ASTM E780-2006(2011) 2500 Standard Test Method for Measuring the Insulation Resistance of Mineral-Insulated Metal-Sheathed Thermocouples and Thermocouple Cable at Room Temperature《.pdf

1、Designation: E780 06 (Reapproved 2011)Standard Test Method forMeasuring the Insulation Resistance of Mineral-Insulated,Metal-Sheathed Thermocouples and Thermocouple Cable atRoom Temperature1This standard is issued under the fixed designation E780; the number immediately following the designation ind

2、icates 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 procedures

3、for measuringthe 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 cab

4、le previouslysealed 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

5、 manner as athermocouple cable.1.2 This test method applies primarily to thermocouplecables conforming to Specifications E585/E585M and E2181/E2181M and to thermocouples conforming to SpecificationsE608/E608M and E2181/E2181M, but may also be applied tothermocouples or thermocouple cables that are s

6、uitable for usein air, whose sheath or thermoelements are comprised ofrefractory metals, that are tested in a dry and chemically inertenvironment, and that may employ compacted ceramic insu-lating materials other than magnesia (MgO) or alumina(Al2O3). Users of this test method should note that speci

7、fica-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 at the time of purchase, or alternatively, judgme

8、ntand experience 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.

9、4 Users 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 asstandard. The values given in parenth

10、eses 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

11、 prior to use.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 and Hydrom-etryE585/E585M Specification for Compacted Mineral-Insulated, Metal-Shea

12、thed, Base Metal ThermocoupleCableE608/E608M Specification for Mineral-Insulated, Metal-Sheathed Base Metal ThermocouplesE1652 Specification for Magnesium Oxide and AluminumOxide Powder and Crushable Insulators Used in theManufacture of Metal-Sheathed Platinum Resistance Ther-mometers, Base Metal Th

13、ermocouples, and Noble MetalThermocouplesE2181/E2181M Specification for Compacted Mineral-Insulated, Metal-Sheathed, Noble Metal Thermocouplesand Thermocouple Cable3. Terminology3.1 DefinitionsThe definitions given in Terminology E344shall apply to the terms used in this test method.1This test metho

14、d 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, 2011. Published May 2011. Originallyapproved in 1992. Last previous edition approved in 2006 as E780 06. DOI:10.1520/E07

15、80-06R11.2For 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.1Copyright ASTM International, 100 Barr Harbor Drive

16、, PO Box C700, West Conshohocken, PA 19428-2959, United States.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 % r

17、elative humidityat 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 elec

18、trical insulation 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 wh

19、ich share 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 measur

20、ements are 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

21、to measuring 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 sufficientl

22、y long to 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 o

23、f moisture 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 thei

24、ntrusion of 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 thermoco

25、uple cable 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 subst

26、antially,measurement 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

27、 displaced 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

28、mineral-insulated, metal-sheathedthermocouples or thermocouple cables to verify that measuredvalues of insulation resistance exceed specified minimumvalues, such as those listed in Specifications E235, E585/E585M, E608/E608M, and E2181/E2181M. Manufacturersand users should be aware, however, that wh

29、en 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

30、 thermocouples 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 no

31、t introduce 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 o

32、f at 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-abl

33、e.6.3 Insulated Copper Connecting Wires, with suitablemechanical-type connectors.6.4 The following apparatus may be required in carrying outthe procedures described in Appendix X1:6.4.1 Heat Source, (for example, a small propane-type torchor an electric heat gun).6.4.2 Moisture Sealant, such as epox

34、y 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 satisfactorily r

35、emove thesheath without damage to the thermoelements is acceptable.3Devcon “5-Minute” Epoxy has been found suitable for this purpose. The solesource of supply of the Devcon “5-Minute” Epoxy known to the committee at thistime is Devcon Corp., Endicott St., Danvers, MA 01923. If you are aware ofaltern

36、ative suppliers, please provide this information to ASTM InternationalHeadquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee,1which you may attend.E780 06 (2011)26.4.4 Optical Magnifier, with a magnification of 5 to 103(for example, a watchm

37、akers loupe).7. Test Specimen7.1 Conduct the insulation resistance measurements on thefull length of mineral-insulated, metal-sheathed thermocouplecable 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, ma

38、ke certainit has fully cured before conducting the test. Take the resistancemeasurements in a dry location at an ambient temperaturebetween 17 and 27C (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 t

39、oensure 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 instrument, make sure the open-circuitresistance between the insulated wire leads is at least1 3 1012V.NOTE 2Large errors can arise in the measurement of

40、 high resistancesdue to electrical current leakage 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 obs

41、erved.8.1.1.2 Adjust the resistance measuring instruments testvoltage to that specified in the invoking specification.8.1.2 Thermoelements to Sheath (applies to thermocouplecable and all thermocouples):8.1.2.1 Electrically connect all the thermoelements withinthe BML or all the thermoelement pairs w

42、ithin the thermo-couple by twisting them together or mechanically short-circuiting them at the end at which the test voltage will beapplied. Make sure 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 elec

43、trically con-nected to ground.8.1.2.2 Connect the positive lead of the measuring instru-ment to the thermoelements or thermoelement pairs, thenegative 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

44、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 indicated by the measuring instrument. De-energizethe test circuit, making sure that any capacitively storedelectric charge has been discharged. Do no

45、t disconnect any testlead wire from either the thermoelements (or thermoelementpairs) or metal sheath without first de-energizing the measuringinstruments 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 unaccepta

46、bly low measurement value, individualmeasurements of the insulation resistance between each ther-moelement or thermoelement pair and the sheath may berequired.(1) Proceed by separating the thermoelements or thermo-element pairs so that they are not in electrical contact with eachother or with the sh

47、eath. The sheath should be electricallyconnected to ground.(2) Connect the positive lead of the measuring instrumentto one of the thermoelements or one 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

48、.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 thermocouple under test.NOTE 3Use of certain compacted ceramic insulating materials, otherthan magnesia or alumina, may result in insulation resistanc

49、e measure-ments that differ significantly depending upon the polarity of the appliedtest voltage. In these cases, the procedures described in 8.1.2.2-8.1.2.5(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. The sheathshould be electrically connected to ground.8.1.3.2 Make electrical connections to any two thermoele-ments