GUM Uncertainty Budgeting for High Temperature MEMS Sensor Lot Acceptance
Lot acceptance for high-temperature MEMS sensors requires GUM uncertainty budgets and guardbanding to mitigate consumer risk across severe thermal envelopes.
Thermocouple Radiation Error
Thermal Discrepancy
Temperature measurements in high-temperature gas flows often deviate from the true fluid temperature due to radiative heat transfer between the sensor and surrounding walls. This difference is known as thermocouple radiation error, which affects the accuracy of combustion and exhaust monitoring systems. The error arises because the sensor junction achieves thermal equilibrium with both the gas and the solid boundaries of the duct.
Radiative Transfer
Energy exchange occurs because the thermocouple junction is typically hotter or colder than the enclosing duct surfaces. When the surrounding walls are cold, the sensor loses heat through radiation, causing it to read a temperature lower than the gas stream. This transfer is governed by the emissivity of the junction and the Stefan-Boltzmann law.
Environmental Influence
High gas velocities increase convective heat transfer, which helps the sensor junction stay closer to the actual gas temperature. In contrast, stagnant or low-velocity flows increase the thermocouple radiation error by reducing the convective coupling.
Error Mitigation
Reducing the magnitude of this offset involves using radiation shields or aspirating the sensor to force high-velocity gas over the junction. Engineers calculate the required correction by measuring the gas velocity, the sensor diameter and the wall temperature to adjust the raw output values. This adjustment is verified by comparing the thermocouple readings to those of optical pyrometers and suction pyrometers in a calibrated test section.