Emphasis on Droplet Spectrum

Dennis R Gardisser, WRK of Arkansas LLC

Sometimes size is a major factor! Pesticide applications are an area where size of the droplet is one of the most important criteria for efficacy and environmental stewardship. Plant or surface physical characteristics, chemical mode of action, timing, weather, application platform and drift potential are some of the considerations for optimum droplet spectrum selection.

A lot of emphasis is placed on application volume. Total volume may not be as important as the optimum droplet spectrum. The term spectrum is used because almost all applications have a range of droplet sizes generated. Fungicide tests were conducted at 3, 5, and 10 GPA. Results indicated that the 5 was better than the 3, but the 3 was better than the 10 GPA. The following year this test was replicated, but additional tests were conducted with the droplet spectrums for the 3 and 10 GPA were also set up with the same droplet spectrum as utilized for the 5 GPA. Test data results were the same for the tests using the previous year’s setup. There was no significant difference in the data of the different volumes when the comparison included on the ones with the same droplet spectrum. Several additional studies have been conducted and selecting the optimum droplet spectrum has been key to performance.

There are a huge number of applications at 2 GPA or less. As the application volume goes down, so must the droplet spectrum to optimize coverage. A key point with droplets is that changes in diameter result in a cubed root relationship with the amount of water volume contained. Doubling the diameter results in eight times the water volume. As an analogy, I’m getting ready to take a trip to Africa and I would much rather sit by a person, in coach fare, that weighs 100 pounds than one that weighs 800 pounds! Small changes also matter. A 26% change in diameter doubles the volume in a droplet.

Low volumes, with small droplets, require the utmost care. Potential deposition and control of small droplets are affected by evaporation, thermal uplifting from the surface or canopy, aircraft or helicopter aerodynamics and air movement. Optimum placement along booms is necessary to achieve uniform swath depositions. Any change in air flow on or around an aircraft must be considered such as: belly clutter, fan driven pumps, gear, sumps, wingtip modifications and application height.

Droplet capture efficiencies at Operation SAFE clinics and research deposition studies diminish rapidly as temperatures go up and humidity goes down. Droplets need to be large enough to overcome thermal and wind effects and settle into the plant canopies. Droplets that are too large reduce coverage potential, especially when low total volumes are being utilized.

All applicators are encouraged to put some witness cards out and observe uniformity across the swath and collection efficiency. This should be done on both types of application surfaces, clean and canopy. The USDA/ARS models are a great tool to get some idea of the expected droplet spectrum from a given setup. This data should be compared to what is actually surviving to be deposited from the release of an actual field application.

Suggested droplet spectrum VMDs, based on field trials are as follows: herbicides – 325±25μ, insecticides and fungicides – 275±25μ. Herbicide applications will certainly work at lower micron sizes; but, the 325 μ size is 65% larger – providing a bit less drift potential. There are some nozzle types that have droplet spectrums with VMDs around 200μ. These droplets are much lighter relative to the 275μ. It takes 2.6 of them to make one 275.

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