Study On Anti-passivation Technology For Soil Moisture Probe Of Solar-powered Intelligent Irrigation Controller

Nowadays, with the increasingly shortage of the water resources, promoting water-saving irrigation has became the inevitable choice to ease the water crisis. To change the traditional irrigation methods to promote the modern water-saving agriculture, it has to develop a reliable, stable, soil moisture probe to support the implementation of Water-saving irrigation technology. However, corrosion and passivation are serious problems to soil moisture probe as it must have been working in surface soil. It will lead to the failure of soil humidity detection system, thus cause crops bad because of drought or a waste of water.In this paper, anti-passivation technology of soil moisture probe of solar-powered intelligent water-saving irrigation controller has been studied on. We developed the graphite-cementitious materials probe, and design and manufacture the shape and surface micro-structure of soil moisture probe. Different environments and application conditions affecting conductivity, mechanical, properties, corrosion resistance and anti-passivation of soil moisture probe have been studied by means of compressive strength test, chemical experiment, electrochemical test, DC probe voltage drop method and SEM.Such conclusions can be drawn in our study.(1) As the increase of the contact area between soil moisture probes and soil, the voltage of probes slightly drops and the resistance decreases, but the change of resistance and the contact area is not a linear relationship. The resistance of probes drops rapidly when the contact area increases as the contact area is small, but as the contact area is larger, the resistance of probes drops slowly when the contact area increases.(2) A slight contraction of the soil when it from wet to dry. It makes the contact area between soil moisture probes and soil slightly smaller. This will result in measured value of resistance is larger than actual value. This measurement error can be reduced by reasonable design of shape and surface micro-structure of soil moisture probe. The "W"-shaped probe and the probe of micro tooth structure have excellent performance in reducing this type of measurement error.(3) The reliability and stability of graphite-cementitious materials probe is significantly higher than metal probe, because it can maintain original conductivity in acid, alkali, salt environment. The acid and the salt environment can erode the graphite-cementitious materials probe,but don't affect its application effect. The graphite-cementitious materials probe was most serious eroded when it works in the environment contained high salt, but Alkaline do not almost affect the graphite-cementitious materials probe.(4) When the soil moisture probes ere inserted into the soil, the resistance of the probes is not stabilized in initial stage, but continuously increases. Initial value of the resistance of metal probes is small, but it increases rapidly. In comparison, initial value of the resistance of graphite-cementitious materials probes is bigger, but its increase is obviously slower. The stability of the graphite-cementitious materials probes is better than metal probes.(5) In the same environment at different times, cross sections of the four probes made of copper coated nickel have been observed with SEM. It can be drawn that the longer, the thicker passive film is. Growth rate of passive film and the time is not a linear relationship. The growth rate of passive film decreases as time increase.