Spray drift, movement of a pesticide through air, during or after application, to a site other than the intended site of application, is considered to be the most challenging problem facing applicators as well as manufacturers of pesticides. After wind velocity and direction, spray droplet size is the most important factor affecting drift. Good coverage is essential for insecticides and fungicides because of the small size of the target organism. Therefore, small-to-medium size droplets are desirable when applying insecticides and fungicides because they provide better penetration into the canopy and better coverage. However, small droplets can drift long distances because of their light weight.
According to a study we conducted in Ohio (Zhu et al., 1994), drift is far less likely to be a problem when spraying with droplets of 200 microns and larger in size. The same study indicates that spray particles under 50 microns in diameter remain suspended in the air indefinitely or until they evaporate. They should be avoided because there is no way to control deposition of these small droplets. (For reference, there are 25,000 microns in one inch, and the average thickness of human hair is 75 to 100 microns). Nozzles which produce relatively high number of small droplets should not be used unless there are no other alternatives.
To combat spray drift, almost all major agricultural nozzle manufacturers have recently introduced new nozzles that are supposed to produce fewer drift-prone droplets than the conventional nozzles. Detailed information on the design and working principles of these new so called "low-drift" nozzles are given in OSU Extension fact sheet, AEX-523. Although manufacturers of low-drift nozzles claim these nozzles to be considerably more effective in reducing spray drift than the standard flat-fan nozzles, no independent data had been available to support their claim. Recently we conducted tests in Ohio to determine effectiveness of two of the low-drift nozzles (Turbo TeeJet and TurboDrop) in reducing drift. Cross sections of these two nozzles are shown in Figures 1 and 2. A more comprehensive study comparing droplet sizes from various low-drift and conventional nozzles was conducted at the University of Tennessee by Womac and Cash (1997). In this publication, only the most significant results from the Ohio study are presented. Detailed information on the Ohio study can be found in the paper by Derksen et al. (1997).
|Figure 1. Turbo TeeJet Nozzle
(Courtesy Spraying Systems Co., Wheaton, IL)
|Figure 2. The TurboDrop Nozzle.
(Courtesy Greenleaf Technologies, Covington, LA)
To determine effectiveness of low drift nozzles, we measured droplet sizes, and deposition distances of droplets in a wind tunnel using the low-drift nozzles and compared the data from these measurements with those obtained from a standard Extended Range (XR) flat-fan nozzle. Test results revealed that the low-drift nozzles produced fewer drift- prone droplets and lower downwind deposits than the standard XR flat-fan nozzle. The TD nozzles produced lower downwind deposits than TT nozzles operated at similar pressures (40 psi); however, a larger orifice TT nozzle operated at a lower pressure (25 psi) performed similar to the TD nozzle operated at 40 psi.
Description of Tests Conducted
Nozzles used in this study and the test conditions are shown in Table 1. All nozzles tested are designed to produce 110 degree spray pattern. Nozzles were subjected to two sets of measurements: droplet size measurements using a laser particle analyzer, and deposition characteristics (distance, quantity) of droplets using a wind tunnel (these procedures are explained below in more detail). Almost all potential users of TurboDrop nozzles have been interested in finding out what happens to the droplet size and drift distances of droplets when the small air induction hole on these nozzles is plugged. To investigate this, in addition to testing TD nozzles under normal recommended operating conditions, additional tests were conducted using both TD02 and TD04 nozzles with the air induction hole covered with tape completely.
|Nozzle||Nominal Flow Rate (gpm) (at 40 psi)||Test Pressure (psi)|
|Standard Flat-Fan (XR)|
|Turbo TeeJet (TT)|
|TD02 (air intake covered)||0.2||40; 60|
a) Droplet Size Measurements
|Figure 3. Diagramatic representation of the Volume Median Diameter (VMD) - (Half of the volume of spray contains droplets larger than the VMD while the other half has smaller droplets). Adapted from Mathew's, 1992.|
Droplet sizes from nozzles at different pressures were measured using a laser droplet sizer. The data obtained from this system included Volume Median Diameter (VMD), and the percent spray volume contained in droplets smaller than 50, 100 and 150 microns. VMD is the most widely used parameter of droplet size. It is defined as the size of droplet that divides the spray volume into two equal parts by volume. In other words, as shown in Figure 3, a representative sample of droplets of a spray is divided into two equal parts by volume so that one half of the volume contains droplets smaller than VMD, and the other half of the volume contains droplets larger than VMD. Typically, a nozzle with a high VMD presents a low risk of drift. However, this may not always be the case. A nozzle with a high VMD may produce more drift-prone droplets than another nozzle with a smaller VMD. When classifying nozzles based on their drift potential, knowing the percent of spray volume contained in small, drift-prone droplets (usually the ones smaller than 150 microns) is as important as knowing their VMDs.
b) Wind Tunnel Measurements
Drift experiments were conducted with single nozzles operated in a wind tunnel operating at 11 miles/hour. The tunnel is constructed of plywood and contains several clear plexiglass windows with a working section of 2 ft wide, 3 ft high, and 12 ft long. The blower is located approximately 15 ft downstream from the end of the working section. The nozzles were mounted with the spray pattern perpendicular to the air flow through the tunnel. Nozzles were operated for 10 seconds.
