Spray Equipment and Calibration (continued)

AE-73 (Revised), September 2004

Other Pesticide Application Equipment

Wiper Applicators

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 unit 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 being 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 so 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.

Figure 21. Typical rope wick applicator showing the components assembled. (11KB b&w illustration)

Injector Sprayers

Injector sprayers continuously meter concentrated pesticide into the spray system as needed. They contain two or more tanks with one or two tanks for concentrated pesticide and a larger tank for carrier. Some units are designed so the volume of pesticide metered is determined by ground speed. Others are adjusted based on a constant travel speed. Any change in speed may cause over or under application.

The advantage of injector sprayers is that no mixed chemical is left over when the application is complete. These units may also be used to save weed control by spot spraying troublesome pests that may be encountered. This is done by adding to the spray solution another pesticide that effectively controls the isolated or patches of pests instead of treating the whole area with both pesticides.

One problem with injection sprayers is the timely injection of the chemical into the system so 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. Injection equipment requires precise measuring equip-ment that is maintained in good condition. Remember, it is more difficult to measure a small amount of chemical on a continuous basis as compared to measuring one larger quantity and mixing it in the spray tank.

Spray Monitors

Spray monitors may be of two types – nozzle monitors and system monitors. Use of the nozzle monitor will alert the operator to a nozzle problem immediately so corrections can be made and skips in the field avoided.

System monitors detect the operating conditions of the total 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 (Figure 22). The monitor can also calculate and display other information – the field 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 which are accurately and regularly calibrated.

Figure 22. Typical sprayer control monitors. (11KB b&w illustration)

Some monitors can also control the flow rate and pressure automatically to compensate for changes in speed or flow. The automatic flow rate controller will respond if there is a change of the monitored rate from the desired flow rate. Flow compensation is usually done by changing the pressure setting within a certain range. If for some reason, such as an excessive speed change or problems with the spray system, the controller is not able to bring the application rate back to the programmed flow rate, 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 distribution and reduced chemical cost.

Swath Markers

Foam and dye marker systems 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 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 the length of the field. The paper may blow across the field unless it can be anchored by applying some moisture from the sprayer on the paper.

Figure 23. Foam marker. (15KB b&w illustration)

Global Positioning System

Technology is now available to automatically determine position using the global positioning system (GPS) (Figure 24). This system, developed by the U.S. Department of Defense, uses a network of 24 satellites orbiting the earth. The user must have a receiver to interpret the signals sent from the satellites and to compute its position. It works whether the receiver is stationary or mobile, anywhere in the world, 24 hours a day.

Figure 24. Global positioning system. (11KB b&w illustration)

Signals from three satellites are required to determine a two-dimensional position on the earth. Altitude determination requires a signal from a fourth satellite. The global positioning system is in use now in aerial and ground application work and holds good potential for improved pesticide application by spotspraying patches of weeds with a chemical injection system or maintaining better swath spacing.

Equipment Guidance Systems

Lightbar-guided 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 GPS positions and a control module. The module takes into account the swath width of the implement and then uses GPS to guide machines along parallel, curved, or circular evenly spaced swaths. 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 principle 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 GPS receiver for locating the position. This receiver must be designed for this purpose and it must operate at a higher frequency (position calculations are usually 5 to 10 times per second) than a GPS receiver designed to record positions for a yield monitor. GPS receivers designed for guidance can be used in conjunction with a yield monitor or for other positioning equipment.

Figure 25. A guidance system. (17KB b&w photo)

Automated steering systems integrate GPS guidance capabilities into the vehicle steering system. Automated steering frees the operator from steering the equipment except at corners and at the ends of fields.

Shielded Spray Boom

Shielded spray booms or completely covered booms show potential for use on broadcast sprayers to increase spray deposition in the target swath. Studies show that shielded booms and individual nozzle shield cones can reduce spray drift by 50 percent or more. Research shows that spray drift with a shielded sprayer operating in a 20 mph wind is equal to or less than an unshielded boom operating in a 10 mph wind. Shields 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 agency that is responsible for enforcing state pesticide laws to be sure they allow spraying during strong wind conditions when shields are used.

The main disadvantage of shielded booms is the increased weight that must be carried on the boom and the added cleanup of the shield when different pesticides are going to be applied with the sprayer. A wheel-carried boom is almost a necessity to carry the extra weight and maintain a stable boom height. Sprayer cleanup should be done in the field or on a sprayer mixing/loading pad that collects washwater so the rinsate can be contained and used as makeup water for future spraying jobs.

Air Assist Sprayers

Air assist sprayers inject pesticides into a high-speed air stream, which helps carry the chemical to the crop to give better penetration of the crop or weed canopy. Studies show that air assist sprayers are capable of carrying spray drops deeper into the plant canopy and help deposit more pesticide on the underside of crop or weed leaves than other sprayers and may improve pest control.

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

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

Spray Drift

Drift of pesticides away from the target is an important and costly problem facing applicators. In addition to the potential damage to non-target areas, drift tends to reduce the effectiveness of chemicals and costs money. Drift can occur by two different means.

