Investigating Heater Resistance Tolerance of the Heat-Pulse Sensor for Accurate Soil Moisture Measurements on Vadose Zone Soil

Integrated precise irrigation management is one of the efficient methods to improve the crop productivity and also helps in water conservation where soil moisture sensors are widely used. Out of available soil-moisture sensors, the dual-probe heat-pulse (DPHP) sensor is the potential candidate due to its optimum price and better accuracy. A DPHP sensor has the heater probe kept at a distance from the temperature sensor probe. Heater probe consist of nichrome wire as the heating element, which is embedded in the stainless-steel tube. In this paper, we examine the acceptable tolerance in the heater resistance across 25 DPHP sensors, for the accurate soil moisture measurement. For this purpose, we used in-house developed 25 DPHP sensors where heater resistance is about 56 $\Omega$ with $\pm$ 5% tolerance. Under laboratory condition, we observed that difference in the measured volumetric water content (VWC) is within $\pm$ 3 % (VWC) when compared with standard gravimetric method. For the field measurements, we developed the automated-self sustained system and deployed 3 systems on the rooftop of the building. Under field conditions, we observed that difference in the measured VWC from the 3 systems is within $\pm$ 3 % (VWC) when benchmarked with standard gravimetric method.