Measured, calculated and inferred welding temperatures are not the same
A thermocouple measures temperature at its measuring junction. Temperature in an inaccessible weld region is the result of a model, boundary conditions and calibration.
Sensor location defines the meaning of the result
A thermocouple cannot measure the temperature of “the weld in general”. It measures the temperature of its own junction. If the sensor is several millimetres away from the target region, the result also reflects heat conduction, sensor contact, response time, the drilled hole and time lag.
In friction stir welding, the region directly below the tool is physically inaccessible. Researchers studying AZ31 therefore combined thermocouples at accessible positions with a calibrated numerical model. Several sensor layouts were compared; for the best arrangement, the authors reported a maximum relative error of 0.4% and combined uncertainty within 5.1%.
What those numbers mean
The reported performance belongs to the specific material, geometry, sensor arrangement and model. It is not the general accuracy of a thermocouple and cannot be assumed for another welding application.
Technical reporting should distinguish between:
- directly measured temperature at the junction,
- calculated temperature from a numerical model,
- inferred temperature derived through correlation or calibration,
- controller set temperature, which may differ from actual workpiece temperature.
Practical WPS question
When a WPS or fabrication specification limits interpass temperature, the measurement location and method must be defined. A calculated value for an inaccessible region is not automatically a valid substitute for the required measurement.
Source
- Yan, F.; Jiang, N.; Zhang, Y.: Multi-objective constrained optimization method for thermal deformation zone temperature measurement of AZ31 magnesium alloy friction stir welding, *Welding in the World*, 7 October 2026.
This is an original explanation of measurement and uncertainty principles. No methodological figures or data tables have been reproduced.