Spray Equipment and Calibration

Centrifugal pumps are commonly used for self-propelled sprayers and large trailed sprayers. They are durable, simply constructed and can readily handle abrasive materials. Because of the high capacity of centrifugal pumps, hydraulic agitators can and should be used to agitate spray solutions even in large tanks.

Centrifugal pumps feature a distinctive “steep performance curve” where discharge volumes decrease rapidly at the upper limits of the pump’s operating pressure range. This is an advantage as it permits controlling pump output without a relief valve. However, since centrifugal pump performance is sensitive to speed (Figure 2), inlet pressure variations may produce uneven pump output under some operating conditions.

Centrifugal pumps should operate at speeds of about 3,000 to 4,500 revolutions per minute (RPM). When driven with the tractor PTO, a speed-up mechanism is necessary. A simple and inexpensive method of increasing speed is with a belt and pulley assembly. A planetary gear system is another option. The gears are completely enclosed and mounted directly on the PTO shaft. Centrifugal pumps can also be driven by a direct-connected hydraulic motor (as in Figure 2, for example) and flow control operating off the tractor hydraulic system. This allows the PTO to be used for other purposes, and a hydraulic motor may maintain a more uniform pump speed and output with small variations in engine speed. Pumps may also be driven by a direct-coupled gasoline engine, which will maintain a constant pressure and pump output independent of vehicle engine speed.

Centrifugal pumps should be located below the supply tank to aid in priming and maintaining a prime. Also, no pressure relief valve is needed with centrifugal pumps. The proper way to connect components on a sprayer using a centrifugal pump is shown in Figure 3. A strainer located in the discharge line protects nozzles from plugging and avoids restricting the pump input. Two control valves are used in the pump discharge line, one in the agitation line and the other to the spray boom. This permits controlling agitation flow independent of nozzle flow. The flow from centrifugal pumps can be completely shut off without damage to the pump. Spray pressure can be controlled by a throttling valve, eliminating the pressure relief valve with a separate bypass line. A separate throttling valve is usually used to control agitation flow and spray pressure. Electrically controlled throttling valves are popular for remote pressure control and are installed in an optional bypass line as shown in Figure 3.

A boom shut-off valve allows the sprayer boom to be shut off while the pump and agitation system continue to operate. Electric solenoid valves eliminate the need for chemical-carrying hoses to be run through the cab of the vehicle. A switch box that controls the electric valve is mounted in the vehicle cab. This provides a safe operator area if a hose should break.

To adjust for spraying with a centrifugal pump (Figure 3), open the boom shut-off valve, start the sprayer and open the throttling control valve until pressure comes up to 10 PSI over the desired spraying pressure. Then adjust the agitation control valve until good agitation is observed in the tank. If the boom pressure has dropped slightly because of the agitation, readjust the main control valve to bring the pressure up to 10 PSI above spraying pressure. Then open the bypass valve to bring the boom pressure down to the desired spray pressure. This valve can be opened or closed as needed to compensate for system pressure changes so a constant boom pressure can be maintained. Be sure to check for uniform flow from all nozzles.

Roller pumps are commonly used with three-point sprayers. They consist of a rotor with resilient rollers that rotate within an eccentric housing. Roller pumps are popular because of their low initial cost, compact size and efficient operation at tractor PTO speeds. They are positive displacement pumps and self-priming. Larger pumps are capable of moving 50 GPM and can develop pressures up to 300 PSI. Roller pumps tend to show excessive wear when pumping abrasive materials, which is a limitation with this pump.

Material options for roller pumps include cast iron and corrosion-resistant silver alloy or Ni-resist housings; PTFE-coated, polypropylene or Teflon rollers; and Viton or Buna-N rubber seals. Corrosion-resistant housings are recommended for pesticides, especially glyphosate or other acidics. Poly rollers are used for all-around spraying; they are suitable for fertilizers and weed and insect control chemicals, including suspensions. PTFE-coated rollers provide additional corrosion resistance. Viton seals are suitable for all purposes; Buta-N seals can be used if pumping applications are limited to fertilizers, plain water or abrasive powders in suspension. Roller pumps should have factory-lubricated sealed ball bearings, stainless steel shafts and replaceable shaft seals.

The recommended hookup for roller pumps is shown in Figure 4. A control valve is placed in the agitation line so the bypass flow is controlled to regulate spraying pressure. Systems using roller pumps contain a pressure relief valve (Figure 5). These valves have a spring-loaded ball, disc or diaphragm that opens with increasing pressure so excess flow is bypassed back to the tank, preventing damage to sprayer components when the boom is shut off.

The agitation control valve must be closed, and the boom shut-off valve must be opened to adjust the system (Figure 4). Start the sprayer, making sure flow is uniform from all spray nozzles, and adjust the pressure relief valve until the pressure gauge reads about 10 to 15 PSI above the desired spraying pressure. Slowly open the throttling control valve until the spraying pressure is reduced to the desired point. Replace the agitator nozzle with one having a larger orifice if the pressure will not come down to the desired point.

Use a smaller agitation nozzle if insufficient agitation results when spraying pressure is correct and the pressure relief valve is closed. This will increase agitation and permit a wider open control valve for the same pressure.

Piston pumps are positive displacement pumps, where output is proportional to speed and independent of pressure. Piston pumps work well for wettable powders and other abrasive liquids. They are available with either rubber or leather piston cups, which permit the pump

to be used for water or petroleum-based liquids and a wide range of chemicals. Lubrication of the pump is usually not a problem due to the use of sealed bearings.

The use of piston pumps for farm crop spraying is limited partly by their relatively high cost. Piston pumps have a long life, which makes them economical for continuous use. Larger piston pumps have a capacity of 25 to 35 GPM and are used at pressures up to 600 PSI. This high pressure is useful for high-pressure cleaning, livestock spraying or crop insect and fungicide spraying. A piston pump requires a surge tank at the pump outlet to reduce the characteristic line pulsation.