VAPOR DRIFT occurs when a chemical vaporizes after being applied to the target area. The vapors are then carried to another area where damage may occur. The amount of vaporization that occurs depends largely on the air temperature and formulation of the pesticide being used. Some products may vaporize rapidly at temperatures as low as 40 degrees Fahrenheit. “Low volatile” esters of 2, 4-D or MCPA may vaporize at 75-90 F. The amine formulations of 2, 4-D or MCPA are essentially “non-volatile.” The dangers of vapor drift can be reduced substantially by choosing the correct herbicide formulation.

PHYSICAL DROPLET DRIFT is the actual movement of spray particles away from the target area. Many factors affect physical drift, but one of the most important is droplet size. Small droplets fall through the air slowly, so they are carried farther by air movement.

Liquid sprayed through a nozzle divides into droplets that are spherical or nearly spherical in shape. The recognized measurement for indicating the size of these droplets is in microns.

Droplets smaller than 100 microns are usually considered highly “driftable.” Drops this size are so small that they cannot be easily seen unless in extremely high concentrations such as on a “foggy” morning.

All spray droplet atomizers available today produce a range of droplet sizes. Some produce a wider range than others. Table 6 shows a typical distribution of droplet sizes for a flat-fan nozzle when spraying water at two different pressures. Most of the droplets produced from a hydraulic spray nozzle are small. Table 6 indicates that more than half of all of the droplets were less than 63 microns in diameter at 20 or 40 PSI. However, little of the total volume is contained in droplets less than 63-micron diameter. Most of the volume is contained in the larger droplets, particularly those ranging in size from 63 to 210 microns. These principles hold true for both pressures, although increasing the pressure caused more of the spray to be contained in small droplets. Even though the volume of small droplets is low, downwind crops can be seriously affected if the crop is susceptible to injury from the pesticide.

Table 6. Droplet size range for a flat fan nozzle at 20 PSI and 40 PSI.
Size Range, microns	Percent of All Droplets		Percent of Total Volume
	20 PSI	40 PSI	20 PSI	40 PSI
0-21	22.4	44.6	0.1	0.4
21-63	37.6	39.5	3.0	10.4
63-105	21.2	10.0	10.7	20.1
105-147	9.2	3.8	16.2	25.4
147-210	7.2	1.9	36.7	35.3
210-294	2.3	0.2	27.5	7.7
over 294	0.2	0.0006	5.8	0.7

The number of droplets deposited per square inch of surface from the ordinary spray nozzle is normally far more than the minimum required to control a specific pest. In some situations, particularly when using fungicides or insecticides, high spray droplet density may be needed. Table 7 shows that coverage or density of droplets on a surface can be theoretically achieved with uniform droplets of various sizes when applied at 1 gallon per acre. Decreasing the drop size from 200 to 20 microns will increase coverage 10 fold. Results of many studies indicate that spray density required for effective weed control varies considerably with plant species, plant size and condition as well as herbicide type, additives and carrier used. Table 7 shows that drop density decreases for drops above 200 microns in diameter at low application rates. Although excellent coverage can be achieved with extremely small drops, decreased deposition and increased drift potential limit the minimum size drop that will provide effective pest control.

Table 7. Spray droplet size and its effect on coverage and drift.
Droplet Diameter (microns)	Type of Droplet	1 gal/A Application		Drift Distance in 10 ft. Fall with 3 mph Wind (ft.)
		Droplets Per In2 (No.)	Coverage Relative to 1,000 Micron Drops
5	Dry Fog	9,220,000	200	15,800
10		1,150,000	100	4.500
20	Wet Fog	144,000	50	1,109
50		9,220	20	178
100	Misty Rain	1,150	10	48
150		342	7	25
200	Light Rain	144	5	15
500		9	2	7
1000	Heavy Rain	1	1	5
*Air temperature of 86° F and 50% relative humidity

Drift potential of various size drops is also shown in Table 7. It can be seen that a non- evaporating 100 micron drop will move 48 feet horizontally in a 3 mile per hour breeze while falling 10 feet. Drops under 50 microns are nearly invisible in the air and can remain suspended for long periods of time. The objective in applying pesticides is to achieve uniform spray distribution while retaining all the spray droplets within the intended spraying area.

Spray liquid may have a velocity of 60 feet per second or more when leaving a nozzle. The speed is reduced due to air resistance and the spray material breaking into small drops. Table 8 shows the distance in which droplets will decelerate to a free fall condition and the length of their life before they disappear due to evaporation. For example, water droplets less than 20 microns in diameter will evaporate in less than one second while falling less than one inch. Droplets over 100 microns in size resist evaporation much more than smaller droplets due to their larger ratio of volume to surface area.

