Ultrasonic Anemometer Zero-Flow Offset in Student-Built Engineering Education Wind-Tunnel Benches: A Hot-Wire Referenced Evaluation Linking Transducer Temperature Drift to Airspeed Reliability Bounds

Student-built wind-tunnel benches used in engineering education often pair low-cost ultrasonic anemometers with hot-wire reference probes, yet the zero-flow offset of the ultrasonic path remains poorly characterised under the thermal conditions these benches actually experience. The model is calibrated against published transducer specifications and against reanalysis temperature records for Nairobi, Kenya (2000–2023; 312 monthly observations; mean 20.146 °C; warming trend 0.2477 °C per decade), which supply a realistic ambient forcing envelope for a teaching laboratory. The analysis derives closed-form bounds on the bias and variance of the indicated airspeed, identifies the temperature-drift threshold beyond which the ultrasonic reading exceeds a specified reliability tolerance, and shows that the hot-wire reference itself contributes a non-negligible uncertainty that must be propagated jointly. The contribution is a design-oriented reliability calculus that allows instructors and student teams to specify transducer thermal compensation, calibration intervals and acceptance thresholds before a bench is commissioned, rather than discovering zero-flow error after the fact.