1、Designation: E780 17Standard Test Method forMeasuring the Insulation Resistance of Mineral-Insulated,Metal-Sheathed Thermocouples and Mineral-Insulated,Metal-Sheathed Cable at Room Temperature1This standard is issued under the fixed designation E780; the number immediately following the designation
2、indicates 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.1. Scope1.1 This test method provides the procedur
3、es for measuringthe room temperature electrical insulation resistance betweenthe thermoelements and between the thermoelements and thesheath, of a mineral-insulated, metal-sheathed (MIMS) ther-mocouple or mineral-insulated, metal-sheathed (MIMS) ther-mocouple cable or between the conductors and betw
4、een theconductors and the sheath, of mineral-insulated, metal-sheathed (MIMS) cable used for industrial resistance thermom-eters. It may be used to measure the insulation resistance ofbulk lengths of mineral-insulated, metal-sheathed MIMS cablepreviously sealed against moisture intrusion or to test
5、athermocouple having an ungrounded measuring junction. Thismethod cannot be used to test a thermocouple having agrounded measuring junction unless the measuring junction isremoved prior to testing, after which the thermocouple may bedealt with in the same manner as a mineral-insulated, metal-sheathe
6、d (MIMS) cable.1.2 This test method applies primarily to thermocouplecables and cable used for industrial resistance thermometersconforming to Specifications E585/E585M, E2181/E2181M,and E2821 and to thermocouples conforming to SpecificationsE608/E608M and E2181/E2181M, but may also be applied tothe
7、rmocouples or MIMS cables that are suitable for use in air,whose sheath or thermoelements or conductors are comprisedof refractory metals, that are tested in a dry and chemicallyinert environment, and that may employ compacted ceramicinsulating materials other than magnesia (MgO) or alumina(Al2O3).
8、Users of this test method should note that specifica-tions dealing with compacted ceramic insulating materialsother than magnesia or alumina, which are described inSpecification E1652, are not currently available. As a result,acceptance criteria must be agreed upon between the customerand supplier a
9、t the time of purchase, or alternatively, judgmentand experience must be applied in establishing test voltagelevels and acceptable insulation resistance values for thesetypes of thermocouples and MIMS cables.1.3 This test method may be used for thermocouples orMIMS cables having an outside diameter
10、of 0.5 mm (0.020 in.)or larger.1.4 Users of this test method should be aware that the roomtemperature insulation resistance of a mineral-insulated, metal-sheathed thermocouple or MIMS cable will change duringshipment, storage, or use if they are not properly sealed.1.5 The values stated in SI units
11、are to be regarded asstandard. The values given in parentheses 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
12、and determine the applica-bility of regulatory limitations prior to use.1.7 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Reco
13、m-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E235 Specification for Thermocouples, Sheathed, Type Kand Type N, for Nuclear or for Other High-ReliabilityApplicationsE344 Terminology Relating to Thermometry an
14、d Hydrom-etryE585/E585M Specification for Compacted Mineral-Insulated, Metal-Sheathed, Base Metal ThermocoupleCable1This test method is under the jurisdiction of ASTM Committee E20 onTemperature Measurement and is the direct responsibility of Subcommittee E20.14on Thermocouples - Testing.Current edi
15、tion approved Jan. 15, 2017. Published March 2017. Originallyapproved in 1992. Last previous edition approved in 2011 as E780 06 (2011).DOI: 10.1520/E0780-17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of A
16、STMStandards 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. United StatesThis international standard was developed in accordance with internationally recognized pr
17、inciples on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1E608/E608M Specification for Mineral-Insulated, Metal-Sheathed Base Meta
18、l ThermocouplesE1652 Specification for Magnesium Oxide and AluminumOxide Powder and Crushable Insulators Used in theManufacture of Base Metal Thermocouples, Metal-Sheathed Platinum Resistance Thermometers, and NobleMetal ThermocouplesE2181/E2181M Specification for Compacted Mineral-Insulated, Metal-
19、Sheathed, Noble Metal Thermocouplesand Thermocouple CableE2821 Specification for Compacted Mineral-Insulated,Metal-Sheathed Cable Used in Industrial Resistance Ther-mometers3. Terminology3.1 DefinitionsThe definitions given in Terminology E344shall apply to the terms used in this test method.3.2 Def
20、initions of Terms Specific to This Standard:3.2.1 bulk material length (BML), na single length offinished thermocouple MIMS cable.3.2.2 dry, adjrefers to a condition of the ambient air attime of test that does not exceed the equivalent of 50 % relativehumidity at 22C 72F.3.2.3 thermocouple, nrefers
21、to a mineral-insulated, metal-sheathed (MIMS) thermocouple.3.2.4 thermocouple cable, nrefers to a mineral-insulated,metal-sheathed (MIMS) thermocouple cable.4. Summary of Test Method4.1 This test method measures the room temperature (22 65C (72 6 10F) dc electrical insulation resistance: (1) in thec
22、ase of a length of MIMS cable, between each of thethermoelements or conductors and between the thermoele-ments or conductors and the sheath; (2) in the case of either athermocouple having a single, ungrounded measuring junctionor a thermocouple having multiple thermoelement pairs whichshare a common
23、, ungrounded measuring junction, between thethermoelement pair(s) and the sheath; (3) in the case of athermocouple having multiple, isolated, ungrounded measur-ing junctions, between each of the thermoelement pairs andbetween the thermoelement pairs and the sheath. The resis-tance measurements are m
24、ade with an instrument such as amegohm bridge or megohmeter as described in 6.2.4.2 In general, because removal of the measuring junctionwould be necessary, measurement of the insulation resistancebetween all thermoelements in a thermocouple is not com-monly undertaken. Testing is limited to measuri
