Capacitive-Soil-Moisture Sensor Ageing in Technology Education Irrigation Laboratories: A Gravimetric Reference Calibration Linking Electrode Polarisation to Irrigation-Scheduling Error Bounds

Capacitive soil-moisture sensors used in undergraduate irrigation laboratories drift as their electrodes polarise, and the resulting bias propagates into irrigation-scheduling decisions that students are taught to make from sensor readings. The framework treats the sensor as a polarisation-dependent transfer function, defines a gravimetric reference calibration that separates reversible dielectric response from irreversible electrode ageing, and propagates the calibrated uncertainty into a scheduling rule. A design-analysis component characterises the operating characteristics of a before-after-control-impact evaluation of the calibration under a declared variance structure with an intraclass correlation of 0.64, between-cluster standard deviation of 6.0 and residual standard deviation of 4.5; power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The analysis indicates that 54 clusters are required to detect a 5 percentage-point improvement at 80% power. The contribution is a calibration-and-error-bound procedure that makes sensor ageing an explicit, teachable quantity in irrigation laboratories rather than an unexamined source of scheduling error.