ASTM E780-06(2011)
(Test Method)Standard Test Method for Measuring the Insulation Resistance of Mineral-Insulated, Metal-Sheathed Thermocouples and Thermocouple Cable at Room Temperature
Standard Test Method for Measuring the Insulation Resistance of Mineral-Insulated, Metal-Sheathed Thermocouples and Thermocouple Cable at Room Temperature
SIGNIFICANCE AND USE
Thermocouples fabricated from thermocouple cable that has been contaminated by moisture or by other impurities may undergo large changes in thermoelectric properties or may fail catastrophically when exposed to high temperatures. Since such contamination usually lowers the electrical resistance between the thermoelements and the sheath substantially, measurement of the insulation resistance can provide a valuable check of insulation quality and cleanliness, and can serve as a basis for rejection of unsuitable material and unreliable components. For manufacturers in particular, low electrical insulation resistance can also be indicative of displaced thermoelements or defects in the metal sheath which will require further investigation, but all users should be aware of these potential defects when faced with an unacceptable insulation resistance measurement.
This test method is primarily intended for use by manufacturers and users of mineral-insulated, metal-sheathed thermocouples or thermocouple cables to verify that measured values of insulation resistance exceed specified minimum values, such as those listed in Specifications E235, E585/E585M, E608/E608M, and E2181/E2181M. Manufacturers and users should be aware, however, that when the insulation resistance is greater than 1 × 108 Ω, disagreement by an order of magnitude in the results obtained with this test method is not unusual. In addition, users of this test method should appreciate that the room temperature insulation resistance of both thermocouple cables and of finished thermocouples may change during shipment, storage, and use, particularly if the end seals are damaged or defective. Consequently, values of insulation resistance determined by this test method may not necessarily be repeatable.
SCOPE
1.1 This test method provides the procedures for measuring the room temperature electrical insulation resistance between the thermoelements, and between the thermoelements and the sheath, of a mineral-insulated, metal-sheathed thermocouple or thermocouple cable. It may be used to measure the insulation resistance of bulk lengths of thermocouple cable previously sealed against moisture intrusion or to test a thermocouple having an ungrounded class 2 junction. This method cannot be used to test a thermocouple having a grounded class 1 junction unless the junction is removed prior to testing, after which the thermocouple may be dealt with in the same manner as a thermocouple cable.
1.2 This test method applies primarily to thermocouple cables conforming to Specifications E585/E585M and E2181/E2181M and to thermocouples conforming to Specifications E608/E608M and E2181/E2181M, but may also be applied to thermocouples or thermocouple cables that are suitable for use in air, whose sheath or thermoelements are comprised of refractory metals, that are tested in a dry and chemically inert environment, and that may employ compacted ceramic insulating materials other than magnesia (MgO) or alumina (Al2O3). Users of this test method should note that specifications dealing with compacted ceramic insulating materials other than magnesia or alumina, which are described in Specification E1652, are not currently available. As a result, acceptance criteria must be agreed upon between the customer and supplier at the time of purchase, or alternatively, judgment and experience must be applied in establishing test voltage levels and acceptable insulation resistance values for these types of thermocouples and thermocouple cables.
1.3 This test method may be used for thermocouples or thermocouple cables having an outside diameter of 0.5 mm (0.020 in) or larger.
1.4 Users of this test method should be aware that the room temperature insulation resistance of a mineral-insulated, metal-sheathed thermocouple or thermocouple cable may change during shipment, storage, or use.
1.5 The values stated in SI units are to be regarded as standard. The values give...
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E780 − 06 (Reapproved 2011)
Standard Test Method for
Measuring the Insulation Resistance of Mineral-Insulated,
Metal-Sheathed Thermocouples and Thermocouple Cable at
Room Temperature
This standard is issued under the fixed designation E780; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.4 Users of this test method should be aware that the room
temperatureinsulationresistanceofamineral-insulated,metal-
1.1 This test method provides the procedures for measuring
sheathed thermocouple or thermocouple cable may change
the room temperature electrical insulation resistance between
during shipment, storage, or use.
