Humidity resistance test system for spray droplet density and size based on NO2- color reaction

Abstract: Agriculture sprayer is widely used in plant protecting. There are many factors affecting the effective utilization of pesticide, such as canopy structure, meteorological condition, machine performance. The quality of spray will also affect the effective utilization of pesticide to some extent. Spray droplet density and spray droplet size are two important parameters which will affect the quality of spray. There are many methods of detecting spray droplet density and spray droplet size, and water sensitive paper is the most frequently-used one for its simpleness, convenience and sensitiveness. However, water sensitive paper has its own disadvantage that it can't be used in wet environment. Hence, a detection system is invented to test spray droplet density and spray droplet size in both dry and wet environments. The detection system consists of the test card and tracer liquid. The test card is composed of starch, calcium sulfate, sulfanilamide, tartaric acid, N-(1-naphthyl) ethylenediamine dihydrochloride and deionized water. These materials become a film on the quantitative filter paper. The surface of test card is white and smooth when it is unused. Tracer liquid contains NO2-, which can react with test cards. When tracer liquid is used to spray the test card, aubergine spots will be generated on it. The information of spray droplet will be recorded on the test card. A series of solutions with different NaNO2 concentrations (0, 7.8, 15.6, 31.3, 62.5, 125, 250 mg/L) are prepared to choose a better concentration. The higher the concentration is, the deeper spots' color shows. The China national standard of food additives (GB2760-2011) indicates that the largest amount of NaNO2 is 150 mg/kg. Considering the food safety and detection sensitivity, the concentration should be close to 150 mg/L but not exceed this value. In the research, 125 mg/L mass concentration of NaNO2 is chosen. Before detecting the spray droplet size, a D2-D1 curve should be drawn, and D2 is the actual diameter of droplet and D1 is the diameter of spot. The microsyringe is used to control the volume; the sphere formula is used to calculate the actual diameter, and the diameter of spot is measured using the micrometer. The droplet volumes range from 0.1 to 2.5 μL. The same volume should be added 3 times on one test card to get average value. The correlation coefficient is 0.995, while the relative standard deviation is from 0.95% to 5.37% which shows a good homogeneity of the test card. Then the same method is used to evaluate the repeatability among different test cards. The regression equation of 3 pieces of random test cards has a determination coefficient of 0.992. The relative standard deviation is from 1.63% to 12.66% which also shows a good repeatability among different test cards. Three pieces of test cards from another batch are chosen randomly, which are added with tracer liquids of 0.6, 1.2 and 1.8 μL. Measure the diameter of spots and meanwhile plug the average value into regression equation to get calculated diameter. They are uniform when actual diameter and calculated one are compared. It indicates that the regression equation has a good universality to calculate the spray droplet size in the test system. The detection method of spray droplet density is as same as water sensitive paper. The veracity of detecting spray droplet density of water sensitive paper is compared with the test system made in this research in 3 different pressures( 0.03, 0.08, 0.13 MPa). It shows a good uniformity of water sensitive paper and the test system. The humidity resistance of the test card is compared with commercial water sensitive paper in the environment with 100% relative humidity for 3 hours. The result shows that commercial water sensitive paper' color turns into blue, while the test card doesn't change. It indicates that the detection system in this paper has superior performance in both dry and wet environments.