Derivation and testing of a model to calculate electrical shunting and leakage errors in sheathed thermocouples

Journal Article · Thu Sep 01 04:00:00 EDT 1977 · Rev. Sci. Instrum.; (United States)

DOI:https://doi.org/10.1063/1.1135218· OSTI ID:7098222

Roberts, M J; Kollie, T G

A three-wire transmission line model was derived to calculate temperature measurement errors caused by electrical shunting and leakage in metal sheathed, oxide insulated, compacted thermocouple assemblies. Input parameters for the model were measured and used to verify experimentally the validity of the model on a 0.5-mm-diam stainless steel sheathed, MgO insulated, Chromel/Alumel thermocouple assembly. When a 1.27-m length of the assembly was heated to 1274 K, the errors due to shunting, calculated using the model, were 6.4% and 9.4% lower than the experimentally measured errors for theromocouple measuring junction temperatures of 273.2 and 371.5 K, respectively. With the 1.27-m length heated to 1373 K, the errors due to shunting, calculated using the model, were 12.6% and 14.9% lower than the experimentally measured errors for measuring junction temperatures of 273.2 and 371.5 K, respectively. For a 1.27-m length of the thermocouple assembly at 1274 K, the measuring junction at 273.2 K, and sheath currents of 20, 40, 60, and 80 mA, the error due to leakage, calculated using the model, was within 6% of the experimentally measured error. With the 1.27-m length heated to 1373 K and all other conditions the same, the calculated error due to leakage was within 9.1% of the measured error. This paper describes the experimental apparatus and measurements, explains the method used to infer parameters from experimental measurements, presents the model, and derives the differential equations that describe the electrical phenomena. These equations are solved exactly for a uniform line. A fortran IV coded numerical solution for nonuniform lines is described. Several illustrative examples are presented to clarify the concepts involved in electrical shunting.

• Research Organization: Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
• OSTI ID: 7098222
• Journal Information: Rev. Sci. Instrum.; (United States), Journal Name: Rev. Sci. Instrum.; (United States) Vol. 48:9; ISSN RSINA
• Country of Publication: United States
• Language: English

Related Subjects

440300* -- Miscellaneous Instruments-- (-1989)
47 OTHER INSTRUMENTATION
ALKALINE EARTH METAL COMPOUNDS
ALLOYS
ALUMEL
ALUMINIUM ALLOYS
CHALCOGENIDES
CHROMEL
CHROMIUM ALLOYS
CORROSION RESISTANT ALLOYS
CURRENTS
ELECTRIC CURRENTS
ERRORS
IRON ADDITIONS
IRON ALLOYS
IRON BASE ALLOYS
LEAKAGE CURRENT
MAGNESIUM COMPOUNDS
MAGNESIUM OXIDES
MANGANESE ALLOYS
MATHEMATICAL MODELS
MEASURING INSTRUMENTS
NICKEL ALLOYS
NICKEL BASE ALLOYS
NUMERICAL SOLUTION
OXIDES
OXYGEN COMPOUNDS
SILICON COMPOUNDS
STAINLESS STEELS
STEELS
TEMPERATURE MEASUREMENT
TESTING
THERMOCOUPLES