1、Designation: E 780 06Standard Test Method forMeasuring the Insulation Resistance of Mineral-Insulated,Metal-Sheathed Thermocouples and Thermocouple Cable atRoom Temperature1This standard is issued under the fixed designation E 780; the number immediately following the designation indicates the year
2、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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method provides the procedures for measuringth
3、e room temperature electrical insulation resistance betweenthe thermoelements, and between the thermoelements and thesheath, of a mineral-insulated, metal-sheathed thermocouple orthermocouple cable. It may be used to measure the insulationresistance of bulk lengths of thermocouple cable previouslyse
4、aled against moisture intrusion or to test a thermocouplehaving an ungrounded class 2 junction. This method cannot beused to test a thermocouple having a grounded class 1 junctionunless the junction is removed prior to testing, after which thethermocouple may be dealt with in the same manner as athe
5、rmocouple cable.1.2 This test method applies primarily to thermocouplecables conforming to Specifications E 585/E 585M andE 2181/E 2181M and to thermocouples conforming to Speci-fications E 608/E 608M and E 2181/E 2181M, but may also beapplied to thermocouples or thermocouple cables that aresuitable
6、 for use in air, whose sheath or thermoelements arecomprised of refractory metals, that are tested in a dry andchemically inert environment, and that may employ compactedceramic insulating materials other than magnesia (MgO) oralumina (Al2O3). Users of this test method should note thatspecifications
7、 dealing with compacted ceramic insulating ma-terials other than magnesia or alumina, which are described inSpecification E 1652, are not currently available. As a result,acceptance criteria must be agreed upon between the customerand supplier at the time of purchase, or alternatively, judgmentand e
8、xperience must be applied in establishing test voltagelevels and acceptable insulation resistance values for thesetypes of thermocouples and thermocouple cables.1.3 This test method may be used for thermocouples orthermocouple cables having an outside diameter of 0.5 mm(0.020 in) or larger.1.4 Users
9、 of this test method should be aware that the roomtemperature insulation resistance of a mineral-insulated, metal-sheathed thermocouple or thermocouple cable may changeduring shipment, storage, or use.1.5 The values stated in SI units are to be regarded as thestandard. The values given in parenthese
10、s are for informationonly.1.6 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 limitations pr
11、ior to use.2. Referenced Documents2.1 ASTM Standards:2E 235 Specification for Thermocouples, Sheathed, Type Kand Type N, for Nuclear or for Other High-ReliabilityApplicationsE 344 Terminology Relating to Thermometry and Hydrom-etryE 585/E 585M Specification for Compacted Mineral-Insulated, Metal-She
12、athed, Base Metal ThermocoupleCableE 608/E 608M Specification for Mineral-Insulated, Metal-Sheathed Base Metal ThermocouplesE 1652 Specification for Magnesium Oxide and AluminumOxide Powder and Crushable Insulators Used in theManufacture of Metal-Sheathed Platinum Resistance Ther-mometers, Base Meta
13、l Thermocouples, and Noble MetalThermocouples1This 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 May 1, 2006. Published May 2006. Originallyapproved in 1992. Last
14、previous edition approved in 1998 as E 780 92 (1998).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 standards Document Summary page onthe ASTM website.1Copyrig
15、ht ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.E 2181/E 2181M Specification for Compacted Mineral-Insulated, Metal-Sheathed, Noble Metal Thermocouplesand Thermocouple Cable3. Terminology3.1 DefinitionsThe definitions given in TerminologyE 3
16、44 shall apply to the terms used in this test method.3.2 Definitions of Terms Specific to This Standard:3.2.1 bulk material length (BML), na single length offinished thermocouple cable.3.2.2 dry, adja condition of the ambient air at time of testthat does not exceed the equivalent of 50 % relative hu
17、midityat 22C 72F.3.2.3 thermocouple, nrefers to a mineral-insulated, metal-sheathed thermocouple.3.2.4 thermocouple cable, nrefers to a mineral-insulated,metal-sheathed thermocouple cable.4. Summary of Test Method4.1 This test method measures the room temperature (22 65C (72 6 10F) dc electrical ins
18、ulation resistance: (1) in thecase of a length of thermocouple cable, between each of thethermoelements and between the thermoelements and thesheath; (2) in the case of either a thermocouple having a single,ungrounded class 2 junction or a thermocouple having multiplethermoelement pairs which share
