1、Designation: E2730 10 (Reapproved 2015)1Standard 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 in
2、dicates the year oforiginal 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.1NOTEEditorial changes were made throughout in June
3、2015.1. Scope1.1 This guide covers methods of calibration and use ofthermocouple 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
4、stated in SI units are to be regarded as thestandard. The values given in parentheses 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 safet
5、y and health practices and determine the applica-bility of regulatory limitations 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 Therm
6、ocouplesE344 Terminology Relating to Thermometry and Hydrom-etryE563 Practice for Preparation and Use of an Ice-Point Bathas a Reference TemperatureE1129/E1129M Specification for Thermocouple ConnectorsE1684 Specification for Miniature Thermocouple Connec-torsE1750 Guide for Use of Water Triple Poin
7、t CellsE2623 Practice for Reporting Thermometer Calibrations2.2 Other References: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 MeasurementBIPM
8、 JCGM 100:2008 Evaluation of Measurement DataGuide to the Expression of Uncertainty in Measurement3. Terminology3.1 DefinitionsThe definitions given in Terminology E344shall apply to this guide.3.2 Definitions of Terms Specific to This Standard:3.2.1 correction, nan offset value added to the result
9、of ameasurement to obtain a correct result.NOTE 1This definition is from Test Method E220.3.2.2 reference junction compensation, nthe electrical cor-rection of the indication of a thermocouple such that thecorrected indication is equivalent to the emf or temperature theinstrument would indicate if t
10、he reference junctions werephysically maintained at 0C.3.2.3 reference junction probe, na probe constructed fromthermocouple materials and high purity copper wire for thepurpose of serving as the reference junction for a thermocoupleassembly. Reference junction probes may be constructed aspart of th
11、e 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 Guide4.1 Calibration of a Reference Junction Probe (RJP) con-sists of establishing the emf error in the RJP relative to thea
12、pplicable thermocouple reference function by placing themeasuring junction and the reference junctions at knowntemperatures while measuring the voltage with a digital volt-meter (DVM) or potentiometer. Three methods are described1This practice is under the jurisdiction ofASTM Committee E20 on Temper
13、atureMeasurement and is the direct responsibility of Subcommittee E20.04 on Thermo-couples.Current edition approved June 15, 2015. Published June 2015. Originallyapproved in 2010. Last previous edition approved in 2010 as E2730 10.DOI:10.1520/E2730-10R15E01.2For referenced ASTM standards, visit the
14、ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. Uni
15、ted States1for 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 thermocouples, and a variety of readout instru-m
16、ents 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 temperature difference. The emf error is
17、then 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 terminal connection. Carefuldesign of
18、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 applica-tion equations are described.NOTE 3
19、Homogeneity is assumed in both the thermocouple and copperwires. Care should be taken to minimize the stress induced over time andduring use on both sets of wire. Cold work in particular should be avoidedin the sections of the copper and thermocouple wire from a distance 5 cm(2 in.) below the top of
20、 the ice/water mixture to a distance 5 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 use of all measuringinstruments involved.
21、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 in a thermocouple circuit. The instrume
22、nt 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 0Creference junctions present in the circuit. There are two
23、typesof instruments that generally apply these techniques: electronicthermometer readouts that use a thermocouple as the sensor,and calibrators designed to calibrate these digital thermometerreadouts. Additionally, the probe and circuit described in thisguide can be used with a voltmeter to emulate
24、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 isrequired to create an accurate ice b
25、ath. 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 and the amount canbe significant in som
26、e 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 in accordancewith Practice E563. Refer to Fig.
