Downwind drift distance for various spray droplets released from different boom heights.
Downwind drift distance for various spray droplets released from different boom heights.

The height of the spray boom during an application plays a critical role in the two major goals of any pesticide application: making an effective application that controls the targeted pest while at the same time mitigating the risk of off-target drift.

Boom height impacts the efficacy of the application by affecting the uniformity of the spray along the length of the boom. The effect on drift is related to exposure of the spray droplets to the wind. We will look at each of these factors separately in greater detail.

Uniformity

Many broadcast applications are made with some type of flat-fan nozzle, such as extended range, pre-orifice, or air induction. These nozzles create a fan-shaped spray pattern that has the heaviest concentration of spray in the center of the pattern, with the amount of spray tapering off to nothing at the edges. Figure 1 shows a spray pattern from an air induction type of flat-fan nozzle captured by a spray table – note the shape of the pattern.

A boom has flat-fan nozzles evenly spaced along its length to make a broadcast application. In order to make that application uniform, the edges of the spray patterns, where the amount of spray is less than in the center, must be overlapped so that they combine to make the total amount of spray applied uniform along the whole length of the boom. This combining of spray patterns is called overlap.

There are three factors that affect overlap. They are nozzle fan angle, nozzle spacing, and boom height. The fan angle of the nozzle determines the total width of the spray pattern, with wider fans creating wider spray patterns. 80-degree (figure 2) and 110-degree (figure 3) fan angles are the most commonly used flat-fan angles in agricultural applications, but there are wider and narrower fan angles available.

The wider the fan angle and spray pattern, the more overlap is created by the nozzle itself. Nozzle spacing is the distance between each nozzle on the boom. The closer the nozzles are spaced together the more the patterns overlap; moving nozzles farther apart reduces overlap.

The final component that determines overlap is the boom height. If the boom is raised, overlap increases because each pattern has more room to spread out. Lowering the boom reduces overlap. While both fan angle and nozzle spacing also determine overlap, it is typically boom height that is used to set and adjust overlap on a sprayer. This is because once nozzles have been selected and placed on the boom, using fan angle or nozzle spacing to adjust overlap is impractical.

If you determine you need to increase overlap, you could change all of your nozzles to a wider fan angle, decrease the nozzle spacing and add more nozzles to the boom, or increase boom height. Increasing the boom height is obviously the best choice.

Most flat-fan nozzles require about 50 percent pattern overlap to make a uniform application. As shown in figure 4, 50 percent overlap means that the length of area that is being overlapped; i.e. receiving spray from two nozzles, should be 50 percent of the nozzle spacing. This means for nozzles spaced 20 inches apart the length of the area of overlap should be 10 inches.

For applicators, the key to achieving the correct overlap is knowing the ideal boom height for the nozzle fan angle and nozzle spacing their sprayer is set up with. Table 1 gives the suggested minimum boom heights needed to achieve sufficient overlap for different fan angles and nozzle spacings for nozzles from one nozzle manufacturer.

Table 1. Minimum boom heights for different nozzle fan angles and spacings.

Fan angle

20 inch spacing

30 inch spacing

40 inch spacing

65 degrees

22-24 inches

33-35 inches

Not recommended

80 degrees

17-19 inches

26-28 inches

Not recommended

110 degrees

16-18 inches

20-22 inches

Not recommended

 

Table 1 highlights how nozzle fan angle, spacing, and boom height interact. For any given fan angle, using a wider nozzle spacing means the boom must be higher in order to achieve sufficient overlap. Using a wider fan angle or a narrower nozzle spacing both allow boom height to be lowered.

Drift

The second major way boom height affects an application is by its impact on drift mitigation. As boom height is increased, the distance the individual spray droplets must travel before they reach the target increases. This increase in distance increases the time in which they are exposed to the wind, allowing them to be blown longer distances downwind.

The impact of boom height on drift can be seen in figure 5, which summarizes the results of measurements taken in a wind tunnel where all environmental factors were controlled. The researchers, from the USDA-ARS facility in Wooster Ohio and the Ohio State University, created droplets of various sizes, released them from different heights, and measured how far downwind the droplets moved before they deposited.

Five different droplet sizes were tested. Droplets are measured by their diameter in microns; 1 micron is equal to one millionth of a meter. As a reference, the human hair is about 100 microns in diameter. Starting with the largest droplet tested, the 300-micron droplet has no downwind movement when the boom is between 0.5 and 1.5 feet.

At heights greater than 1.5 feet, there is very slight downwind movement. The 200-micron droplet behaves very similarly except it does move downwind slightly at a release height of 1.5 feet, and has a greater increase in drift distance as boom height increases. The 150 micron shows a similar trend but with greater drift distances.

The 100-micron droplet has a very sharp increase in downwind drift distance as boom height increases, and highlights the importance of keeping the boom as low as possible. While there are many nozzle types and adjuvants available that can dramatically reduce the percentage of spray volume being released in droplets less than 100 microns, there are still some droplets in that size class, and a low boom height can help reduce the likelihood these droplets can move off target.

The 50-micron droplet shows a drift pattern that at first appears to be perplexing and not what would be predicted. It shows a substantial increase in drift distance as the boom is raised from 0.5 to 1 foot, but at higher boom heights its drift distance remains fairly constant. The prediction would be that it would continue to have further increases in drift distance as the boom is raised, so why does this not occur? The answer lies in the solution used for the tests: water. With no nonvolatile component in the spray solution, the 50-micron droplets evaporate very quickly. After 0.5 feet in boom height, the 50-micron droplet cannot survive long enough to deposit. The final distance given is how far it traveled before it evaporated completely.

Summary

The ideal boom height is one that provides the correct amount of overlap for the nozzle fan angle and spacing used while at the same time mitigating the risk of drift as much as possible. Check with your nozzle manufacturer to find the ideal boom height needed to achieve the correct amount of overlap and then strive to maintain this boom height throughout the application. Doing so will help to ensure an accurate, uniform application, which mitigates the risk of off-target drift. A final thought is that using a wider fan angle and a narrower nozzle spacing allows you to lower the boom height.