The connection diagram for a piston pump is shown in Figure 6. It is similar to a roller pump, except that a surge tank has been installed at the pump outlet. A damper is used in the pressure gauge stem to reduce the effect of pulsation. The pressure relief valve should be replaced by an unloader valve (Figure 7) when pressures above 200 PSI are used. This reduces the pressure from the pump when the boom is shut off, so less power is required. If an agitator is used in the system, agitation flow may be influenced when the valve is unloading.

Open the throttling control valve and close the boom valve to adjust for spraying (Figure 6). Then adjust the relief valve to open at a pressure 10 to 15 PSI above spraying pressure. Open the boom control valve and make sure flow is uniform from all nozzles. Then adjust the throttling control valve until the gauge indicates the desired spraying pressure.

Diaphragm pumps are commonly used with ATV sprayers and nutrient application systems. They can handle abrasive and corrosive chemicals at high pressures and permit a wide selection of flow rates.

Large diaphragm pumps can produce high pressures (to 850 PSI) along with high volume (60 GPM), but the price of diaphragm pumps is relatively high. Therefore, diaphragm pumps are best suited for applications where their attributes are required, such as when spraying highly abrasive or corrosive chemicals or when both high pressure and volume are required.

The spray system hookup for diaphragm pumps is the same as for piston pumps (Figure 6). Be sure the controls and all hoses are large enough to handle the high flow and that all hoses, nozzles and fittings are capable of handling high pressure.

The nozzle is a critical part of any sprayer. Nozzles perform three functions:

1. Regulate flow
2. Atomize the mixture into droplets
3. Disperse the spray in a desirable pattern.

There is no single perfect nozzle. There are a variety of nozzle designs, and each is best suited for certain purposes and less desirable for others. The chart in Table 2 compares various nozzles, their droplet sizes and their effectiveness for broadcast spraying. Table 3 compares nozzle characteristics for banding or directed spraying.

Nozzles determine the rate of pesticide distribution at a particular pressure, forward speed and nozzle spacing. Drift can be minimized by selecting nozzles that produce the largest droplet size while providing adequate coverage at the intended application rate and pressure.

Nozzles are made from several types of materials. The most common are brass, plastic, nylon, stainless steel, hardened stainless steel and ceramic. Brass nozzles are the least expensive but are soft and wear rapidly. Nylon nozzles resist corrosion, but some chemicals cause thermoplastic to swell. Nozzles made from harder metals usually cost more but are usually more durable. The durability of various nozzle materials compared to brass is shown in Figure 13. Nozzles wear with use and flow rate. It is important to check and replace worn nozzles regularly, because worn nozzles may increase pesticide application cost and cause crop injury, illegal rates or residue. For example, a 10% increase in flow rate may not be readily noticeable; however, spraying 160 acres with a pesticide that costs $35 per acre at the increased rate would cost an extra $3.50 per acre or $560 more for the field.

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NDSU Photo

Wear rates of various nozzle materials.

Each nozzle on a sprayer should apply the same amount of pesticide. Streaking may occur if one nozzle applies more or less than adjacent nozzles. Nozzle flow rates must be monitored by regularly collecting the flow from each nozzle under operating conditions and comparing the outputs. If the discharge from a nozzle varies more than 10% above or below the average of all the nozzles, replace it.

Do not mix nozzles of different materials, types, discharge angles or gallon capacity on the same sprayer. Any mixing of nozzles will produce uneven spray patterns.

Care must be used when cleaning clogged spray nozzles. The nozzle should be removed from the nozzle body and cleaned with a soft-bristled nozzle cleaning brush. Blowing the dirt out with compressed air is also an excellent method. Do not use a small wire or jackknife tip to clean the nozzle orifice, as it is easily damaged. An interdental toothbrush is inexpensive, highly effective and will not damage the nozzle orifice.

Table 2. Nozzle guide for broadcast spraying.

Table 3. Nozzle guide for band and directed spraying.

Nozzle flow rate is a function of the orifice size and pressure. Manufacturers’ catalogues list nozzle flow rates at various pressures and discharge rates per acre at various ground speeds. In general, as pressure goes up flow rate increases, but not in a one-to-one ratio. To double the flow rate, you must increase the pressure four times. Many spray control systems use this principle to control output. They increase pressure to maintain correct application rates with an increase in speed. Use caution in speed changes as the spray system pressures may need to operate above recommended nozzle operating ranges, producing excessive driftable fines.

Once the spray material leaves the nozzle orifice, only droplet size, droplet number and the velocity of drops can be measured. Droplet size is measured in microns. A micron is one millionth of a meter; 1 inch contains 25,400 microns. To give this measure some perspective, a human hair is approximately 56 microns in diameter.

All hydraulic nozzles produce a range of droplet sizes – some large droplets to many small droplets. The volume median diameter VMD) is a summary statistic used to characterize the range of droplet sizes produced by a nozzle. The VMD is the droplet size where 50% of the nozzle’s volume is composed of droplets smaller than the VMD and 50% of the volume is in larger droplets. The VMD should not be confused with the NMD (number median diameter), which is usually a smaller number. The NMD is the median size that divides the spectrum of droplets into an equal number of smaller and larger drops.

Nozzle design affects the droplet size, and droplet size is a primary consideration when choosing an appropriate nozzle for a pesticide application. Large droplets are less prone to drift, but small droplets may be more desirable for better coverage. Pressure also affects droplet size – higher pressures produce smaller droplets.

The size of the spray droplet has a direct influence on the efficacy of the applied chemical, so selecting the proper nozzle type to control spray droplet size is an important management decision. When the average droplet diameter is reduced to half its original size, eight times as many droplets can be produced from the same flow. A nozzle that produces small droplets can theoretically cover a greater area with a given flow. However, extremely small drops may not deposit on the target, as evaporation is reducing their size during travel to the target and air currents in the drop pathway may interrupt the drop movement and carry the drop off-target. Environmental conditions of wind and humidity, which is best expressed as Delta T, can have a major effect on drop deposit on the target when small drops are used to apply pesticides.

