Inaccurate pesticide application rates, spray patterns and droplet size can lead to pesticide movement from the targeted area and reduce the effectiveness of the pesticide. A recent study in Nebraska revealed that two-thirds of the applicators were applying pesticides improperly (application rate errors greater than 5 percent). A similar study conducted in North Dakota indicated that 60 percent of tested sprayers had calibration errors greater than 10 percent. Although inaccurate tank mixing causes some of these errors, a majority of the problems result from improper spray equipment calibration and worn nozzles.Nozzle Selection
Inaccurate applications can be due to nozzle wear, therefore it's important to select the correct nozzle material. Wear-resistant materials such as tungsten, carbide, ceramic and hardened stainless steel help nozzles maintain a constant flow rate after a long period of usage. Nozzles made from less durable materials (plastic, brass) demonstrate increased flow rates after only a short period of spraying. For example, after 50 hours of spraying, a brass nozzle can exhibit an increased flow rate of 10 to 15 percent, whereas a hardened stainless steel nozzle will increase only about 2 percent. The increased flow rates result from an increased nozzle orifice area. The added cost to purchase a more durable nozzle can pay for itself many times over by reducing over-application that results from nozzle wear.
Nozzle size depends on the desired application rate, ground speed and nozzle spacing. For each nozzle type and spray angle, the manufacturer recommends spray height and nozzle spacing. Nozzle spacings of 20 and 30 inches are most common. The desired flow rate from the nozzle can be determined from the following equation:
GPM = (GPA x MPH x w) / 5940Calibration Procedure
GPM is the nozzle flow rate in gallons per minute,
GPA is the application rate in gallons per acre,
MPH is the ground speed of the sprayer in miles per hour (MPH = (ft/min)/88), and
w is the nozzle spacing in inches for broadcast spraying.
In this situation, applying pesticide to 30 acres with one tankful would be in compliance with the label. Solid applied herbicides generally work better with larger volumes of spray mixture. One full container of chemical will cover 10 acres. If 15 gallons of carrier per acre are applied, the applicator would get 20 acres per refill and use two containers of pesticides.
The tractor with spray rig is set as if spraying the first 300-foot pass in 42.5 seconds. The second pass is a bit faster at 42.7 seconds. The average time is 42.6 seconds.
Spray nozzles are spaced at 30 inches. Using the formula acreage output rates to nozzle output, application will be approximately 0.364 gallons per minute per nozzle.
Experience shows that the pump can handle this volume and nozzles are rated for this application. Field application is now ready.
The nozzle output now can be checked at the field's edge. Once adjustments are made and each nozzle checks within 5 percent of the desired output, fill the tank with pesticide and water.
Ten acres should be covered by the time the half-tank level is reached using the example above.
A standard nozzle with a flow rate of 0.4 gallons per minute at 40 psi is easy to obtain. The 15-gallon per acre application rate can be obtained by operating the sprayer at the recommended 40 psi and a higher ground speed.
MPHOr the ground speed can be kept constant at 4.8 miles per hour, and the nozzle pressure reduced using the following relationship.
= (GPM x 5940) /( GPA x w)
= (0.4 GPM x 5940) / (15 GPA x 30 in)
= 5.3 MPH
Pressure new / pressure rated =The new pressure is 33 psi. Use small adjustments in pressure to obtain the desired nozzle flow rate within the recommended operating pressure. Operating a nozzle at excessively high pressures will produce small spray droplets susceptible to drift. Operating at excessively low pressures produces larger, less- effective spray droplets and poor spray pattern uniformity down the length of the boom.
(GPM desired / GPM rated)2 =
Pressure new / 40 psi =
(.364 GPM / .4 GPM)2
If calibrating with water and spraying solutions that are heavier or lighter than water (8.3 pounds per gallon), conversion factors must be used (Table 1). In the example above, to obtain a nozzle flow rate of 0.364 GPM with a solution that weighs 10 pounds per gallon, the nozzle should produce 0.364 GPM x 1.10 or about 0.40 GPM when spraying water.Spray System Checks
Erroneous flow rates can result from damaged, worn or plugged nozzles or strainers, and spray hose restrictions between the pressure gauge and the nozzle. Clean nozzles with a toothbrush, not a pocket knife. Never blow out a nozzle with the mouth.
Check the pressure along the length of the boom. If a large pressure difference is found, look for restrictions or install a larger diameter spray hose (see Table 2). An accurate pressure gauge is worth the extra cost.Field Checking
Check spray uniformity by spraying water on a concrete surface and observing the amount of streaking that occurs when the water dries. Spray patterns that result in excessive accumulation below the nozzle are produced by:
Pesticide drift is a major concern of land application of pesticides. In addition to reduction in effectiveness, pesticide drift can damage non-target areas. One method to decrease drift is to use a low volatile formulation that is less likely to volatize and drift.
Pesticide drift also can be controlled by reducing the number of small droplets emitted from the sprayer. Nozzle type, angle and orientation, boom height, and operating pressure can influence the production of driftable drops. A droplet of 100 microns in diameter can drift about 50 feet in a 3-mile per hour breeze where a 10 micron droplet can drift 3000 feet. Spray thickeners can reduce drift as well as spraying at low temperatures and high humidity.Useful Formulas and Equivalents
per acre = (5940 x gallons/minute/nozzle) / (MPH x nozzle
Gallons per minute per nozzle = (gallons/acre x MPH x nozzle spacing) / 5940
Ounces per minute per nozzle = (gallons/acre x MPH x nozzle spacing x 32) / 1485
|Weight of solution||Specific gravity||Conversion factors|
|7.0 lbs. per gallon||.84||.92|
|8.0 lbs. per gallon||.96||.98|
|8.3 lbs. per gallon*||1.00||1.00|
|9.0 lbs. per gallon||1.08||1.04|
|10.0 lbs. per gallon||1.20||1.10|
|11.0 lbs. per gallon||1.32||1.15|
|12.0 lbs. per gallon||1.44||1.20|
|14.0 lbs. per gallon||1.68||1.30|
|* Suitable for most water soluble pesticides.|
|Flow in GPM||Pressure drop in PSI (in 10-foot length) without couplings|
|1/4" I.D.||3/8" I.D.||1/2" I.D.||3/4" I.D.||1" I.D.|
|(1) From Spraying Systems.|
|Spray angle (degree)||20-inch spacing||30-inch spacing|
Publication #: 5.003
Cooperative Extension, Colorado State University. Published
December 1992. Reviewed September 1992. Copyright 1992. For
more information, contact your county Cooperative Extension
Paul D. Ayers, Colorado State University Cooperative Extension agricultural engineer, agricultural and chemical engineering; and Bruce Bosley, Cooperative Extension County Director, Morgan County Cooperative Extension.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Milan Rewerts, interim director of Cooperative Extension, Colorado State University, Fort Collins, Colorado. Cooperative Extension programs are available to all without discrimination. No endorsement of products named is intended nor is criticism implied of products not mentioned.
Disclaimer and Reproduction Information: Information in NASD does not represent NIOSH policy. Information included in NASD appears by permission of the author and/or copyright holder. More