Author: W. Lai

Publisher:

Published: 1962

Total Pages: 138

ISBN-13:

Presented in this report is a study of the errors in surface-temperature measurements made with a thermocouple (TC), when both TC wires and surface are exposed to radiant energy. The errors arise from (1) the net heat transfer at the TC junction, and (2) the quality and stability (change in calibration) of the TC materials. The effects of electrical disturbances and mechanical vibrations were also investigated. Results from steady-state experiments show that analysis of the heat-transfer errors must include the effects of (1) specimen thermal conductivity and absorptivity, (2) orientation of the TC with respect to convection currents, (3) proximity of the TC wires and the specimen surface, (4) relative changes of absorptivity and over-all thermal conductivity due to thermal aging of the TC material, TC insulation, and the specimen to which the TC is attached, and (5) heat flux level. Some experiments were performed to study the response of surface TC's to transient heating. The results, for a single specimen thickness, showed that the transient response is greater for large TC wire diameter during the initial transient heating period bu becomes independent of TC wire diameter when the heat flux rate reaches steady state. It was shown that moisture absorbed by the TC insulation has a negligible effect on the heat transfer error. The magnitude of errors arising from instability, which is independent of the quality of TC materials, depends upon the exposure condition and duration. The calibration of commercial thermocouples, whether noble metal or base metal, will probably change lesss than 1F after exposure at less than 1000F in air for 1000 hours. Twisting the TC wires together, and shielding power cables, are sufficient to reduce pickup noise from thyratron power regulators in our instrumentation to an acceptable level. Failure of TC's due to vibration at 40 CPS and 2 to 3 g acceleration for up to 4 hours is unlikely up to temperatures of about 3000F. A bibliography is appended.