An automated, self sustained soil moisture measurement system using low power dual probe heat pulse (DPHP) Sensor

This paper presents an automated, self sustained, low power in-situ soil moisture measurement system for dual probe heat pulse (DPHP) sensors. The deployed DPHP sensor has two probes: a heater probe and a temperature probe spaced 3 mm apart. Change in the temperature, due to heating, is a function of the soil moisture content. DPHP sensor is integrated with a low power instrumentation board (5mW) for signal amplification, which stores the data in a micro SD card. System automatically turns on the DPHP sensor as well as the instrumentation board at every 1 hour interval for 3 minutes heating cycle, 3 minutes cooling cycle and 54 minutes sleep cycle. Average power consumption during heating cycle is 170mW, one of the lowest powers reported till date. Li-ion battery rated at 3.7V, 1150 mAh capacity is used to power the entire system. The battery is charged by a solar panel, fixed on the top of the system and rated at 6V and 450mA. A PYTHON based GUI is developed for data analysis and presentation utility. Since moisture measurement depends on the structure, mineral and porosity of the soil the system is equipped with a data base in the form of look up tables (LUTs), already produced for various common types of agricultural soil. These LUTs are used for mapping the measured values of change in the temperature from the proposed system to the corresponding soil moisture content. The developed system is able to measure soil moisture content ranging from 1% to 40%. This paper also presents the charging and discharging profile of the battery during in-situ field testing, automatically generated by the system.