19、a common, ungrounded class2 junction, between the thermoelement pair(s) and the sheath;(3) in the case of a thermocouple having multiple, separate,ungrounded class 2 junctions, between each of the thermoele-ment pairs and between the thermoelement pairs and thesheath. The resistance measurements are
20、 made with an instru-ment such as a megohm bridge or megohmeter as described in6.2.4.2 In general, because removal of the hot junction would benecessary, measurement of the insulation resistance betweenall thermoelements in a thermocouple is not commonly under-taken and testing is limited to measuri
21、ng the insulationresistance between thermoelement pairs where possible, andbetween the thermoelement pairs and the sheath of the ther-mocouple.4.3 Special preparation of a thermocouple will not normallybe required, provided that the extension lead wires are clean,undamaged, and sufficiently long to
22、permit connection of thetest instrument.4.4 Athermocouple cable having effective end seals in placeand its thermoelements accessible may be tested withoutfurther preparation. If preparation of the thermocouple cable isrequired, special precautions may be necessary to prevent theintrusion of moisture
23、 and other contaminants that can affect theinsulation resistance. The repeatability of the test method canprimarily depend upon how well this is achieved. Preparationusually involves removing 10 to 30 mm (0.4 to 1.2 in.) of thesheath from each end of the thermocouple cable, preventing theintrusion o
24、f any moisture into, or expelling any moisture from,the compacted mineral insulation, and sealing the ends withepoxy resin or other suitable moisture sealant. Users of this testmethod may refer to Appendix X1 for information.5. Significance and Use5.1 Thermocouples fabricated from thermocouple cable
25、 thathas been contaminated by moisture or by other impurities mayundergo large changes in thermoelectric properties or may failcatastrophically when exposed to high temperatures. Sincesuch contamination usually lowers the electrical resistancebetween the thermoelements and the sheath substantially,m
26、easurement of the insulation resistance can provide a valu-able check of insulation quality and cleanliness, and can serveas a basis for rejection of unsuitable material and unreliablecomponents. For manufacturers in particular, low electricalinsulation resistance can also be indicative of displaced
27、 ther-moelements or defects in the metal sheath which will requirefurther investigation, but all users should be aware of thesepotential defects when faced with an unacceptable insulationresistance measurement.5.2 This test method is primarily intended for use bymanufacturers and users of mineral-in
28、sulated, metal-sheathedthermocouples or thermocouple cables to verify that measuredvalues of insulation resistance exceed specified minimumvalues, such as those listed in Specifications E 235, E 585/E 585M, E 608/E 608M, and E 2181/E 2181M. Manufacturersand users should be aware, however, that when
29、the insulationresistance is greater than 1 3 108V, disagreement by an orderof magnitude in the results obtained with this test method is notunusual. In addition, users of this test method should appreciatethat the room temperature insulation resistance of both ther-mocouple cables and of finished th
30、ermocouples may changeduring shipment, storage, and use, particularly if the end sealsare damaged or defective. Consequently, values of insulationresistance determined by this test method may not necessarilybe repeatable.6. Apparatus6.1 WarningAll tools and apparatus used must be cleanand must not i
31、ntroduce oil or other contaminants into theinsulation. The presence of such contaminants may invalidatethe test results obtained using this test method.6.2 Megohmeter or Megohm Bridge, with a test voltagerange between 50 and 500 VDC, measurement ranges from1 3 105V to 1 3 1012V, and an accuracy of a
32、t least 610 % ofreading. Both the positive and negative connection terminalsand test leads are to be electrically “floating,” that is notconnected to earth ground potential.6.2.1 Other resistance-measuring instruments or circuitsthat satisfy the electrical requirements given in 6.2 are accept-able.6
33、.3 Insulated Copper Connecting Wires, with suitablemechanical-type connectors.6.4 The following apparatus may be required in carrying outthe procedures described in Appendix X1:E7800626.4.1 Heat Source, (for example, a small propane-type torchor an electric heat gun).6.4.2 Moisture Sealant, such as
34、epoxy resin3or hot meltglue that when properly applied will provide an effective sealagainst moisture intrusion for the end(s) of the thermocouple orthermocouple cable at temperatures up to 200C (392F).6.4.3 Metal-Sheathed Cable StripperAny commerciallyavailable cable stripper that will satisfactori