the thermoelements, and between the thermoelements and the
1.5 The values stated in SI units are to be regarded as
sheath,ofamineral-insulated,metal-sheathedthermocoupleor
thermocouple cable. It may be used to measure the insulation standard. The values given in parentheses are for information
only.
resistance of bulk lengths of thermocouple cable previously
sealed against moisture intrusion or to test a thermocouple
1.6 This standard does not purport to address all of the
having an ungrounded class 2 junction.This method cannot be
safety concerns, if any, associated with its use. It is the
usedtotestathermocouplehavingagroundedclass1junction
responsibility of the user of this standard to establish appro-
unless the junction is removed prior to testing, after which the
priate safety and health practices and determine the applica-
thermocouple may be dealt with in the same manner as a
bility of regulatory limitations prior to use.
thermocouple cable.
2. Referenced Documents
1.2 This test method applies primarily to thermocouple
2.1 ASTM Standards:
cables conforming to Specifications E585/E585M and E2181/
E235Specification for Thermocouples, Sheathed, Type K
E2181M and to thermocouples conforming to Specifications
and Type N, for Nuclear or for Other High-Reliability
E608/E608M and E2181/E2181M, but may also be applied to
Applications
thermocouples or thermocouple cables that are suitable for use
E344Terminology Relating to Thermometry and Hydrom-
in air, whose sheath or thermoelements are comprised of
etry
refractory metals, that are tested in a dry and chemically inert
E585/E585M Specification for Compacted Mineral-
environment, and that may employ compacted ceramic insu-
Insulated, Metal-Sheathed, Base Metal Thermocouple
lating materials other than magnesia (MgO) or alumina
Cable
(Al O ). Users of this test method should note that specifica-
2 3
E608/E608MSpecification for Mineral-Insulated, Metal-
tions dealing with compacted ceramic insulating materials
Sheathed Base Metal Thermocouples
other than magnesia or alumina, which are described in
E1652Specification for Magnesium Oxide and Aluminum
Specification E1652, are not currently available. As a result,
Oxide Powder and Crushable Insulators Used in the
acceptance criteria must be agreed upon between the customer
Manufacture of Metal-Sheathed Platinum Resistance
and supplier at the time of purchase, or alternatively, judgment
Thermometers, Base Metal Thermocouples, and Noble
and experience must be applied in establishing test voltage
Metal Thermocouples
levels and acceptable insulation resistance values for these
E2181/E2181M Specification for Compacted Mineral-
types of thermocouples and thermocouple cables.
Insulated, Metal-Sheathed, Noble Metal Thermocouples
1.3 This test method may be used for thermocouples or
and Thermocouple Cable
thermocouple cables having an outside diameter of 0.5 mm
3. Terminology
(0.020 in) or larger.
3.1 Definitions—ThedefinitionsgiveninTerminologyE344
shall apply to the terms used in this test method.
This test method is under the jurisdiction of ASTM Committee E20 on
Temperature Measurement and is the direct responsibility of Subcommittee E20.04
on Thermocouples. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2011. Published May 2011. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1992. Last previous edition approved in 2006 as E780–06. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0780-06R11. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E780 − 06 (2011)
3.2 Definitions of Terms Specific to This Standard: between the thermoelements and the sheath substantially,
3.2.1 bulk material length (BML), n—a single length of measurement of the insulation resistance can provide a valu-
finished thermocouple cable. able check of insulation quality and cleanliness, and can serve
as a basis for rejection of unsuitable material and unreliable
3.2.2 dry, adj—a condition of the ambient air at time of test
components. For manufacturers in particular, low electrical
that does not exceed the equivalent of 50% relative humidity
insulation resistance can also be indicative of displaced ther-
at 22°C [72°F].
moelements or defects in the metal sheath which will require
3.2.3 thermocouple, n—refers to a mineral-insulated, metal-
further investigation, but all users should be aware of these
sheathed thermocouple.
potential defects when faced with an unacceptable insulation
3.2.4 thermocouple cable, n—refers to a mineral-insulated,
resistance measurement.
metal-sheathed thermocouple cable.