Water-sensitive paper can be used to assess droplet size and density. Experience has shown that for low volume sprays with medium-sized droplets, insecticides should have a density of not less than 20 to 30 droplets/cm2, herbicides 20 to 40 droplets/cm2 and fungicides 50 to 70 droplets/cm2. Drop number and size can be estimated with a hand lens or the SnapCard mobile app.

Nozzle bodies are typically equipped with check valves that produce a quick shutoff and prevent dripping at the nozzle during turns or transport. Diaphragm check valves (Figure 14) are best at stopping nozzle drip. Ball check valves are more susceptible to corrosion than diaphragm check valves and are not as trouble-free. Check valves cause a pressure drop of 5 to 10 PSI, depending on the spring pressure in the valve. Check valves allow the nozzles to be changed without material leaking from the boom.

Every spray pattern has two basic characteristics: the spray angle and the shape of the pattern.

Most agricultural nozzles have an angle from 65 to 120 degrees. Narrow angles produce a more penetrating spray; wide-angle nozzles can be mounted closer to the target, spaced farther apart on the boom, or provide overlapping coverage (Figure 15).

Though there are a multitude of spray nozzles, there are only three basic spray patterns: the flat fan, the hollow cone and the full cone. Each of these has specific characteristics and applications.

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NDSU Photo

Basic nozzle spray angles and patterns.

Flat-Fan Spray Nozzles

Flat-fan nozzles are widely used for broadcast spraying of herbicides and some insecticides. They produce a tapered-edge flat-fan spray pattern. Less material is applied along the edges of the spray pattern, so the patterns of adjoining nozzles must be overlapped to give uniform coverage over the length of the boom. For maximum uniformity, overlap should be about 30% to 50% of the nozzle spacing at the target level (Figure 16). Normal operating pressure is defined by nozzle specifications.

Lower pressures produce larger droplets, which reduces drift potential, while higher pressures produce small drops for maximum plant coverage, but small drops are more susceptible to drift. Extended range flat fan nozzles will operate over a range of 15 to 60 PSI without causing a significant effect on the width of the spray pattern. Lower operating pressure produces larger droplets and reduces the drift potential, while the higher pressures produce fine drops with higher drift potential. Extended range nozzles work well with automatic spray controllers, but realize that droplet size will change as the controller adjusts system pressure.

Flat-fan nozzles are available in several spray discharge angles. The most commonly used configurations are listed in Table 4. Proper spray boom height depends on nozzle discharge angle and is measured from the target to the nozzle. For postemergence pesticides, the target is the growing crop and not the soil surface (Figure 17).

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NDSU Photo

Proper overlap with a flat-fan type nozzle on a20-inch nozzle spacing.

There are two drift-reducing nozzle designs that generate a flat-fan pattern. The pre-orifice nozzle features a chamber ahead of the final orifice that reduces the operating pressure at the outer orifice, which effectively reduces the production of drift-susceptible fines. The air induction nozzle, also known as an air inclusion or Venturi nozzle, features two orifices: one to meter liquid flow and the other, a larger orifice to form the pattern. Between these two orifices is a Venturi or jet, which is used to draw air into the nozzle body. Within the nozzle body, liquid pressure decreases and air mixes with the liquid. The resulting spray consists of large, air-filled droplets and very few drift-prone droplets. For further details on drift-reducing nozzles, refer to NDSU Extension publication AE1246: Selecting Spray Nozzles with Drift-Reducing Technology.

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NDSU Photo

Spray height — nozzle tip to target — flat fan.

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NDSU Photo

Discharge pattern of an “Even” spray nozzle.

Table 4. Minimum suggested spray heights.

“Even” Flat Fan Spray Nozzles

“Even” flat-fan nozzles apply a uniform coverage across the entire width of the spray pattern (Figure 18). They should be used for banding pesticides over the row and should be operated between 30 and 40 PSI. This nozzle should not be used for broadcast applications. The width of the band is dependent upon the nozzle height above the target and spray pressure, as shown in Table 5.

Table 5. Height for banding with even flat fan nozzles.

Flooding Fan Nozzle

Flood fan nozzles produce a wide-angle, flat-spray pattern and are used for applying herbicides and mixtures of herbicides and liquid fertilizers. The nozzle spacing for applying herbicides should be 60 inches or less. These nozzles are most effective in reducing drift when they are operated within a pressure range of 10 to 25 PSI. Relative to flat-fan nozzles, pressure changes have a greater impact on spray pattern width of flood nozzles. Also, the distribution pattern is not as uniform as that of the regular flat-fan nozzle. The best distribution is achieved when the nozzle is mounted at a height and angle to obtain at least 100% overlap (double coverage). When set for 100% overlap, a change in nozzle pressure distorts the spray pattern.

A “turbo floodjet” nozzle from TeeJet Technologies produces larger drops and a more uniform spray pattern than a standard flood tip. It is designed to reduce drift and provides uniform deposition with 30% to 50% overlap instead of 100% required by standard flood nozzles. The turbo flood nozzle is designed for use with soil-incorporated herbicides and liquid fertilizer and can be operated at pressures ranging from 10 to 40 PSI.

Flood nozzles can be mounted so they spray straight down, straight back or at any angle in between (Figure 19). Studies indicate the most uniform pattern is obtained when the spray is directed straight back, but this will produce the greatest chance for drift of the small droplets. Directing the spray straight down will minimize the drift potential but will produce the most irregular spray pattern. The best compromise position is to set the nozzle at a 45-degree angle with the sprayed surface. Care should be taken so that incorporation equipment does not intercept or interfere with the spray discharge pattern.

