Значение и использование
5.1 This test method is important because the accuracy of a temperature measurement by a thermocouple is directly related to the emf stability of the thermoelements.
5.2 This test method is used to verify that the tested thermoelements meet the intended requirements.
5.3 This test method is useful in comparing the emf stability of two base metal thermoelements under the same conditions. The test and reference emf may be measured either simultaneously or alternately.
5.4 The relative stabilities of base metal thermoelements determined by this test method are valid only under the specified test conditions. Results would be affected by changes in the following conditions: (1) temperature profile or gradient along the length of the thermoelements; (2) abundance, velocity and composition of the air surrounding the test pieces; (3) thermoelectric inhomogeneity of the test thermoelements; (4) stability of the platinum thermoelement.
5.5 The test method does not address the determination of base metal thermoelement stabilities over a series of temperature changes.
5.6 The reliability of this test method depends on the emf stability of the reference platinum thermoelement. For testing the relative emf stability of base-metal thermoelements, a reference element of platinum that has sufficient thermoelectric stability to determine any significant change in emf of base-metal thermoelements shall be used. To ascertain that the experimental method protects the platinum sufficiently from degradation, the method shall be validated by performing the procedure described in Appendix X1 prior to the actual test.
5.7 The test result does not apply to applications in which the temperature distribution, for a given measuring junction temperature, changes with time.
1.1 This testmethod measures emf stability of base-metal thermoelement materialsin air referenced to platinum at specified constant elevatedtemperatures using dual, simultaneous, emf indicators, or using asingle emf indicator, with the test and reference emf measuredalternately. This test is conducted over a period of weeks.
1.2 A calibratedplatinum-rhodium/platinum thermocouple is used as a referencestandard to establish the test temperature.
1.3 The usefullife of a thermocouple depends on the stability of the emfgenerated at given temperatures for a required time interval. Thismethod provides a quantitative measure of the stability ofindividual thermoelements. By combining the results of the positive(P) and negative (N) thermoelements, the stability of athermocouple comprised of both P and N thermoelements may beobtained. The emf of an individual thermoelement is measuredagainst platinum, which may be the platinum leg of theplatinum-rhodium/platinum reference thermocouple, or an additionalplatinum reference.
Note 1—Some thermoelements may show insignificant emf drift whileundergoing relatively rapid oxidation. In these cases, failure ofthe thermoelement may be indicated only by a large rise in theelectrical resistance between joined thermoelements, as measured atthe reference junctions.
Note 2—See ASTM MNL 12 for recommended upper temperature limits inair.
Note 3—This test method is only applicable for initially newthermoelements. Base-metal thermoelements exposed to temperaturesabove 200 °C become thermoelectrically inhomogeneous,and stability testing of inhomogeneous thermoelements will giveambiguous results.
1.4 Thisstandard does not purport to address all of the safety concerns, ifany, associated with its use. It is the responsibility of the userof this standard to establish appropriate safety and healthpractices and determine the applicability of regulatory limitationsprior to use.