Study On Soil Moisture Sensor Probe Made Of Graphite Cementitious Composite Material Used In Solar Automatic Irrigation Controller

China is a country which is facing severe water scarcity. However, due to thetraditional irrigation methods which based on manual control, the efficiency ofirrigation is much lower than developed countries, resulting in a serious waste of waterresources. The development of automated water-saving irrigation technology whichbased on the detection of soil moisture situations, is very important for the protection ofwater resources in China.Soil moisture sensor probe which can measure soil moisture accurately and inreal-timely is the basic and key part of the automated water-saving irrigation equipment,while metal probes have a serious problem of corrosion and passivation as working inthe soil which is a severe environment of the soil of high humidity and complexchemical compositions. To solve this problem, we developed a new type ofanti-corrosion and anti-passivated graphite cementitious composite probe. Probes madeof the composite material have a micro-dissolution erosion effect when barried in soil.This effect can constantly renew the probe surface, which can guarantee itsanti-passivated anti-corrosion effect.In this research, we studied the conductive properties of soil moisture sensorprobe made of this new type of graphite cementitious composite material which used insolar automatic irrigation controller. The probes detect soil moisture by a electricalresistance method. The resistance values of probes were negatively correlated with soilmoisture. DC two-probe method was used for the determination of the different graphitecontent graphite cementitious composite in different moisture conditions andcompositions. By designing different conductive inner core structure of probes and theshape of probes, we comparaed and analyzeed the conductive properties of differentstructures of the soil probe under different humidity conditions.(1) The effect on electrical conductivity of matching ratio of graphite and cement.The resistivity of the material decreases with increasing graphite content, but when graphite content increaed to25%, the resistivity reduce slowed down. The resistivity ofthe composite material increaes with the increases of load voltage. This effect is moreobvious when the graphite content is smaller. When the graphite content deceased to5%,the effect reached the extremum. The resistivity of the composite material increaseswith its own humidity decreases, this effect significantly reduced to a very low levelwhen the graphite content increaes to about25%.(2) Different graphite cementitious composite materials made of differentmatching ratio are suitable different kinds of soil mositure probes. Materials which thegraphite content is higher than25%have extremely low resistivity and the resistivitychanges little, makes them suitable for the production of two-pins probes. Materialswhich the graphite content is less than15%have a higher resistivity and as themicroporous structure of the cement is easy to absorb soil moisture, the resistivity ofmaterials changes significantly with the changes in soil moisture, makes them suitablefor the production of one-pin probes.(3) The inner core structure of the moisture probe will affect the resistivity valuresof the probe. As the sensor body matains the same graphite content, the resistivityvalures of the probe decrease with the decrease of the contact area size between sensorbody and its inner core. The more complex of the core structure the lower the sensitivityof the probe. According to our experimental data, single strand copper core is better thanthe linear core, double-stranded twisted copper core and rectangular cross-sectioncopper core, etc.(4) The relationship between the resistance value feedbacked by the new probeand the contact area size between the sensor and soil. When the contact area is small,the measured resistance value decreases rapidly with increasing of the contact area.(5) Good probe structure will reduce the error brought by the shrinking-swellingphenomenon when soil moisture changes.The conical probe has the best effect ofreducing errors..