Temperature measurement errors with type K (Chromel vs Alumel) thermocouples due to short-ranged ordering in Chromel

Journal Article · Fri Oct 31 23:00:00 EST 1975 · Rev. Sci. Instrum.; (United States)

DOI:https://doi.org/10.1063/1.1134086· OSTI ID:7356923

Kollie, T G; Horton, J L; Carr, K R; Herskovitz, M B; Mossman, C A

After annealed, type K(Chromel vs Alumel) thermocouples were heated above 200/sup 0/C, their temperature measurements were in error up to 1.3 percent, as determined by comparison calibrations to working standard 90 percent Pt--10 percent Rh/Pt thermocouples or platinum resistance thermometers. Reannealing the type K thermocouples removed the errors. The errors were due to changes in the thermal emf vs temperature relationship of the type K thermocouples, which from previous work of others can be attributed to short-ranged ordering of the Chromel thermoelements. Reannealing the thermocouples removed the errors because the order--disorder transformation is reversible; that is, short-ranged ordering of the Ni and Cr atoms of the Chromel alloy occurs between 200/sup 0/ and 600/sup 0/C, and disordering occurs above 600/sup 0/C. The traveling gradient method was used to determine the effects of heat treatment on the thermal emf of type K thermocouples, to investigate the kinetics of ordering of Chromel, and to determine the amount of order produced by heat treatments. Though the order--disorder transformation could not be stopped, results demonstrate that there exists an optimum amount of order in the Chromel thermoelement to yield a repeatable thermal emf vs temperature relationship for a type K thermocouple in a specific application. When the experimental conditions of the application were controlled carefully, isothermal heat treatment to produce the optimum order in the Chromel thermoelements and calibration of the type K thermocouples prior to use essentially eliminated the temperature measurement errors due to order.

• Research Organization: Oak Ridge National Lab., TN
• OSTI ID: 7356923
• Journal Information: Rev. Sci. Instrum.; (United States), Journal Name: Rev. Sci. Instrum.; (United States) Vol. 46:11; ISSN RSINA
• Country of Publication: United States
• Language: English

Related Subjects

440300* -- Miscellaneous Instruments-- (-1989)
47 OTHER INSTRUMENTATION
ALLOYS
ALUMEL
ALUMINIUM ALLOYS
ANNEALING
CALIBRATION
CHROMEL
CHROMIUM ALLOYS
ERRORS
HEAT TREATMENTS
IRON ADDITIONS
IRON ALLOYS
MANGANESE ALLOYS
MEASURING INSTRUMENTS
NICKEL ALLOYS
NICKEL BASE ALLOYS
SILICON COMPOUNDS
TEMPERATURE MEASUREMENT
THERMOCOUPLES