Proper application of herbicides helps ensure crop safety, weed control performance, and cost effectiveness. For these reasons, calibration and maintenance of spray equipment are essential. Over-application of herbicides is costly and may result in crop injury or carryover. Under-application may result in poor weed control. Similarly, sprayers that are not well-maintained may deliver an uneven spray pattern, resulting in weedy "streaks" through the field.
The procedures for maintaining and calibrating spray equipment are really quite simple and consist of two major steps: (1) selection of the proper nozzle tip, and (2) calibrating the equipment to deliver the correct amount of spray.
Selecting the Proper Nozzle Tip
There are a number of different nozzle tips available for applying herbicides, including flood, flat fan, hollow cone, raindrop, and whirl chamber. All these types may be used for applying herbicides to the soil, but foliar-applied postemergence herbicides should be applied only with flat fan, hollow cone, or similar nozzle tips that deliver medium to fine droplets at relatively high pressures. Flood and raindrop nozzles deliver a coarse spray with large droplets which may not give adequate coverage for many postemergence herbicides. A number of new nozzle types have been introduced recently, including the Driftgard and Turbo Floodjet among others. These nozzles can provide adequste coverage with postemergence herbicides while greatly reducing particle drift. Consult a professional with knowledge of spray technology for more information on proper nozzle selection and use.
Nozzle tips are available in a variety of materials which vary considerably in price and wear life. The most common materials are hardened stainless steel, stainless steel, thermoplastics, and brass. Hardened stainless steel is the most wear-resistant material, but it is also the most expensive. Stainless steel and hardened stainless steel tips have excellent wear-resistance with either abrasive or corrosive materials (e.g. wettable powders, liquid fertilizer solutions). Thermoplastic tips have generally shown good resistance to abrasion and corrosion, but may vary in wear life depending on the specific material used to mold the tips. Brass tips are relatively inexpensive, but wear out rapidly when used to apply abrasive materials such as wettable powders. Brass also corrodes rapidly when exposed to some liquid fertilizers (e.g. 28% nitrogen solution).
Nozzles on the spray boom should be spaced according to spray tip manufacturers' recommendations because the correct amount of spray overlap between adjacent nozzles is critical to achieve a uniform spray pattern. For the same reason, it is important that the height of the boom be adjusted according to nozzle tip manufacturers' recommendations. Manufacturers also specify the correct pressure range in which the nozzle tips should be operated. The pressure and size of the nozzle tip orifice determine the spray output, and nozzle tip sizes should be matched with the desired spray application rate and ground speed. Nozzle tip manufacturers have selection guides that simplify this process. The formula given below may be used both for tip selection and for calibration of the sprayer once the tips are installed.
Formula for Nozzle Tip Selection and Calibration
A single formula may be used both for nozzle tip selection and sprayer calibration. The formula is:
| GPM (per nozzle) = | GPA x MPH x W |
| 5940 |
A. Nozzle tip size (orifice) selection:
Once a total carrier volume and speed are decided on and the nozzle spacing is known, substitute those numbers in the above formula. Select a nozzle that will give the required flow rate when the nozzle is operated within the recommended pressure range. Recommended carrier volumes (GPA) usually are specified on all herbicide labels and typically range from 10 to 40 gallons per acre. Ground speed (MPH) should be accurately determined, since speedometers on many tractors are unreliable.
B. Measuring ground speed:
Mark off a distance of 200 feet in the field to be sprayed or in a field with similar surface conditions. At the engine throttle speed (rpm) and gear to be used for actual spraying, determine the time required to travel the 200 feet. Use the table below to determine actual speed in MPH.
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C. Calibrating the sprayer
Install the selected nozzle tips in the sprayer, turn the sprayer on, and collect the output from a single nozzle for one minute in a container marked in ounces. The number of ounces collected in one minute can be converted to GPM by dividing by 128 (1 gallon = 128 ounces). If the GPM collected from the nozzle is below that required by the above formula, then increase the spray pressure. Decrease pressure if the output is too large. Check each nozzle separately for the correct output. Ideally, they should all be within 5% of the correct output.
Maintaining Spray Equipment
Check nozzle flow rates frequently and adjust the pressure to compensate for small changes in nozzle output resulting from normal wear. It is also important to recalibrate each time a different material is applied - for example, when changing from application of a wettable powder to a soluble liquid product, or from a water carrier to liquid fertilizer. Since each of these spray mixtures have different densities, actual flow rates can be quite different at a single pressure setting. Replace nozzle tips and recalibrates when output has changed 10% or more from that of new nozzle tips or when the spray pattern becomes uneven.
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