5.2 This test method is primarily intended for use by
manufacturers and users of mineral-insulated, metal-sheathed
4. Summary of Test Method
thermocouples or thermocouple cables to verify that measured
4.1 This test method measures the room temperature (22 6
values of insulation resistance exceed specified minimum
5°C (72 6 10°F)) dc electrical insulation resistance: (1) in the
values, such as those listed in Specifications E235, E585/
case of a length of thermocouple cable, between each of the
E585M, E608/E608M, and E2181/E2181M. Manufacturers
thermoelements and between the thermoelements and the
and users should be aware, however, that when the insulation
sheath; (2)inthecaseofeitherathermocouplehavingasingle,
resistance is greater than1×10 Ω, disagreement by an order
ungroundedclass2junctionorathermocouplehavingmultiple
ofmagnitudeintheresultsobtainedwiththistestmethodisnot
thermoelementpairswhichshareacommon,ungroundedclass
unusual.Inaddition,usersofthistestmethodshouldappreciate
2 junction, between the thermoelement pair(s) and the sheath;
that the room temperature insulation resistance of both ther-
(3) in the case of a thermocouple having multiple, separate,
mocouple cables and of finished thermocouples may change
ungrounded class 2 junctions, between each of the thermoele-
during shipment, storage, and use, particularly if the end seals
ment pairs and between the thermoelement pairs and the
are damaged or defective. Consequently, values of insulation
sheath. The resistance measurements are made with an instru-
resistance determined by this test method may not necessarily
ment such as a megohm bridge or megohmeter as described in
be repeatable.
6.2.
6. Apparatus
4.2 Ingeneral,becauseremovalofthehotjunctionwouldbe
necessary, measurement of the insulation resistance between
6.1 Warning—All tools and apparatus used must be clean
all thermoelements in a thermocouple is not commonly under-
and must not introduce oil or other contaminants into the
taken and testing is limited to measuring the insulation
insulation. The presence of such contaminants may invalidate
resistance between thermoelement pairs where possible, and
the test results obtained using this test method.
between the thermoelement pairs and the sheath of the ther-
6.2 Megohmeter or Megohm Bridge, with a test voltage
mocouple.
range between 50 and 500 VDC, measurement ranges from
5 12
4.3 Specialpreparationofathermocouplewillnotnormally
1×10 Ωto1×10 Ω, and an accuracy of at least 610% of
be required, provided that the extension lead wires are clean,
reading. Both the positive and negative connection terminals
undamaged, and sufficiently long to permit connection of the
and test leads are to be electrically “floating,” that is not
test instrument.
connected to earth ground potential.
6.2.1 Other resistance-measuring instruments or circuits
4.4 Athermocouplecablehavingeffectiveendsealsinplace
that satisfy the electrical requirements given in 6.2 are accept-
and its thermoelements accessible may be tested without
able.
further preparation. If preparation of the thermocouple cable is
required, special precautions may be necessary to prevent the
6.3 Insulated Copper Connecting Wires, with suitable
intrusionofmoistureandothercontaminantsthatcanaffectthe
mechanical-type connectors.
insulation resistance. The repeatability of the test method can
6.4 Thefollowingapparatusmayberequiredincarryingout
primarily depend upon how well this is achieved. Preparation
the procedures described in Appendix X1:
usually involves removing 10 to 30 mm (0.4 to 1.2 in.) of the
6.4.1 Heat Source,(forexample,asmallpropane-typetorch
sheathfromeachendofthethermocouplecable,preventingthe
or an electric heat gun).
intrusion of any moisture into, or expelling any moisture from,
6.4.2 Moisture Sealant, such as epoxy resin or hot melt
the compacted mineral insulation, and sealing the ends with
glue that when properly applied will provide an effective seal
epoxyresinorothersuitablemoisturesealant.Usersofthistest
againstmoistureintrusionfortheend(s)ofthethermocoupleor
method may refer to Appendix X1 for information.
thermocouple cable at temperatures up to 200°C (392°F).
5. Significance and Use
5.1 Thermocouplesfabricatedfromthermocouplecablethat
Devcon “5-Minute” Epoxy has been found suitable for this purpose. The sole
source of supply of the Devcon “5-Minute” Epoxy known to the committee at this
has been contaminated by moisture or by other impurities may
time is Devcon Corp., Endicott St., Danvers, MA 01923. If you are aware of
undergo large changes in thermoelectric properties or may fail
alternative suppliers, please provide this information to ASTM International
catastrophically when exposed to high temperatures. Since
Headquarters.Your comments will receive careful consideration at a meeting of the
such contamination usually lowers the electrical resistance responsible technical committee, which you may attend.
