In an ideal crop-spraying world, nozzles would deliver a spray pattern with maximum coverage and no drift.
Spray nozzle manufacturers have been edging closer to that ideal over the past couple of decades, first with low-drift nozzles and, in more recent years, air-induction or venturi nozzles.
The question is: Can air-induction nozzles deliver better performance than conventional nozzles or earlier-generation low-drift nozzles by minimizing spray-drift risk more than their predecessors while maximizing plant coverage? That's critical for fungicide, insecticide and post-emergence herbicide applications.
Judging by many of the results from studies by university researchers around the country, the answer is yes. Compared with conventional nozzles, air-induction or venturi nozzles improve drift control without compromising plant coverage and spray efficacy under appropriate application volumes, operating pressures and ground speeds.
Kansas State University Extension Agricultural Engineer Robert Wolf, using a computerized spray droplet measuring system called DropletScan, has conducted nozzle studies that have led him to this conclusion.
“In general, the air-induction/venturi nozzles — compared to the conventional nozzles — do provide a higher quality of spray in drift potential.” In all cases, he says, the droplet-size spectrum from air-induction/venturi nozzles was larger, with a smaller portion of the volume in droplets considered highly drift-prone.
UNIVERSITY OF NEBRASKA Extension Cropping Specialist Robert Klein has arrived at a similar conclusion. He used a laser system to analyze spray pattern droplet size distribution. Some of the newer air-induction nozzles allow lower pressures than other low-drift nozzles without compromising spray-drift control, Klein's comparisons show. With the newer air-induction nozzles, he says, “I feel we can use around 30 psi. It's just nicer when you can use a little bit lower pressure.” And, in general, they produce spray patterns with a smaller portion of the spray material in drift-prone droplets, Klein adds.
Most nozzle manufacturers offer low-drift nozzles, says Erdal Ozkan, professor and Extension agricultural engineer at Ohio State University. Spray material in standard flat fan nozzles is carried through a straight bore, from boom to tip. In contrast, spray material in most low-drift nozzles is carried from the boom through a pre-orifice, into a “pocket” machined into the nozzle body, and out of a discharge orifice that's larger than the pre-orifice, he explains. That creates droplets large enough to resist drift but not so large that they concentrate too much of the spray material in too few droplets for adequate plant coverage.
Manufacturers have been building on that nozzle design with air-induction or air-assist nozzles. Air-induction nozzles, like most low-drift nozzles, have a pre-orifice and an exit orifice, according to Ozkan. But they also have an air inlet port that draws air into the nozzle body for blending with the spray solution.
In an air-induction nozzle, “The air-fluid mixture forms a larger spray droplet to help transport the droplets to the target. By increasing the size of the spray droplet, spray drift is reduced by minimizing smaller, ‘driftable’ fines,” Wolf explains.
Klein, Wolf and Ozkan are among a number of university researchers who have been analyzing how well low-drift spray nozzles — including those with air-induction — strike a balance between droplets coarse enough to minimize drift but fine enough to provide adequate plant coverage.
One measurement useful in understanding nozzle performance is the volume median diameter (VMD). A spray nozzle pattern's VMD is the droplet size at which all droplets that size or smaller carry half the spray material, while the other half of the spray material is carried in the other droplets. For example, a spray pattern with a VMD of 500 microns would mean half the spray volume — not half the droplets — is carried by droplets 500 microns or smaller. The other half is carried by droplets larger than 500 microns.
Wolf says a droplet size spectrum with a VMD of 300-500 microns is desirable for herbicides. A spectrum with a VMD of 200-300 microns is preferred for fungicides and insecticides, he adds. He doesn't like to see droplet sizes smaller than 200 microns. “That's where I draw the line. New (spray nozzle) technology has done a good job of eliminating those smaller droplets,” he says.
Two-hundred microns are about the width of two human hairs, based on a size reference offered by Ozkan, who says a human hair is roughly 75-100 microns.