Hollow Cone Nozzles

Hollow cone nozzles are designed for air blast spraying or for directed sprays in a banded application and are used in situations where complete coverage of the leaf surface is important, such as with insecticides or fungicides. The hollow cone pattern provides a fine spray pattern, which achieves thorough coverage only when directed toward the target or combined with air assist. These nozzles usually operate in the pressure range of 40 to 100 PSI or more, depending on the nozzle being used and the pesticide applied. Spray drift is higher with hollow cone nozzles than with other nozzles, as small droplets are produced.

A hollow cone nozzle produces a spray pattern with more of the liquid concentrated at the outer edge of the pattern (Figure 15) and less in the center. Any nozzle producing a cone pattern, including the whirl-chamber type, will not provide uniform distribution for broadcast applications.

“Raindrop” nozzles from Delavan feature a wide-angle, hollow cone, drift-reduction design that produces large drops at pressures of 20 to 60 PSI. Although featuring a hollow cone design, these nozzles are designed to replace conventional flood nozzles in broadcast applications.

Full Cone Nozzles

The full cone nozzle produces a swirl and a counter swirl inside the nozzle that results in a full cone pattern. Full cone nozzles produce large, evenly distributed drops and high flow rates. A wide full cone tip maintains its spray pattern over a range of pressures and flow rates. It is a low-drift nozzle and is often used to apply soil-incorporated herbicides.

Photo Credit:

NDSU Photo

Various positions for mounting flood nozzles.

For broadcast application, flat-fan nozzles should be properly spaced and adjusted on the sprayer. Nozzle discharge angle, nozzle distance from the sprayed surface and nozzle spacing on the boom must all be considered for good spray coverage. Refer to Table 4 for proper nozzle adjustments. Figure 20 shows some of the spray pattern problems that may result from common boom adjustment errors.

Several types of wiper applicators are available commercially. One consists of a long horizontal tube or pipe (3 to 4 inches in diameter), which is filled with a systemic herbicide (Figure 21). A series of short, overlapping ropes or a wetted pad on the tube is in contact with the herbicide and becomes saturated by wicking action. Another type is the roller applicator, which consists of a tube 8 to 12 inches in diameter turned by a hydraulic motor. The tube is covered with carpet that is continuously wetted. These units are mounted on the front or rear of a tractor on a three-point hitch that is hydraulically adjusted so it can be set at a height where the pad applies herbicide to weeds taller than the crop but does not contact the crop. Best results are obtained with double coverage of wiper applicators. The second pass should be in the opposite direction to the first pass so two sides of the plant are covered.

Direct injection sprayers continuously meter concentrated pesticide into the spray system as needed. They contain a large main tank and pump, for water or other carrier, and one or more smaller tanks and pumps for concentrated pesticide. Modern systems automatically adjust chemical injection flow rates to maintain proper per-acre application rates regardless of ground speed or active boom sections. However, users must ensure that chemical pump capacity is great enough to support desired ground speeds at their desired per-acre application rates. High-capacity pumps will allow for faster ground speeds.

The advantage of direct injection sprayers is that no mixed chemical is left over when the application is complete. These units may also be used to reduce pesticide costs by targeting pesticide applications within a limited portion of a field, such as border sprays of insecticide introduced during postemergence herbicide applications. The number of pesticides available for a given application depends on the number of installed product tanks, however, as each pesticide concentrate should be stored in its own tank. Mixing multiple pesticides in a single injection tank is a risky practice due to the potential for physical and chemical incompatibility.

One problem with injection sprayers is the timely injection of the chemical into the system so that it is discharged at the proper time. Lead time on injection may vary due to the size of the hoses on the sprayer, travel speed, the amount of liquid being applied and the point of injection of the chemical into the system. Direct injection requires precise measuring equipment that is maintained in good condition. Remember, it is more difficult to measure a small amount of chemical on a continuous basis than to measure one larger quantity and mix it into the spray tank.

Spray monitors may be of two types: nozzle flow monitors and system monitors. A nozzle flow monitor will alert the operator to a partially or fully blocked nozzle or a worn nozzle in an overflow condition, so that corrections can be made.

System monitors detect the overall operating conditions of the sprayer. They are sensitive to variations in travel speed, pressure and flow rate. These values, along with operator input such as swath width and gallons of spray in the tank, are fed to a computer that calculates and displays the travel speed, pressure and application rate. The monitor can also calculate and display other information such as the machine capacity in acres per hour, the acres covered, the remaining mix in the tank and the distance covered. To function properly, the monitor must have suitable sensors that are accurately and regularly calibrated. Spray system monitor functionality is oftentimes integrated into a multifunctional in-cab display, but standalone units also exist (Figure 22).

Some monitors can also automatically control the flow rate and pressure to compensate for changes in speed or flow. The automatic flow rate controller will respond if there is a deviation of the monitored rate from the desired flow rate. Flow compensation is usually done by changing the pressure setting within a certain range. If the controller is not able to bring the application rate back to the programmed flow rate for some reason, such as an excessive speed change or problems with the spray system, the unit will signal the operator that a problem exists. Monitors are helpful in precise chemical application work and should result in better pest control, more efficient spray distribution and reduced chemical cost.

Although rendered obsolete in most situations due to the prevalence of satellite-based guidance, foam and dye marker systems can be used to aid uniform spray application by marking the edge of the spray swath (Figure 23). This mark shows the operator where to drive on the next pass to reduce skips and overlaps, and it is a tremendous aid in non-row crops, such as spraying tilled fields for applying pre-emergent pesticides. The mark may be continuous or intermittent. Typically, 1-2 cups of foam are dropped every 25 feet. The foam or dye requires a separate tank and mix, a pump or compressor, a delivery tube to each end of the boom and a control to select the proper boom end. Another marker is the paper type. This unit drops a piece of paper intermittently over the length of a field. The paper may blow across the field unless it can be anchored by applying some moisture from the sprayer to the paper.

Global Navigation Satellite System (GNSS) is the collective term for the four fully operational positioning satellite constellations with global coverage (Figure 24). The Global Positioning System (GPS), developed by the U.S. Department of Defense, was the world’s first GNSS constellation and remains the best-known. Others are BeiDou (China), Galileo (European Union) and GLONASS (Russia).

