, in V/C(Ref. Tabl
49、e 1), andTRJ= reference junction temperature (as-sumed 0.000 C, 0.010 C, or measuredby reference thermometer, as appli-cable).7.2.2 Mode 2Use of the RJP in the calibration of thereference junction compensation circuit of a thermocouple-measuring instrument using a voltage calibrator or multifunc-tion calibrator. Refer to Fig. 5.7.2.2.1 Prepare the reference point temperature source.FIG. 3 Schematic Diagram of Calibration Method CTABLE 1 Seebeck Coefficients at 0 C for Some CommonThermo