E780 − 06 (2011)
6.4.3 Metal-Sheathed Cable Stripper—Any commercially providing an unacceptably low measurement value, individual
available cable stripper that will satisfactorily remove the measurements of the insulation resistance between each ther-
sheath without damage to the thermoelements is acceptable. moelement or thermoelement pair and the sheath may be
6.4.4 Optical Magnifier, with a magnification of 5 to 10× required.
(for example, a watchmaker’s loupe). (1)Proceed by separating the thermoelements or thermo-
elementpairssothattheyarenotinelectricalcontactwitheach
7. Test Specimen other or with the sheath. The sheath should be electrically
connected to ground.
7.1 Conduct the insulation resistance measurements on the
(2)Connect the positive lead of the measuring instrument
full length of mineral-insulated, metal-sheathed thermocouple
to one of the thermoelements or one of the thermoelement
cableorontheintactthermocouplesensorassemblyundertest.
pairs,thenegativeleadtothemetalsheath,recordthetime,and
energize the test circuit. Proceed as directed in 8.1.2.3 and
8. Procedure
8.1.2.4.
8.1 Resistance Measurement:
(3)Repeattheproceduredescribedin8.1.2.5(2)foreachof
8.1.1 Ifepoxyresinhasbeenusedasasealant,makecertain
the thermoelements within the BML or for each of the
ithasfullycuredbeforeconductingthetest.Taketheresistance
thermoelement pairs within the thermocouple under test.
measurements in a dry location at an ambient temperature
NOTE 3—Use of certain compacted ceramic insulating materials, other
between 17 and 27°C (62 and 82°F).
than magnesia or alumina, may result in insulation resistance measure-
NOTE 1—Surface adsorption of atmospheric moisture on the end seals
ments that differ significantly depending upon the polarity of the applied
may be a problem in conducting the test, and great care must be taken to test voltage. In these cases, the procedures described in 8.1.2.2-8.1.2.5(3)
ensure that the end seals are clean and dry when tests are conducted.
should be repeated using the opposite polarity connections and a second
set of test results recorded.
8.1.1.1 When insulated copper lead wires are used with a
resistance measuring instrument, make sure the open-circuit 8.1.3 Thermoelement to Thermoelement (applies only to a
resistance between the insulated wire leads is at least 1×10 thermocouple cable):
12Ω. 8.1.3.1 Separate the thermoelements so that they are not in
electricalcontactwitheachotherorwiththesheath.Thesheath
NOTE 2—Large errors can arise in the measurement of high resistances
should be electrically connected to ground.
duetoelectricalcurrentleakageeffects.Electricalresistancemeasurement
techniques for high resistances should be used to minimize current
8.1.3.2 Make electrical connections to any two thermoele-
leakage. Consult the operator’s manual of the resistance measuring
ments from the test voltage terminals of the measuring instru-
instrument for proper measurement techniques and safety precautions to
ment with the positive and negative lead wires.
be observed.
8.1.3.3 Record the time and energize the test circuit.
8.1.1.2 Adjust the resistance measuring instrument’s test
8.1.3.4 Select the lowest range of the measuring instrument
voltage to that specified in the invoking specification.
that will provide an on-scale reading.
8.1.2 Thermoelements to Sheath (applies to thermocouple
8.1.3.5 Maintain the applied test voltage until the measured
cable and all thermocouples):
valuestabilizesorforamaximumtimeof1minandrecordthe
8.1.2.1 Electrically connect all the thermoelements within
reading indicated by the measuring instrument. De-energize
the BML or all the thermoelement pairs within the thermo-
the test circuit, making sure that any capacitively stored
couple by twisting them together or mechanically short-
electricchargehasbeendischarged.Donotdisconnectanytest
circuiting them at the end at which the test voltage will be
lead wire from either of the thermoelements without first
applied. Make sure that no thermoelement or thermoelement
de-energizing the measuring instrument’s test circuit.
pair is in contact with the sheat
...
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