Signals from three GNSS satellites are required to determine a two-dimensional position on the earth. Altitude determination requires a signal from a fourth satellite. A GNSS receiver interprets the signals sent from one or more GNSS constellations and computes its position. GNSS positioning is now commonplace in aerial and ground pesticide application, as it enables equipment guidance, accurate swath spacing and automatic boom sectional control.

Lightbar guidance and automated steering systems help maintain precise swath-to-swath widths. Guidance systems identify an imaginary A-B starting line, curve or circle for parallel swathing using GNSS positions and a control module. The module accounts for the swath width of the implement and then uses GNSS to guide machines along parallel, evenly spaced swaths. These swaths may be straight, curved or circular. Lightbar guidance systems include a display module that uses audible tones or lights as directional indicators for the operator. The guidance system allows the operator to monitor the lightbar to maintain the desired distance from the previous swath.

Guidance systems require two principal components: a light bar or screen, which is essentially an electronic display showing a machine’s deviation from the intended position (Figure 25), and a GNSS receiver for locating the position. This receiver must feature a suitably rapid position refresh rate (usually 5 or 10 times per second) to enable equipment guidance. GPS receivers designed for guidance can be used in conjunction with a yield monitor or for other positioning equipment.

Automated steering systems integrate GPS guidance capabilities into the vehicle’s steering system. In addition to following an A-B starting line, modern systems feature capabilities such as automated end-of-row turns and the ability to establish a custom pattern based on your initial pass.

The advent of GNSS guidance has enabled automatic sectional control for sprayers (Figure 26). A sprayer equipped with automatic sectional control uses GNSS data from the guidance system to automatically shut off boom sections when they enter previously sprayed areas or designated no-spray zones, such as grassed waterways. This prevents chemical overapplication, which saves money, increases efficiency and minimizes crop injury.

The degree of chemical savings achieved with automatic sectional control depends on field shape and the number of sprayer boom sections. A greater number of boom sections will lead to greater savings. Irregularly shaped fields will yield greater savings than square or rectangular fields.

Shielded or hooded (completely covered) spray booms increase spray deposition in the target swath when used on broadcast sprayers (Figure 27). Studies show that shielded booms and individual nozzle shield cones can reduce spray drift by 50% or more. Qualified hooded/shielded sprayers are a drift reduction technology according to the EPA. Certain pesticides, depending on their label language, are eligible for reduced use restrictions when using a qualified sprayer. However, shields and hoods DO NOT eliminate all drift; they only reduce the amount. Be aware of susceptible crops downwind, and use caution when spraying. Be sure to check with the state department of agriculture or enforcement agency for state pesticide laws to be sure they allow spraying during strong wind conditions when shields or hoods are used.

The main disadvantages of shielded or hooded booms are poor nozzle visibility in case of plugs, the increased weight that must be carried on the boom and the added cleanup when different pesticides are going to be applied with the sprayer. Sprayer cleanup should be done in the field or on a sprayer mixing/loading pad that collects washwater so that the rinsate can be contained and used as makeup water for future spraying jobs.

Air assist sprayers inject sprayed pesticide into a high-speed air stream, which helps deliver the spray to the crop and gives better penetration of the crop or weed canopy (Figure 28). Studies show that air assist sprayers can carry spray droplets deeper into the plant canopy and help deposit more pesticide on the underside of the crop or weed leaves compared to other sprayers, which may improve pest control.

NDSU studies showed that, at the same application rate, air assist sprayers improved leaf coverage by about 5% over conventional sprayers in a full potato plant canopy.

Air assist sprayers may have a high drift hazard early in the growing season when the plant canopy is small. This is due to dissipation of the air blast when hitting the ground, as the resulting upward rebound of the air that can carry the small spray drops up to drift away. It is recommended to reduce the air velocity in small or young crop canopies due to drift risk, but it can be difficult to achieve the proper settings. Spray drift hazard is considerably lower when used to apply pesticides to full plant canopies later in the growing season.

Maintaining a constant application rate when ground speed fluctuates is a fundamental concern when applying pesticides. Pulse-width modulation (PWM) sprayers provide rate control by changing the intermittent on/off cycling of an electronically actuated solenoid valve mounted upon each nozzle body. Application rate is modified by altering the duty cycle, which is expressed as the percentage of time that the nozzle remains on. Cycling occurs so rapidly, typically at 10 to 30 Hz (1 Hz = 1 cycle per second), that spray pattern and quality are not affected.

The primary benefit of a PWM sprayer is that the operator’s chosen droplet size will be consistently applied throughout the field, as flow rate changes do not require pressure adjustments. This contrasts with a sprayer rate controller, which modifies nozzle flow rate in response to fluctuating ground speed by adjusting system pressure, which in turn alters droplet size. Additional benefits of PWM sprayers include turn compensation, individual nozzle sectional control and faster nozzle shutoff response.

For more information on PWM sprayers and their use, refer to UGA Cooperative Extension Circular 1277: Pulse Width Modulation Technology for Agricultural Sprayers or the Sprayers101 article Rate Controllers and a Pulse Width Modulation Update.

Sense-and-act sprayers use sensors to detect the presence of weeds along the spray boom and send an activation signal to the nozzles when weeds are detected (Figure 29). Green-on-brown systems detect green vegetation against a soil background, such as in fallow, and have been commercially available in the U.S. since the early 2000s. Contemporary examples include Trimble Weedseeker 2 and WEED-IT Quadro. Green-on-green systems, such as John Deere See & Spray or Greeneye, detect weeds within a growing crop and were released commercially within the U.S. in 2022.

Sense-and-act sprayers offer numerous opportunities for weed management but require careful consideration of sprayer configuration. Settings requiring configuration include weed detection sensitivity, nozzle selection, spray length when a nozzle is activated and the choice of single-nozzle activation, where only the nozzle in the weed’s lane is turned on, versus overlapping-nozzle activation, where one adjacent nozzle on each side is also turned on. Configurations that minimize product usage, such as using an even-spray nozzle with single-nozzle activation and a short spray length, leave little room for error in assuring that each detected weed is effectively sprayed. It is infeasible to simultaneously maximize weed control and minimize product usage; achieving one will require compromising the other.

