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 E 235
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 E 585/E 585M and E 2181/E 2181M and to thermocouples conforming to Specifications E 608/E 608M and E 2181/E 2181M, 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 (Al
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 i......
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