27、 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 temperature source using
28、a triple point of water cell in accordance with 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 Source Met
29、hodPrepare the reference point temperature source using a variabletemperature source (calibration bath or dryblock calibrator) setto 0.0C and verify using a reference thermometer. Refer toFig. 3.7.1.2 Prepare the room temperature source using a variabletemperature source (calibration bath or dry-blo
30、ck calibrator)set to 25C and verify using a reference thermometer. Thetemperature of 25C 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 25C will beused to designate the ambient temperature. There
31、 are manycases where the terminal ends may be at a temperature higherthan ambient temperature. Connections inside an instrument orcontrol box can reach temperatures of 40C or higher. The RJPcan be calibrated at multiple temperatures and the resultingRJP correction can be modeled as a first- or secon
32、d-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 connection can b
33、eachieved. 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 25C nomi-nal. Place the refere
34、nce junction probe in the reference pointtemperature 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 regardless of the type
35、 connection or temperature stabi-lizing method used.7.1.5 Connect the copper leads of the RJP to the voltmeter.7.1.6 Allow the setup to stabilize as indicated on thevoltmeter; the stability required depends upon the level ofuncertainty required.E2730 10 (2015)127.1.7 Measure the temperature of the 2
36、5C source (TMJ) andthe reference point temperature source (TRJ) if using method C.7.1.8 The RJP error in microvolts is given by Eq 1.ERJP error5 Eobserved2 *TRJTMJSABT!dT (1)where:ERJP error= RJP error, in V,Eobserved= voltage indication, in V,TMJ= ambient source temperature, in C, as measuredby the
37、 reference thermometer,TRJ= reference junction temperature, in C (assumedto be 0.000C in Method A, 0.010C in MethodB, and measured by reference thermometer inMethod C), andSAB(T) = Seebeck coefficient at temperature T, in V/C.NOTE 7Use the correct value for the SABbased on the actualtemperature of t
38、he reference point temperature source. Values given arebased on the ice Melting Point (MP) (0.000C).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 Eobserved2 Eexpected(2)where:ERJP error= R
39、JP error, in V,Eobserved= voltmeter indication, in V, andEexpected= thermocouple voltage, in V, at the ambientsource temperature, as computed by the refer-ence function or interpolated from the thermo-couple table.7.1.10 The RJP error in microvolts is algebraically approxi-mated using Eq 3 for metho
40、d B.ERJP error5 Eobserved2 Eexpected10.010C 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 the refer-ence function or interpolated from the thermo-couple table, andSAB(0C) = S
41、eebeck coefficient at 0C, in V/C (refer toTable 1).7.1.11 The RJP error in microvolts is algebraically approxi-mated using Eq 4 for method C.ERJP error5 Eobserved2 Eexpected1SAB0C! 3 TRJ(4)where:FIG. 1 Schematic Diagram of Calibration Method AE2730 10 (2015)13ERJP error= RJP error, in V,Eobserved= v
42、oltmeter indication, in V,Eexpected= thermocouple voltage, in V, at the ambientsource temperature, as computed by the refer-ence function or interpolated from the thermo-couple table,TRJ= reference junction temperature, in C, as mea-sured by the reference thermometer, andSAB(0C) = Seebeck coefficien
43、t at 0C, in V/C (refer toTable 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 co
44、rrection in C using Eq6.TRJP correction52ERJP error!SAB0C!(6)where:TRJP correction= RJP correction, in C,ERJP error= RJP error, in V, andSAB(0C) = Seebeck coefficient at temperature 0C, inV/C (refer to Table 1).WarningEq 6 may provide incorrect results for thermo-couples having significantly differe
45、nt values of SABat 0C and25C7.2 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
46、for t1must 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.
47、1.2 Connect 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.FIG. 2 Schematic Diagram of Calibration Method BE2730 10 (2015)147.2.1.4 Place the RJP p
48、robe into the reference point tem-perature source.7.2.1.5 Place the thermocouple measuring junction in thelocation to be measured.7.2.1.6 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. Warning
49、See 8.2.2 regarding the potential for additional error using thismethod.EMJ5 Eobserved1ERJP correction1SAB0C! 3 TRJ(7)where:EMJ= corrected thermocouple voltage, in V,Eobserved= voltmeter indication, in V,ERJP correction= reference junction probe correction, in V,SAB(0C) = Seebeck coefficient at 0C, in V/C (refer toTable 1), andTRJ= reference junction temperature (assumed0.000C, 0.010C, or measured by referencethermometer, as applicable).7.2.2 Mode 2Use of the RJP in the calibration o
