How to calibrate your sprayer

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Proper calibration can help growers make sure chemical applications are getting to the target at the proper rate. Worn nozzles or other equipment malfunctions can mean some areas of the target receive too much or too little chemical, thus having a negative effect on weed or pest control.

In calibrating a sprayer, the speed and pressure at which the sprayer is operated, the nozzle being used, and the height at which the nozzle is held (which controls the width of the band sprayed) must be kept constant.

The parts of the spray system include the pump, pressure regulator, pressure gauge, screens (strainers) and nozzles. Many herbicides are made up of particles suspended in the spray solution. They cause a lot of abrasion and wear on piston pumps. The best pumps to use with these products are diaphragm or centrifugal pumps because they will last much longer than other pumps. Pressure regulators are needed to maintain a constant pressure during application. Herbicides should be applied at 15 to 40 psi. The pressure gauge makes it possible to reset the pressure regulator from one operation to another.

The nozzle tip is a critical component of a sprayer. Although relatively inexpensive, this part of the system must not be neglected. The nozzle affects the flow rate, breaks up the mixture into droplets and disperses the droplets in a specific pattern.

To prevent plugging and excessive wear of nozzles, screens should always be used to remove large particles from the spray mixture. Check the manufacturer’s recommendation for the nozzle to be used. A general recommendation is to use a 50-mesh screen. Screens with ball check valves are recommended for use when applying postemergence herbicides because they prevent dripping from the nozzle after the control valve has been closed.

A sprayer remains accurately calibrated only as long as the openings of the nozzles remain unchanged. As nozzles wear, the openings become larger and the rate and uniformity of herbicide applied become more variable. To minimize these problems, the operator should buy high-quality nozzles, inspect them regularly and replace them as needed. To clean a clogged nozzle, use compressed air or a soft-bristled brush, such as a toothbrush. Never use wire or a nail because the nozzle orifice can be easily damaged.

On an area that best represents the average topography for the area to be sprayed, measure and mark off the calibration distance that coincides with the band width, if band applying, or with the nozzle spacing (width covered by a single nozzle) if broadcast applying (see Table 1).

Calibration distance feet

Fill the sprayer with water only and record the number of seconds required to travel the calibration distance at a comfortable, steady speed. With a tractor-mounted sprayer, note and record the engine rpm and the gear selection so that the same speed is used during calibration and application. For backpack sprayers, it is important that the person making the application is the person that calibrates the sprayer, because the calibration is based on that person’s pace.

Gear rpmtime in seconds

While maintaining the selected application pressure, collect the spray output from one nozzle for the same number of seconds needed to travel the calibration distance.

Amount of water collected in fluid ounces

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The number of fluid ounces collected equals the gallons per acre (GPA) output of the spraying system.

If using a boom, repeat Step 3 twice more, collecting water from a different nozzle each time. The average number of ounces collected for each of the three nozzles is equal to the gallons of water applied per acre for that boom, speed and pressure. If the difference in output of any of the nozzles tested varies more than 10% from the others, check all of the nozzles on the boom and replace as needed.

To determine the amount of chemical to add to the spray tank, divide the capacity of the tank by the number of gallons of water applied per acre to determine the area that can be covered with a tankful of spray.

Tracey Harpster is an Extension Educator, Penn State Pesticide Safety and Education; and Jim Sellmer is a Professor of Horticulture at Penn State. For more read “Controlling Weeds in Nursery and Landscape Plantings” here: https://extension.psu.edu/controlling-weeds-in-nursery-and-landscape-plantings.