Evaluating capacitive wetness sensors for measuring deposition in electrostatically charged spraying operations

Abstract

Standard methods for measuring pesticide deposition include water sensitive paper and fibre (paper or cord) collectors, which require time consuming manual steps to gather, process and interpret results. Measurement of pesticide deposition is important in evaluating the efficacy of a spray, which affects the total crop yield. A lack of knowledge about spray quality often results in the need to spray to excess, which can contaminate the soil and damage neighbouring ecosystems and the atmosphere. The suitability of PHYTOS 31 capacitive leaf wetness sensors (Meter Group, Pullman, WA, USA) for spray sensing applications was analysed. The best fit calibration function for droplets of ~ 1 μL volume was linear (R2 = 0.985). The best fit calibration function for drops > 10 μL was a weighted sum of a linear and power function (R2 = 0.984). Thus, the calibration function should be chosen according to the anticipated size of droplet. The PHYTOS 31 sensors and water sensitive papers were used in a field test, comparing electrostatically charged and uncharged sprays. Both measurement techniques show that charged spray is more effective in covering parts of the plants which do not have a direct line of sight to the nozzle. The techniques disagree on quantitative coverage measurement, with the water sensitive papers reporting areal coverage two to four times higher than the capacitive sensors. High-speed video observations show that this is due to contraction (de-wetting) of the drop on the capacitive sensors. The surface of the capacitive sensors is more hydrophobic than the papers. The hydrophobicity of the capacitive sensors appears to be a better match to plant leaves than that of the papers. Capacitive sensors are a suitable measurement tool for evaluating liquid coverage in the field, but further calibration is necessary to be confident in the quantitative data.