35、ly remove thesheath without damage to the thermoelements is acceptable.6.4.4 Optical Magnifier, with a magnification of 5 to 103(for example, a watchmakers loupe).7. Test Specimen7.1 Conduct the insulation resistance measurements on thefull length of mineral-insulated, metal-sheathed thermocouplecab
36、le or on the intact thermocouple sensor assembly under test.8. Procedure8.1 Resistance Measurement:8.1.1 If epoxy resin has been used as a sealant, make certainit has fully cured before conducting the test. Take the resistancemeasurements in a dry location at an ambient temperaturebetween 17 and 27C
37、 (62 and 82F).NOTE 1Surface adsorption of atmospheric moisture on the end sealsmay be a problem in conducting the test, and great care must be taken toensure that the end seals are clean and dry when tests are conducted.8.1.1.1 When insulated copper lead wires are used with aresistance measuring ins
38、trument, make sure the open-circuitresistance between the insulated wire leads is at least1 3 1012V.NOTE 2Large errors can arise in the measurement of high resistancesdue to electrical current leakage effects. Electrical resistance measurementtechniques for high resistances should be used to minimiz
39、e currentleakage. Consult the operators manual of the resistance measuringinstrument for proper measurement techniques and safety precautions tobe observed.8.1.1.2 Adjust the resistance measuring instruments testvoltage to that specified in the invoking specification.8.1.2 Thermoelements to Sheath (
40、applies to thermocouplecable and all thermocouples):8.1.2.1 Electrically connect all the thermoelements withinthe BML or all the thermoelement pairs within the thermo-couple by twisting them together or mechanically short-circuiting them at the end at which the test voltage will beapplied. Make sure
41、 that no thermoelement or thermoelementpair is in contact with the sheath at either end of the cable orat the cold junction. The sheath should be electrically con-nected to ground.8.1.2.2 Connect the positive lead of the measuring instru-ment to the thermoelements or thermoelement pairs, thenegative
42、 lead to the metal sheath, record the time, and energizethe test circuit.8.1.2.3 Select the lowest range of the measuring instrumentthat will provide an on-scale reading.8.1.2.4 Maintain the applied test voltage until the measuredvalue stabilizes or for a maximum time of 1 min and record thereading
43、indicated by the measuring instrument. De-energizethe test circuit, making sure that any capacitively storedelectric charge has been discharged. Do not disconnect any testlead wire from either the thermoelements (or thermoelementpairs) or metal sheath without first de-energizing the measuringinstrum
44、ents test circuit.8.1.2.5 In the event of a BML or thermocouple (havingmore than one separate, ungrounded class 2 junction only)providing an unacceptably low measurement value, individualmeasurements of the insulation resistance between each ther-moelement or thermoelement pair and the sheath may be
45、required.(1) Proceed by separating the thermoelements or thermo-element pairs so that they are not in electrical contact with eachother or with the sheath. The sheath should be electricallyconnected to ground.(2) Connect the positive lead of the measuring instrumentto one of the thermoelements or on
46、e of the thermoelementpairs, the negative lead to the metal sheath, record the time, andenergize the test circuit. Proceed as directed in 8.1.2.3 and8.1.2.4.(3) Repeat the procedure described in 8.1.2.5(2) for each ofthe thermoelements within the BML or for each of thethermoelement pairs within the
47、thermocouple under test.NOTE 3Use of certain compacted ceramic insulating materials, otherthan magnesia or alumina, may result in insulation resistance measure-ments that differ significantly depending upon the polarity of the appliedtest voltage. In these cases, the procedures described in 8.1.2.2-
48、8.1.2.5(3)should be repeated using the opposite polarity connections and a secondset of test results recorded.8.1.3 Thermoelement to Thermoelement (applies only to athermocouple cable):8.1.3.1 Separate the thermoelements so that they are not inelectrical contact with each other or with the sheath. T
49、he sheathshould be electrically connected to ground.8.1.3.2 Make electrical connections to any two thermoele-ments from the test voltage terminals of the measuring instru-ment with the positive and negative lead wires.8.1.3.3 Record the time and energize the test circuit.8.1.3.4 Select the lowest range of the measuring instrumentthat will provide an on-scale reading.8.1.3.5 Maintain the applied test voltage until the measuredvalue stabilizes or for a maximum time of 1 min and record thereading indicated by the measuring instrument. De-energizethe test circuit,