It's not enough to know the droplet size spectrum (size range of smallest to largest droplets) in a nozzle's spray pattern. It's also important to know the distribution of spray material among the various droplet sizes within a spray nozzle's pattern. For example, a 400-micron droplet carries eight times as much spray volume as a 200-micron droplet, Wolf points out. If too much of the spray volume is concentrated in droplets too large and too few, plant coverage may suffer.
IN KLEIN'S SPRAY pattern comparisons with water applied at 40-psi operating pressure, just 7-9% of the spray volume was in drift-prone droplet sizes of 210 microns or smaller from the two air-induction nozzles (Spraying Systems' AI 11005 and AIXR 11005). In contrast, 16% of the spray volume was in the drift-prone droplet size for two low-drift nozzles without air induction (Spraying Systems' turbo flat fan TT11005 and turbo flood TF 2.5). And it was 33% for the extended range flat-fan nozzle (XR 11005).
Most spray patterns are determined, based on water as the spray material. But droplet size spectrum for a given nozzle type is generally altered by the spray material being applied and the sprayer operating pressure, Klein and Wolf say. That effect shows up in Klein's comparisons for the above nozzles in applications involving Roundup WeatherMax in combination with several different spray adjuvants.
A VMD of around 500 microns, with drift-prone droplet sizes limited to 6% or less of the spray volume, was achieved by the two air-induction nozzles in applications of Roundup WeatherMax with adjuvant combinations of 2% ammonium sulfate plus Interlock and 2% ammonium sulfate plus In-Place.
Incidentally, Klein adds, air-induction nozzles work well with some of the new drift-retarding spray adjuvants, which have been developed specifically for those nozzles. On the other hand, some of those adjuvants may not work well with other nozzles, he adds.
And how do spray nozzle patterns translate into efficacy? Weed control was comparable among an air-induction nozzle, two other low-drift nozzles and two flat-fan nozzles included in Nebraska comparisons. However, the air-induction nozzle (AI110025) and two low-drift nozzles (turbo flat fan TT11003 and turbo flood TF2) produced significantly less spray volume in drift-prone droplet sizes than the two flat-fan nozzles (XR1103 and XR11004): 13-22% vs. 31-53%. All five nozzles produced 80% or slightly higher weed control with Roundup WeatherMax on non-biotech corn, oil sunflower, velvetleaf, green foxtail and waterhemp.Continue on Page 2
WOLF EMPHASIZES THAT air-induction nozzles lose their advantages if operated at the wrong pressure. Earlier-generation air-induction nozzles — what he calls Group I — that include TurboDrop XL from Greenleaf Technologies, AI from Spraying Systems, Ultra-Lo-Drift from Precision/Lurmark and Raindrop Ultra from Delavan, should be operated at 60-80 psi, according to Wolf.
Later-generation air-induction nozzles — what he calls Group II — that include AIXR from Spraying Systems and Guardian Air from Hypro should be operated at 40 psi, according to Wolf. He also includes the AirMix air-induction nozzle from Greenleaf Technologies in Group II, even though the AirMix was introduced about the time the Group I nozzles were introduced, according to Wolf.
Group II nozzles provide better coverage than the Group I nozzles, he says. However, while Group II nozzles have less drift potential than turbo chamber and extended range nozzles, they don't reduce drift potential as much as the earlier-generation Group I air-induction nozzles, he adds.
Wolf emphasizes the importance of matching appropriate pressure with the type of nozzle you're using for optimum coverage while minimizing drift. Spray effectiveness and drift management are the two key issues in choosing nozzles, Klein says.
“These new (air-induction) nozzles are a little bit higher priced, but to me, that's the business end of the sprayer,” Klein adds. With the various combinations of pre-orifice and exit orifice sizes in these new nozzles, he says, “you can do anything you want to” with them to achieve a balance between droplet sizes large enough to resist drift but small enough to give good plant coverage.