Although well-established internationally, particularly in Southeast Asia, spray drones (Figure 30) are an emerging technology in North Dakota and across North America. Advantages of spray drones include their lower cost relative to large ground sprayers and their ability to apply products when landscape characteristics or field conditions do not allow ground travel. Disadvantages of spray drones include their limited capacity and productivity relative to large ground sprayers and uncertainties around spray drift and application efficacy.

Research is ongoing to better understand the spray application characteristics of drones. Drone sprays, consisting of relatively small droplets that are highly concentrated due to low carrier volume (e.g., 1 to 3 gpa) and delivered to the target by air assist generated from propellor downwash, are substantially different than those delivered by a ground sprayer. Studies suggest that obtaining a consistent and uniform spray pattern with a drone is challenging, as propellor downwash tends to concentrate spray directly beneath the vehicle while also causing considerable lateral movement of spray droplets. Relatedly, characterizing spray drift from drones is another research focus.

Due to the characteristics of drone sprays, it is critical for drone spray operators to calibrate their drones for effective swath width. Failure to do so may lead to within-field striping due to inadequate coverage caused by overestimating the effective swath width. Swath width calibration is best performed within the crop canopy, as vegetation characteristics modify the movement and deposition of spray droplets. Further details on calibrating a drone can be found in the Sprayers101 articles How to Calibrate a Drone and RPAS Swathing in Broad Acre Crop Canopies.

Spray drone operators must also meet all applicable state and federal licensing and operational requirements for unmanned aerial application, which are more extensive than those for ground application of pesticides.

The amount of chemical solution applied per acre depends on forward speed, system pressure, nozzle size and nozzle spacing on the boom. A change in any one of these will change the application rate.

The forward speed and pressure must be adjusted correctly to set the sprayer for any given per-acre rate. The nozzle size should be changed to make a large change in application rates, and all nozzles should discharge an equal amount of spray. If any of these adjustments are incorrect, poor results will be obtained.

The first thing to accomplish with sprayer calibration is to select the nozzle type and size appropriate for your spraying task. You can base the nozzle type decision on spraying conditions and guidelines as recommended in Tables 2 and 3.

Once you’ve selected the type of nozzle, the next step is to calculate the nozzle size.

Nozzle selection should not be based on “gallons per acre” as advertised by some manufacturers. A nozzle that is identified as a 10-gallon nozzle will deliver this amount per acre only for one specific condition, such as when the nozzle spacing is 20 inches on the boom, the sprayer is traveling at 4 mph and the boom pressure is 30 PSI. If the spacing, speed or pressure varies from these set values, the nozzle will not deliver the specified gallons per acre.

Choice of nozzle size should instead be based on a gallons-per-minute calculation. Basing calculations on gpm allows the operator to make spraying decisions based on the crop and field conditions.

Calibration Method No. 1

Equation 1

gpm = gpa x mph x w / 5940

gpm = gallons per minute, the nozzle flow rate.

gpa = gallons per acre, a decision made based on label recommendations, field conditions, spray equipment and water supply.

mph = the ground speed you select, miles per hour.

w = band width or spacing between nozzles in inches.

5940 = a constant to convert gallons per minute, miles per hour and nozzle spacing in inches to gallons per acre.

As an example, assume you are going to use 80-degree flat-fan nozzles. You desire to apply 20 gallons per acre, nozzles are spaced 20 inches apart and the speed you prefer to drive is 6 mph. What size nozzle in gallons per minute is needed for this spray application?

gpm = 20 gpa x 6 mph x 20 inches / 5940

gpm = 0.4 gallons per minute per nozzle

Specifications from manufacturers’ catalogues for fan nozzles (Table 13) show that the XR8004 and AIXR11004 will provide 0.4 gallons per minute at 40 PSI. Another choice is the XR8005 or AIXR11005 at 25 PSI, or the XR8006 or AIXR10006 at 18 PSI. The lower pressures will provide larger drops with less drift potential than spraying at 40 PSI. The AIXR nozzle will produce larger droplets than the XR nozzle at all pressures. However, larger drops will reduce coverage as compared to smaller drops. Be sure to check the pesticide label for instructions on suitable nozzle types and operating pressures.

Table 13. Fan nozzle output.

Once you have determined the proper size tip, put these nozzles on the sprayer and operate it with water. Test for leaks, other sprayer problems, spray pattern uniformity and calibration.

Equation 2

If a set of nozzles is available for use, the previous formula, after rearranging the values, can be used to determine the sprayer application rate in gallons per acre.

gpa = gpm x 5940 / mph x w

Example: What is the application rate in GPA of a sprayer with the following parameters:

• 0.2 gpm flow rate
• 20-inch nozzle spacing
• 6 mph travel speed

gpa = 0.2 gpm x 5940 / 6 mph x 20 in = 9.9

Sprayer calibration is extremely important, as it ensures that pesticide is being applied uniformly and at the correct rate. Sprayers must be calibrated even when new or when nozzles are replaced. They should also be recalibrated after a few hours of use, as new nozzles wear and flow rate will increase rapidly. Calibration should be done by measuring the amount of pesticide applied to a part of an acre and calculating how much would be applied to an entire acre. Be sure to check the flow rate of all nozzles on the sprayer so they are all applying the same amount. Each nozzle is spraying a separate strip through the field. If one nozzle is applying more or less than it should, streaking due to under- or overapplication may occur.

Operate the sprayer using the same throttle setting you use when spraying and when making the speed check. This will ensure the pump is delivering the same volume as it will when you’re spraying.

