Development of frost detecting sensor using time domain reflectometry

Crops are subject to frost damage every year. A method for detecting frost occurrence immediately would allow for earlier and more effective management strategies to be employed and ultimately reduce crop damage. We have developed a frost detecting sensor based on time domain reflectometry (TDR) that determines relative permittivity by measuring the propagation time of electromagnetic waves. Frost that formed on a sensor could be detected using the difference between the relative permittivity of water, 80, and ice, 3.5. Two types of frost sensor, A and B, were developed. Sensor A was made with a glass-epoxy print-circuit board by etching two zigzag electrodes. Sensor B was made with a glass-composite print circuit board by etching two spiral electrodes. Sensor A was placed on Peltier devices to cool to approximately -3℃, artificially inducing frost. This experiment was conducted in an acrylic box, 30 × 45 × 30 cm, filled with air at 100% relative humidity. Relative permittivity and temperature of the sensor were measured at 10 s intervals. Sensor B was also placed on the Peltier devices to cool to approximately -3℃, also artificially inducing the occurrence of frost. The experiment with sensor B was conducted in a constant temperature and humidity incubator. The relative humidity varied to 60, 70, and 80 %. Relative permittivity and temperature of sensor B were measured with 10 s intervals. The relative permittivity changed when moisture condensed on both sensors and when condensed moisture turned to ice under the conditions of 60, 70, 80, and 100 % R.H. The differences of relative permittivity before and after frost formation were between 0.042 and 0.14. The TDR sensors distinguished frost from dew. As R.H. increased, the amount of frost or dew formed, measured as the relative permittivity, increased as well. We need to verify the shape of the sensor and improve the thickness of the printed circuit board in order to increase the sensitivity.