ASTM E220-19 pdf free.Standard Test Method for Calibration of Thermocouples By Comparison Techniques.
4.1 Comparison calibration consists of measuring the emf of the thermocouple being calibrated in an isothermal medium while simultaneously measuring the temperature of the medium with a reference thermometer. The reference thermometer may he any thermometer with sufficient accuracy at the temperature of calibration.
S. Signilicance and Use
5.1 For users or manufacturers of thermocouples. this test method provides a means of verifying the emf-ieniperature characteristics of the material prior to use.
5.2 This test method can be used to calibrate a thermocouple for use as a reference, or it can be used to calibrate thcmiocouples representing a hatch of purchased. assembled ihcrnmocouples.
5.3 This test niethod can be used for the verification of the conformance of thermocouple materials to temperature tolerances for specifications such as the tables in Specification E23() or other special specifications as required for commercial. military, or research applications.
6. Interferences
6.1 Sincc the success of this test method depends largely upon the ability to maintain the measuring junction of the thermocouple being calibrated and the reference thermometer at the same temperature. considerable care must be taken in choosing the media and conditions under which the compariSons are made Stirred liquid baths, uniformly heated metal blocks, lube furnaces, and dry fluidized baths, properly used. are acceptable temperature comparison environments. In thc case of large diameter thermoelements and sheathed thermocouples. special attention must be given to effects of thermal conduction.
6.2 Voltage measurement instruments with sufficiently high input impedance must be used for measuring thcnnocouplc emf to eliminate instrument loading as a significant source of error. The ratio of input impedance to thermocouple loop resistance should be significantly (at least l0) greater than the ratio of the measured cml to the desired cml uncertainty.
6.3 The test method relies on the assumption that test thermoelements are homogeneous. If so, their output voltage at a given measuring junction temperature is independent of temperature variations along the length of the thermocouple. Departures from this ideal contribute to uncertainty in the use of test results. The effects typically are negligibly small for new, unused thermocouple material, hut not for used thermocouples, especially those of base-metal composition. The effects of inhomogcneity can be identified, but not accurately quantified, by the techniques described in Appendix X4 in this test method and in section 8.2 of Guide t2846. Descriptions of the tcsting of used thermocouples may he found Guide E2846 and Manual MNL 12 I ll.
6.4 This test method presumes that the tested thermocouples arc suitable for use in air throughout the range of calibration temperatures. To avoid oxidation of the thermoelements. refractory-metal thermocouples that have not been hermetically sealed in a sheath suitable for use in air should be tcstcd in an inert gas environment at temperatures above approximately 500 C. In this case. use of this test method is recommended in combination with the furnaces and related procedures described in Test Method E452.
7. Apparatus
7.1 The choice of apparatus used for the comparison test will depend primarily on the temperature range to be covered and on the desired calibration uncertainty. The apparatus required for the application of this test method will depend in detail upon the temperature range being covered but in all cases shall be selected from the equipment described as follows.ASTM E220 pdf download.