By John Garr, Alan Corr, and Jerry Green of GarrCo Products
Operation S.A.F.E. clinics are an excellent way for ag aviators to access spray application experts and learn how to set up their aircraft for uniform and effective applications. If you have a new airplane or have not had your planes tested recently, it is a good idea to attend a clinic soon. Before going, we recommend you watch the National Agricultural Aviation Research & Education Foundation (NAAREF) preparation video (http://www.agaviation.org/operationsafe) and fill out an online form before going. This article is our attempt to make fly-in reports and their terms like CV, VMD, spread factor, histogram, and droplet size easier to understand.
Each liquid test or series consists of three passes over the flight line. Two reports are generated at an Operation S.A.F.E. clinic: one describes your spray pattern, as measured by the string, and the other describes your droplet size, as measured by the water sensitive cards.
Spray pattern report
The first thing to look at in the pattern analysis report is the aircraft speed and elevation, usually determined for all three passes. These measurements should confirm that you are flying the way you intend.
The target rate is the gallons per acre (GPA) spray volume that the applicator is wanting to apply. Computer software calculates the GPA based on nozzle flow rate, number of nozzles, ground speed, boom pressure, and swath width.
A fluorometer analyzes the amount of dye deposited across a string to determine the uniformity of the spray pattern. The graphs show the pattern for each pass as if you are looking at the rear of the aircraft, so the left side of the graph is the left side of the aircraft. There is also a graph that shows the average pattern of all three passes.
You should start by examining the average pattern. Make sure it looks uniform with no holes or spikes that would cause a non-uniform application. If you see a problem spot, check the individual patterns to see if it occurs in all three passes. A consistent pattern problem is likely an issue with aircraft setup. You will need to examine the aircraft and adjust nozzle placement configuration to correct the issue. If you only see a pattern problem in a single pass, then it is more likely the issue could have been caused by the wind or how the aircraft was flown.
Two graphs show how uniform deposition is across the field using the averaged spray pattern and a selected swath width. One graph shows the spray deposition and CV (coefficient of variation) for a racetrack application. The other graph shows the spray deposition and CV for a back and forth application. The CV is a statistical measure that indicates the relative variation in the volume across the pattern. The fly-in software calculates the CV based on the average pattern and specified swath width. The lower the CV, the more uniform the deposition is. A CV of 0 would indicate a perfectly uniform application. The CV should be less than 20 percent.
If your CV is high, there are two likely causes. The first is that the effective swath width is not appropriate. You may be trying to make a too wide or too narrow swath. The S.A.F.E. report shows what happens to your CV as you widen or narrow up your swath width. The swath table contains calculated GPA rates, and racetrack and back-and-forth CVs for a range of swaths, which allow the pilot to determine how increasing or decreasing swath width will impact the uniformity of deposition. The second reason for a high CV is large spikes or gaps in your spray pattern. Determine what caused the pattern to be non-uniform, correct it, and then retest the aircraft.
Droplet size report
Water-sensitive cards measure the droplet size. Usually, seven cards are spread parallel to the string in 8-foot increments on the last pass. A negative card number on the report indicates the position to the left of the center of the aircraft, and a positive number indicates the position to the right. A computer program calculates the spray droplet diameter based on the stain diameter on the cards.
Key definitions of terms in the report are:
VMD (Volume Median Diameter): Half of the spray volume contains droplets this size and larger expressed in microns, and the other half of the spray volume contains smaller droplets. A micron is one-millionth of a meter. For reference, a human hair is roughly 100 microns in diameter. The VMD number is the midpoint of the spray droplet size distribution. If the pesticide label calls for a 350-micron droplet, then your VMD should be close to 350 microns, but two other measurements, VD(0.1) and VD(0.9), can be more important in connection with the drift potential and efficacy of your application.
VD(0.1): Ten percent of spray volume contains droplets less than or equal to this diameter expressed in microns, ninety percent are equal or larger. For instance, if your VD(0.1) is equal to 150 microns, this means 10 percent of the total volume of spray from your nozzle came out in droplets 150 microns in diameter or smaller. The higher the VD(0.1), the fewer number of fine droplets and the less risk of drift.
VD(0.9): Ninety percent of the spray volume contains droplets less than or equal to this diameter expressed in microns, ten percent are equal or larger. For example, if your VD(0.9) was 600, then 90 percent of the spray volume were droplets 600 microns and smaller, and 10 percent was larger. The larger the VD(0.9), the less potential for drift. Bigger droplets will penetrate into thick canopies better than smaller droplets. Proper adjuvant selection will improve deposition and help ensure droplets will not bounce off and retained on the leaves. Knowing the pesticide mode-of-action helps determine the appropriate droplet spectrum.
Spread factor is used to determine the actual droplet size created by the nozzle and not the size of the stain on the water-sensitive cards. When water spray droplets land on the water-sensitive cards, they usually spread. In addition, many tank-mixed ingredients such as surfactants, oils and pesticide products can cause spray droplets to spread even more. In a commercial application, oil adjuvants, including oil drift adjuvants, can cause droplets to spread on spray card and make it appear the spray droplets were larger than what they actually were in the air if the spread factor is not taken into account.
The Deposition versus Distance graph indicates the GPA deposition across the spray pattern. The Coverage versus Distance graph indicates the percentage of card surface covered by droplets across the spray pattern. These graphs provide similar information as the string analysis – a visual representation of the spray pattern. Preference should be given to the string analysis in measuring pattern uniformity because the string measures the amount of spray multiple times for every inch of string while the water sensitive cards only measure the amount of spray every 8 feet.
The % Vol. Histogram of Droplet Diameters shows the spray volume contained in various droplet size ranges. The # Drops Histogram is the corresponding graph that shows the number of droplets in various ranges. The report contains information for each card that verifies uniform droplet size across the spray pattern. The first step in analyzing the droplet size report is to make sure that the VMD, VD(0.1), and VD(0.09) are appropriate for the applications you want to make. The next step is to look at the individual cards. If you have a card that has a droplet size much smaller or larger than average, it could indicate a potential problem with a leak or nozzles not functioning correctly. The droplet size in the center of the aircraft is often smaller than average because of higher speed air associated with the prop.
The goal of these clinics is for you to leave a fly-in with your system properly set up, or with a plan to do so when you get back to the hangar. The spray application parameters do not have absolute number targets at these clinics. Spray optimization is a balance of factors that depends on your application equipment and goals, the targeted pests, crop and the products you are applying. Operation S.A.F.E. analysts will help ensure you understand your tests results and get the most out of your aerial applications.