Moisture sensor based on standing wave ratio for agriculture industry

In this paper, we have developed a prototype of moisture sensor based on a measurement of standing wave ratio (SWR) in the transmission line with four types of probes: 4-rod, 3-rod, cylindrical, and embedded-coplanar-strip probes. The sensors can measure moisture in agricultural products whose conductance is unstable by applying SWR technique using high frequencies. The highest operating temperature for our developed sensor is 85°C which is higher than those of 65-75°C for other commercial sensors. The targeted applications are measurements of water content of moist soil in plantation and of organic fertilizer pellets in a silo. After having conducted simulations and experiments, we found that 3-rod and 4-rod probes are preferable for use in agriculture farms to the other two types of the sensors because they are more accurate and durable when inserted into the ground. On the other hand, embedded-coplanar-strip probe was more suitable for measurements in silos because the flat shape of the probe allows the organic fertilizer pellets to pass through easily without sticking onto the probe. The best measurement accuracy with ±9.3% deviation can be obtained by the embedded-coplanar-strip sensor when used with organic fertilizer pellets, the best accuracy with ±3.2% deviation by the 3-rod sensor, the best accuracy with ±3.8% deviation by the 4-rod sensor, and the best accuracy with ±6.1% deviation by the cylindrical sensor all when used with porous media such as moderate saline loam soil.