Catch the spray material from each nozzle in a measuring container for one minute. Carefully measure the discharge from each nozzle. It is usually easier to make measurements in ounces per minute rather than gallons per minute. Multiply the gallons per minute flow rate given in a nozzle catalog by 128 to convert it to fluid ounces per minute. Many nozzle catalogs also list flow rate in ounces per minute as well as gallons per minute.

Equation 3

1 gal = 128 ounces

In our example from Equation 1, we needed 0.4 gpm per nozzle.

0.4 gpm x 128 oz/gal = 51 oz/min

Compare this calculated number of ounces with the measured values. Any nozzles that are not within 5% of the average output should be cleaned if they’re plugged or replaced if worn. If an individual nozzle discharge rate exceeds the manufacturer’s specification by 10% or greater, it is worn out and should be replaced.

If the average output is less than is needed, adjust the output by raising or lowering the pressure. A simple and fast method to check nozzle flow rate is with the use of a nozzle flow calibrator, as shown in Figure 32. It is faster than collecting flow in a measuring container and very accurate.

Speed Check

An accurate speed is essential for good sprayer operation. Tractors or sprayers that use GPS or radar to measure ground speed should give accurate readings. Equipment that uses mechanical speedometers, such as older tractors or ATVs, does not give accurate readings, so they need to be checked. Use a tape measure to stake out a measured distance. Then record the time it takes to drive the loaded sprayer over this distance (Figure 33) at the throttle setting and the gearing that you will use for spraying. Do this with the sprayer at least half-full of water and on the same surface that will be sprayed – calibrating on loose soil or hard roads will not give accurate speeds when operating in fields.

Table 14. Seconds to drive 300 feet converted to miles per hour.

Equation 4

speed (mph) = distance (feet) x 60 / time (seconds) x 88

A speed check over a 300-foot distance is accurate and easy to do. Table 14 converts the time (in seconds) to travel 300 feet into miles per hour.

The following calibration method eliminates guesswork and enables you to quickly and accurately determine how the sprayer must be set up to deliver the GPA that you require. This method permits the sprayer to be set up and calibrated by operating the sprayer over a short distance in the field. It ensures that the nozzles will provide the uniform output that is needed.

This method involves spraying over a measured distance, starting with a full tank of water. Traveling over a longer distance will provide more accurate results.

This formula can be used to calibrate over any distance. This method works well when you have a field of a known length such as ½ mile (2640 feet) or 1 mile (5280 feet). Other distances of measured length can also be used.

1. Start with a full tank of water.

2. Spray a known distance in the field in
which you will be spraying.

3. MEASURE the gallons of water required
to refill the tank.

4. Use the following formula to figure gallons per acre (GPA).

GPA = Number of gallons to refill the tank x 43,560 ft2 ÷ acre / Width of boom coverage (ft) x distance of travel (ft)

Example: After spraying a ½ mile distance,

18.25 gallons are required to refill the tank. The sprayer covers a 50-foot swath.

GPA = 18.25 (gal) x 43,560 (ft2/ac)

50 (ft) x 2640 (ft)

GPA = 6

A good way to double-check your calibration is to determine how much pesticide was applied to a certain acreage.

For example, if 100 acres were sprayed and 600 gallons of chemical mix were used, this was an application rate of 6 gallons per acre. This system is very simple and has the advantage of measuring the amount of spray that was truly applied to an area. Keep in mind, this should not be your only calibration method.

OUNCES = GALLONS METHOD

This method of calibration is very easy and can be used to check and fine-tune a sprayer quickly, but it does require driving a distance in the field. Before calibrating the sprayer with any method, you must check nozzle output for uniformity. Correct any nozzles that vary in flow rate by more than ±5%. Also check that the pressure gauge is reliable and that the pressure is properly set. Then proceed as follows:

1. For broadcast application, determine the distance, in inches, between nozzles. For banded applications, determine the band width in inches. For directed applications, collect the discharge from all nozzles per row.

2. From Table 15, determine the distance needed to equal 1/128 acre. Mark this distance off in a field you will be spraying.

3. Measure the time (in seconds) needed to drive the required distance at normal operating speed with all equipment attached and the spray tank ½ full.

4. Collect the discharge from all nozzles directing spray to one row for the time measured in step 3. All chemicals added together in ounces is gallons per acre. If broadcast spraying is being done, the ounces collected from one nozzle is gallons per acre.

Table 15. Calibration distance, ounces
to gallons, 1/128 acre.

1. Determine travel speed in the field you will be spraying. Drive at a uniform speed and measure the time to drive 300 feet.

2. Convert the time to travel speed in MPH from Table 14.

3. Check for nozzle uniformity, and if variation is more than 10% from the average, replace the nozzles. Determine the application rate in GPA by using one of the three calibration methods.

4. The answer is the application rate per
total acre.

5. Rate x row width (in) = Rate per treated acre

Total acre band width (in)

Example: In nozzle manufacturer charts, gallons per acre means volume applied to the area sprayed (treated acre). Depending on row spacing and band width, this area is some fraction of the total field. The following shows the higher volume discharged in a treated acre when the broadcast rate is determined:

• 5 GPA measured with the broadcast calibration method
• 30-inch row spacing
• 10-inch band width

5 GPA x 30-inch row spacing / 10-inch band width = 15 GPA being applied in the band

As an alternative to calculating the conversion from a per-total-acre rate to a per-treated-acre rate, Table 16 shows the concentration effect of directing the spray from the broadcast rate to band application. Multiply the GPA found on the broadcast basis times the appropriate conversion factor in Table 16.

With 15 GPA being applied per treated acre (in the row), MIX THE CHEMICAL IN THE SPRAY TANK BASED ON THIS RATE. Do not mix it on the 5 GPA (total acre) rate or you will be applying chemical in the row at three times the desired concentration. If you do not want to apply water in the row at 15 GPA, select a smaller nozzle. Refer to the charts in the nozzle manufacturer’s catalog.

Table 16. Conversion factor to convert broadcast rate (rate per total acre) to band rate (rate per treated acre).

