Detecting Frost and Dew Formations Using Time Domain Reflectometry.

Early- and late-frosts often cause large damages on crop production.  To date there’s no instrument to directly detect a frost but to predict a frost based on air temperature forecast.  Time domain reflectometry, TDR, has long been used to measure soil water content based on changes in dielectric constant of bulk soil because each soil constituent has its unique value of dielectric constant, e.g., 3-10 for soil particles, 3 for ice, 80 for water, and 1 for air.  Since frost consists of ice particles and air, and dew does water droplets and air, we thought that TDR could directly detect frost and dew formations.  A pair of 55cm-long circular electrodes etched out on a print circuit board, PCB, was used as a detector.  The end of the electrodes was connected to a 2m-long 50ohm-coaxial cable and to a TDR instrument, TDR100 of Campbell Scientific or 1502C of Tektronix.  Experiments were conducted in the laboratory and in the field.  The PCB detector was placed on Peltie devices that made freezing temperature.  When dew formed on the PCB detector, dielectric constant increased.  After temperature on the PCB detector continued to decrease, the dew changed into frost, and dielectric constant decreased.  In the field experiments, there was no dew formed before a frost formation, but frost directly formed on the PCB detector.