Research Of Measurement Technique On Soil Water Content Using A High Frequency Capacitance Based On An Additional Resistance

With the rapid economical development of industry and agriculture, the water resource crisis has become much more serious problem. Furthermore, it has affected the quality of life for most citizens in the world and has become the important restraining factor in social evolution and economic development. According to the statistics, in 2007, the national total water consumption is 518.9 billion m3, out of which, the agricultural water use is 360.2 billion m3, accounting for 61.9%; the industrial water use is 140.2 billion m3, accounting for 24.1%; the domestic water use is 71.0 billion m3, accounting for 12.2%, and ecology and the environment complement water is 10.5 billion m3, accounting for 1.8%.Water conservation, especially agricultural water saving irrigation has practical significance for further promoting sustainable utilization of water resources and China sustainable economic and social development. Agricultural water saving irrigation need to measure rapidly the water content, therefore, soil water status measurement is of great long-term and strategic significance.The current methods to measure soil water content can be categorized as follows: Karl Fischer method, weight method, tensiometry method, neutron radiation method, time domain reflection(TDR) method, frequency domain reflection(FDR) method, infrared remote sensing method, gypsum method, capacitance method, resistance method, standing wave ratio(SWR) method, and so on. Some of the methods are only implemented in laboratory and some others have good performance, but the cost is high. Whereas it is apparent that the capacitance method has many advantages, such as low cost, easy fabrication and wide dynamic response range. Therefore the method of measurement soil water content based on a capacitance sensor is feasible to be popularized in the vast rural areas in China.Capacitance soil water sensor is implemented to measure soil water content, which is depended mainly on the great differences of the dielectric constant in a soil sample. Therefore, one capacitance sensor based on the soil dielectric properties for measuring soil water content can be constructed. The capacitance changes of the sensor resulted from the soil water content changes. When the capacitance of sensor is confirmed, the soil water content will be determined accordingly. According to the soil dielectric properties, besides the capacitance characteristics, soil capacitance sensor has some certain dielectric loss, equivalent to resistance loss. In order to achieve the capacitance value of soil water content sensor, the usual method is to reduce the impact of resistance loss, but in fact it is not possible to eliminate fundamentally the inherent loss. So the author proposes a method to obtain the capacitance value using a high-frequency capacitance soil water sensor based on an additional resistance, the soil water content can be solved precisely by mathematical calculation.In this paper, by analyzing the soil dielectric properties, a novel method is discussed to solve the capacitance value of soil sensor under a high frequency signal by the introduction of an additional resistance, and soil water content will be achieved rapidly. According to the above principle, a parallel-plate capacitance sensor is fabricated to measure soil water content and a signal acquisition and processing system controlled by a single-chip using a high frequency capacitance based on an additional resistance is designed and developed. The system is easy to operate and fast to measure, the measurement values are shown on the screen directly and the results are accurate. At the same time, the system has an extend function of PC to process the data, applying LabVIEW powerful data processing capabilities, PC can achieve real-time data processing and display the measurement result. In order to verify the feasibility, the actual experiment to measure the soil water sample is carried out, the result shows that the capacitance value out of the soil water content capacitance sensor achieved by using this method is accurate, the measurement error is small; soil water content is nearly linear with the capacitance of soil water capacitance sensor.