A useful practice is to carry an auxiliary tank of clean water on the sprayer that can be used to wash down and rinse the sprayer in the field. This leaves the diluted spray material in the field and permits the sprayer to return to the pad “clean,” eliminating an accumulation of chemical wash water that would need to be disposed of later. A suggested plumbing arrangement showing water tank and valve locations is shown in Figure 36. The water tank and wash nozzles can be added to most sprayers.

For complete instructions on cleaning spray equipment, the Purdue Extension publication PPP-108: Removing Herbicide Residues from Agricultural Application Equipment is highly recommended. Presented here is a brief summary of its recommendations.

Step 1: Spray Out Booms Every Night. Herbicide mixtures that remain in the sprayer for extended periods can embed into hoses, fittings and tanks and cause crop injury later. When finishing for the day, completely empty the sprayer when in the field. Then, add a volume of clean water 10 times the sump’s remnant, circulate and spray it out in the field. Repeat.

Step 2: Perform First Rinse in the Field. As a rule of thumb, rinse with a volume of water equal to 10% of the tank’s capacity or two to three times the gallons per acre rate (e.g., 30 to 50 gallons water for a 15 GPA rate). Never drain surplus pesticide or rinsate where it can run off into streams, lakes or other surface water, or where it can contaminate wells and groundwater.

Step 3: Remove and Clean All Screens. Screens usually contain pasty material that did not completely dissolve into solution. Rinsing is insufficient to remove these residues – the screens must be removed and cleaned. Cleaning screens early in the process allows the later cleaning and rinsing steps to more effectively decontaminate the sprayer.

Step 4: Remove and Clean Boom End Caps. The end cap is the section of the boom between the last nozzle and the end of the boom. Similar to screens, these regions can accumulate herbicide residue and must be removed and cleaned to eliminate contamination.

Step 5: Perform Second Rinse With Water. A second flush of clean water will remove most, if not all, of the loose residue that remains in the sprayer.

Step 6: Add Tank Cleaner. Tank cleaners should be used as the last step to break down or neutralize small amounts of herbicide residual in the sprayer. Commercial tank cleaners are superior to detergents, bleach or ammonia. Tank cleaners must remain in the sprayer long enough to work; follow label instructions or leave in the sprayer overnight. After the soaking period, thoroughly flush the tank cleaner from the system.

Step 7: Perform Final Rinse and Flush. Some operators perform a final water flush to ensure the system is clean and use a pressure washer to clean residues from the sprayer surface. This may be most appropriate for the end of the season.

Lastly, it is good practice to implement a written checklist or standard operating procedure for sprayer cleanout in your own business or farm operation. This helps to ensure that the process is conducted thoroughly and correctly.

For complete instructions on winterizing spray equipment, the Purdue Extension publication PPP-121, Preparing Spray Equipment for Winter Storage and Spring Startup, is highly recommended. What follows is a brief summary of its recommendations.

When it is time to store your sprayer, remove as much water as possible from your sprayer system and then add antifreeze to prevent the remaining water from freezing and damaging sprayer parts. Ensure that the burst point temperature of your chosen antifreeze is adequate to provide the protection you need. Check your sprayer’s operator manual or manufacturer guidance for the proper antifreeze to use. In the absence of specific information, RV antifreeze made from propylene glycol offers many advantages over other types of antifreeze, including its ease of use, nontoxicity, and ability to not damage plastic or metal parts.

As a rule of thumb, add antifreeze at a volume 1.5 to 2 times the capacity of the sprayer’s plumbing. Add the antifreeze to the sprayer tank and circulate it throughout the sprayer until the color of the antifreeze exiting the nozzles is consistent with the color you poured into the tank. For future reference, record how much antifreeze it took to winterize the sprayer.

Returnable, reusable containers are recommended if they are available because they eliminate disposal problems. Recycling is a solution for nonreturnable containers. According to the Ag Container Recycling Council, 257 million pounds of ag chemical containers have been recycled since its inception in 1992. When recycling is not an option, proper disposal of empty pesticide containers is very important. Empty containers should not be left around as they pose a hazard to animals, people and the environment.

Empty liquid containers must be triple-rinsed or power-rinsed before disposal. After completely draining its contents into your sprayer, rinse the container by filling it at least 1/10 full of water, capping it and shaking until all inside surfaces have been rinsed. Empty the rinse water into the spray tank. Drain the container completely (at least 30 seconds) and repeat the rinse process two more times, adding the rinse water to the spray tank.

Triple-rinsing is slow and tedious. An easier and faster way is to use a power rinse device that attaches to a hose and pierces the bottom or side of the container (Figure 37). The water spray rinses the container as it drains. A 60-second spray rinse is usually better than a triple rinse. Special rotating nozzles are also available to flush out containers and rinse spray tanks. Rinsed containers should be crushed and disposed of in a waste handling system or recycled if allowable.

If burning of packages is allowed by local ordinances, burn no more than one day’s accumulation at a time. The smoke and fumes from pesticides can be toxic. Burn containers in a location where smoke and fumes do not move toward people or an inhabited area. An alternative to burning is to place empty paper and cardboard containers in a plastic trash bag and dispose of them in an approved waste handling facility.

Approved procedures for disposal of excess chemicals and empty containers have changed frequently. Disposal techniques that are legal today may not be acceptable tomorrow. Check with local authorities on proper methods to use.

Injection metering of spray chemicals is another approach to solving many of the handling and disposal problems associated with rinsate or surplus spray tank mix.

Injection sprayers are designed so that tank mixing is unnecessary. Because the tank contains only clean water, tank washout between sprays and disposal of any unused chemical mixture is eliminated.

In place of tank mixing, chemicals from a concentrate container are metered and injected into the water being pumped through the sprayer, giving the correct proportion of chemical and water for the needed spray. Injection can take place at various points on the sprayer, depending upon the design. When spraying is complete, the containers of concentrate can be moved to storage. With minimum cleanup, the sprayer is ready for another use.