<![CDATA[Reliable water-level monitoring in public facilities is essential to support effective water management and ensure the availability of water in storage tanks. This study aims to design and evaluate a wireless water-level monitoring system based on an ESP32 microcontroller and a waterproof JSN-SR04T ultrasonic sensor, implemented at the Al Bayan Mosque, Politeknik Negeri Lhokseumawe. The proposed system adopts a master–slave architecture, in which the ESP32 Slave performs sensor data acquisition and processing, while the ESP32 Master receives and processes measurement data transmitted via Bluetooth. The system was evaluated over a measurement range of 25–200 cm through initial sensor characterization, linear regression-based calibration, water-level measurement testing, and Bluetooth communication performance testing using Packet Delivery Ratio (PDR) and packet loss as evaluation parameters. The results demonstrate that sensor calibration significantly improved measurement accuracy, reducing the Mean Absolute Error (MAE) from 1.811 cm to 0.216 cm and the Root Mean Square Error (RMSE) from 2.035 cm to 0.254 cm, corresponding to improvements of 88.07% and 87.53%, respectively. Following calibration, the sensor exhibited a relative error ranging from 0.00% to 0.60%. Water-level testing produced an MAE of 0.514%, an RMSE of 0.612%, a mean relative error of 1.465%, and a coefficient of determination (R²) of 0.99988. However, the maximum relative error of 6.88% occurred at a low water level of 12.50%. Bluetooth communication testing achieved a PDR of 100% with 0% packet loss up to a distance of 11 m, both under Line-of-Sight (LOS) conditions and with one wall as an obstruction. These results indicate that the proposed system provides accurate water-level measurement and reliable wireless data transmission, making it suitable for real-time water-tank monitoring applications.]]>
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