Collectors were placed both on the wind tunnel floor and at the exit of the wind tunnel to measure drift distances of droplets on the ground, and airborne droplets leaving the wind tunnel. A screen with approximately 50% opening was used as a target to capture airborne droplets. A fluorescent tracer (Rhodamine WT) was mixed with distilled water (3.8 mg/gal) in a spray tank and used to determine airborne drift deposits. The amount of fluorescent tracer on each target was determined from the fluorescence values measured with a fluorometer.
The following general conclusions can be stated based on the limited number of tests we conducted. It is important to note that the data presented in this publication applies only to the specific sizes of these nozzles tested and the actual test conditions. Using the same type of a nozzle at a different pressure setting may provide results that may contradict the general conclusions from this study listed below.
- Compared to standard XR flat-fan nozzles, both TurboDrop and Turbo TeeJet low-drift nozzles produced much fewer drift-prone droplets, and drift distances of droplets measured in the wind tunnel were much shorter.
- Droplet size measurements along the long axis of spray patterns showed that Turbo TeeJet and XR nozzles produced droplet sizes much more uniform in size across the spray pattern than TurboDrop nozzles. The variation in droplet size across the pattern was greater with 0.2 gpm nominal flow rate nozzles (at both 40 and 60 psi) than nozzles with a nominal flow rate of 0.4 gpm.
- VMDs from TurboDrop nozzles were nearly twice as high as those from Turbo TeeJet and more than twice as high as those from XR flat-fan nozzles (with all tests conducted using 0.2 and 0.4 gpm nozzles). However, when a TT05 nozzle was operated at a lower pressure (25 psi) to deliver 0.4 gpm output, its VMD was nearly equal to that from a TD02 nozzle operating at 60 psi, and slightly less than the VMD from a TD04 nozzle operating at 40 psi (see Figure 4).
Figure 4. Volume Median Diameters (VMD) of nozzles obtained using the Laser Droplet Size Analyzer with one continuous scanning of spray patterns (tap water).
- Depending on the operating pressure and nominal flow rate, VMDs of droplets from TT nozzles tested were 16 to 54% greater than those from XR nozzles.
- Compared to all the other nozzles tested, with the exception of TT05, TurboDrop nozzles produced fewer drift-prone droplets. The spray volume contained in droplets smaller than 100 microns was less than 5% for all TD nozzles. TT05 nozzle at 25 psi produced approximately 4% of the spray volume in droplets smaller than 100 microns (see Figure 5).
Figure 5. Percent spray volume contained in droplets smaller than 100 micron.
- Covering the air intake hole of TD nozzles at 40 psi reduced their VMD 7 to 8%, and increased the spray volume contained in droplets smaller than 100 microns by 19 to 25%.
- Based on visual observations, TD nozzles, when operated at 40 psi pressure (the lowest recommended for these nozzles), the spray angle seemed to be narrower (close to 90 degrees) than the angle expected (110 degrees) from the tips used with these nozzles.
- The TD and TT nozzles produced fewer downwind ground and airborne deposits than the standard flat-fan nozzle (XR) when operated at similar flow rates and pressures (see Figures 6 and 7).
Figure 6. Airborne spray deposits 7 ft from nozzles with nominal flow rates of 0.2 gal/min. Figure 7. Airborne spray deposits 7 ft from nozzles with nominal flow rates of 0.4 gal/min.
- The low-drift TD nozzles produced fewer downwind deposits than TT nozzles when operated at similar flow rates and pressures.
- Tests with the low-drift TT05 nozzle showed that even with low-drift nozzles, drift can be reduced by using larger orifice nozzles operated at lower pressure.
- Obstructions covering the air intake hole of the TD nozzle can change its operating characteristics but does not appear to significantly affect downwind spray drift.
In this publication, we summarized only the major findings from our study. Detailed information on droplet size and drift deposits in the wind tunnel from all the nozzles tested can be found in the paper by Derksen et al. (1997).
Derksen, R.C., H.E. Ozkan, R.D. Fox and R.D. Brazee. 1997. Effectiveness of TurboDrop and Turbo TeeJet nozzles in drift reduction. ASAE Paper No. 971070, ASAE, 2950 Niles Road, St. Joseph, MI 49085.
Mathews, G.A. 1992. Pesticide application methods. John Wiley and Sons, Inc., 605 Third Ave., New York, NY 10158.
Zhu, H., D.L. Reichard, R.D. Fox, R.D. Brazee and H.E. Ozkan. 1994. Simulation of drift of discrete sizes of water droplets from field sprayers. Transactions of the ASAE, Vol.37, No.5, pages 1401-1407.
NOTE: Disclaimer—This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.