25、ng the insula-tion resistance between the thermoelement pairs and the sheathof the thermocouple and, where possible, the thermoelementpairs.4.3 Special preparation of a thermocouple will not normallybe required, provided that the extension lead wires are clean,undamaged, and sufficiently long to per
26、mit connection of thetest instrument.4.4 AMIMS cable having effective end seals in place and itsthermoelements or conductors accessible may be tested with-out further preparation. If preparation of the MIMS cable isrequired, special precautions may be necessary to prevent theintrusion of moisture an
27、d 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 MIMS cable, preventing theintrusion of any moist
28、ure 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 thathas be
29、en 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,measurement
30、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 ther-moele
31、ments or conductors or defects in the metal sheath whichwill require further investigation, but all users should be awareof these potential defects when faced with an unacceptableinsulation resistance measurement.5.2 This test method is primarily intended for use bymanufacturers and users of mineral
32、-insulated, metal-sheathed(MIMS) thermocouples or MIMS cables to verify that mea-sured values of insulation resistance exceed specified minimumvalues, such as those listed in Specifications E235, E585/E585M, E608/E608M, E2181/E2181M, and E2821. Manufac-turers and users should be aware, however, that
33、 when theinsulation resistance is greater than1108, disagreement byan order of magnitude in the results obtained with this testmethod is not unusual. In addition, users of this test methodshould appreciate that the room temperature insulation resis-tance of both MIMS cables and of finished thermocou
34、ples willchange during shipment, storage, and use if the end seals aredamaged 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 introduce oil or othe
35、r 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 from1105 to11012, and an accuracy of at least 610 % ofreading. Both
36、 the positive and negative connection terminalsand test leads are to be electrically “floating” (not connected toearth ground potential).E780 1726.2.1 Other resistance-measuring instruments or circuitsthat satisfy the electrical requirements given in 6.2 are accept-able.6.3 Insulated Copper Connecti
37、ng Wires, with suitablemechanical-type connectors.6.4 The following apparatus may be required in carrying outthe procedures described in Appendix X1:6.4.1 Heat Source, (for example, a small propane-type torchor an electric heat gun).6.4.2 Moisture Sealant, such as epoxy resin3, wax, or hotmelt glue
38、that when properly applied will provide an effectiveseal against moisture intrusion for the end(s) of the thermo-couple or MIMS cable at temperatures up to 66C (150F).Additional sealants, with higher temperature ratings, are avail-able. The higher temperature sealants require additional pro-cedures
39、to ensure a proper seal.6.4.3 Metal-Sheathed Cable StripperAny commerciallyavailable cable stripper that will satisfactorily remove thesheath without damage to the thermoelements or conductors isacceptable.6.4.4 Optical Magnifier, with a magnification of 5 to 10(for example, a watchmakers loupe).7.
40、Test Specimen7.1 Conduct the insulation resistance measurements on thefull length of mineral-insulated, metal-sheathed (MIMS) cableor 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
41、 cured before conducting the test. Take the resistancemeasurements in a dry location at room temperature (22 6 5C(72 6 10F).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
42、 dry when tests are conducted.8.1.1.1 When insulated copper lead wires are used with aresistance measuring instrument, make sure the open-circuitresistance between the insulated wire leads is at least11012.NOTE 2Large errors can arise in the measurement of high resistancesdue to electrical current l
43、eakage effects. Electrical resistance measurementtechniques for high resistances should be used to minimize 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 measurin
44、g instruments testvoltage to that specified in the invoking specification.8.1.2 Thermoelements or Conductors to Sheath (applies toMIMS cable and all thermocouples):8.1.2.1 Electrically connect all the thermoelements or con-ductors within the BML or all the thermoelement pairs withinthe thermocouple
45、by twisting them together or mechanicallyshort-circuiting them at the end at which the test voltage will beapplied. Verify that no thermoelement, conductor, or thermo-element pair is in contact with the sheath at either end of thecable or at the cold junction. The sheath should be electricallyconnec
46、ted to ground.8.1.2.2 Connect the positive lead of the measuring instru-ment to the thermoelements or conductors or thermoelementpairs, the negative lead to the metal sheath, record the time, andenergize the test circuit.8.1.2.3 Select the lowest range of the measuring instrumentthat will provide an
47、 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 indicated by the measuring instrument. De-energizethe test circuit, making sure that any capacitively storedelectric charge has been discharged. Do not
48、disconnect any testlead wire from either the thermoelements or conductors (orthermoelement pairs) or metal sheath without first de-energizing the measuring instruments test circuit.8.1.2.5 In the event of a BML or thermocouple (havingmore than one isolated, ungrounded measuring junction) pro-viding
49、an unacceptably low measurement value, individualmeasurements of the insulation resistance between eachthermoelement, conductor, or thermoelement pair and thesheath may be required.(1) Proceed by separating the thermoelements, conductors,or thermoelement pairs so that they are not in electrical contactwith each other or with the sheath. The sheath should beelectrically connected to ground.(2) Connect the positive lead of the measuring instrumentto one of the thermoelements, conductors, or one of thethermoelement pairs, the negative lead to
