Steady state error estimation and modification of a shielded thermocouple

For high accuracy measurement of aero-engine exhaust temperature, a conjugate heat transfer (CHT) numerical simulation method was employed to estimate different sources of steady state errors and their variation characteristics of a typical shielded thermocouple working at different Mach number condition namely 0.2,0.3,0.4,0.5,0.6. The results showed that under the Mach number less than 0.3 working condition, the radiation error took the highest proportion of total error, while at the 0.6 Mach number, the conduction error was the main factor affecting the accuracy of measurement. To improve the measurement accuracy of the shielded thermocouple, the influences of the length to diameter ratio of the wire and inlet to outlet area ratio of the shield on the conduction error and radiation error were separately studied. The results showed that the conduction error can be significantly reduced by reasonably increasing the length to diameter ratio of the wire, and as the decrease of inlet to outlet area ratio, the velocity error increased while radiation error decreased there existed a best value of inlet to outlet area ratio to minimize the overall value of the steady state error. Finally, the length to diameter ratio of the wire equal to 12 and the inlet to outlet area ratio equal to 6 were selected to modify the primary shielded thermocouple. After modification, the overall value of the steady state error was reduced by more than 50% in these 5 working conditions, thus the measurement accuracy of the shielded thermocouple was improved.