Table 8. Evaporation and deceleration of various size droplets*.
Droplet Diameter (microns)	Deceleration Distance (in)	Terminal Velocity (ft/sec)	Time to Evaporate (sec)	Fall Distance (in)	Final Drop Dia. (microns)
20	>1	.04	0.3	<1	7
50	3	.25	1.8	3	17
100	9	.91	7	96	33
150	16	1.7	16	480	50
200	25	2.4	29	1,512	67
*Conditions assumed: 90° F, 36% relative humidity, 25 psi, 3.75% pesticide solution

With water carriers, spray droplets will decrease in size due to evaporation during their fall. Figure 26 shows the trajectories of evaporating spray droplets falling through stable air at 77 F, having 55 percent relative humidity in a 1-mile per hour crosswind. Droplets less than 100 microns in size obtain a horizontal trajectory in a very short time and the water in the droplet disappears. The active ingredient in these droplets become very small aerosols, most of which will not reach the ground until picked up in falling rain. From Figure 26, it can be concluded that there is a rapid decrease in drift potential of droplets as they increase to about 150 or 200 microns. The size drop where drift potential decreases depends on wind speed, but generally lies in the range of 150 to 200 microns for wind speeds of 1 to 7 miles per hour. For typical ground applications of herbicides with water carriers, droplets of 50 microns or less will completely evaporate to a residual core of pesticide before reaching the target. Droplets greater than 150 microns will have no significant reduction in size before deposition on the target. Evaporation of droplets between 50 and 150 microns are significantly affected by temperature, humidity, and other climatic considerations.

Figure 26. Evaporation rate of water droplets. (12KB b&w graph)

Drift is not always harmful. It depends on the pesticide being used, the targeted pest and the non-target organisms or objects that are downwind or adjacent to your target area. Keep in mind that if you have considerable drift downwind you will be losing pesticide. Drift from most herbicides should be kept to a mini-mum and all drift reducing techniques should be used if the chemical permits. When using an insecticide for mosquito control, “drift” may be desirable. In this situation, a small driftable droplet is needed to get into small areas to do an effective job.

Several factors affect droplet size and potential drift. They include:

1. Wind direction
2. Wind speed
3. Air stability
4. Nozzle type
5. Flow rate
6. Spray pressure
7. Nozzle spray angle
8. Boom height
9. Relative humidity and temperature
10. Spray thickeners
11. Shielded booms

Table 9. Comparison of the distances downwind for 1 percent of the application rate to be deposited.
Run No. and Comparison	Pressure, psi	Wind Speed, mph	Pct. Deposited at		Downwind Distance, ft.*
			4 ft.	8 ft.
Regular flat fan at 14” height	40	3.5	3.1	.6	7
Regular flat fan at 27” height	40		5.9	1.5	13
Regular flat fan at 25 psi	25	9.9	10.3	3.1	15.5
Regular flat fan at 40 psi	40		9.1	3.6	17
Regular flat fan at 18”	30	5.3	9.3	2.2	14
Low-pressure flat fan at 18”	15		5.7	1.4	11
Regular flat fan and 6 oz. Nalco-Trol	30	8.2	3.3	.5	7
Regular flat fan with no thickner	30		8.3	3.1	16.5
Flooding flat fan at 13”	10	4.2	1.3	.6	5.5
Regular flat fan at 18”	30		3.5	1.1	9
Raindrop nozzle at 18”	40	10.3	4.8	.6	7
Regular flat fan at 18”	30		10.2	3.3	16
*Downwind distance for deposit to drop to 1 percent of application volume.

Table 10. Effect of spray angle on droplet size with flat fan nozzles.
Spray Angle (degrees)	Nozzle Type (at 40 psi)	Nozzle Pressure
		15 psi	40 psi	60 psi
		(volume median diameter in microns)
40°	.5 GPM Flat Spray	900	810	780
65°	.5 GPM Flat Spray	600	550	530
80°	.5 GPM Flat Spray	450	360	330
110°	.5 GPM Flat Spray	390	310	290

Wind direction: Pesticides should not be applied when the wind is blowing toward an adjoining susceptible crop or to a crop in a vulnerable stage of growth. Wait until the wind blows away from any susceptible crop, plants or sensitive areas downwind.

Wind speed: The amount of herbicide lost from the target area and the distance it moves both increase as wind velocity increases. However, severe drift injury can occur with low wind velocities, especially under temperature inversion conditions.

Air stability: Air movement largely determines the distribution of spray droplets. Wind is generally recognized as an important factor, but vertical air movement is often overlooked. Temperature inversion is a condition where cool air near the soil surface is trapped under a layer of warm air. A high inversion potential occurs when ground air is 2 to 5 F cooler than the air above. Under inversion conditions, little vertical mixing of air occurs, even with a breeze. Spray drift can be severe under inversion conditions since small spray droplets may fall slowly or may stay suspended due to the cool dense air and will move with a gentle breeze into an adjoining area.

Spray drift may occur even with relative calm conditions under stable air or an inversion condition, especially with small spray droplets. Some of the most severe drift problems have occurred with low wind velocities, inversion conditions and small spray droplets. Avoid spraying under inversion conditions. Spray drift potential can be reduced by increasing droplet size, by using larger orifice nozzles and/or lower spray pressures with extended range nozzles.

Another cause of spray drift is from “lapse” being greater than a 3.2 F, decrease per 1,000 feet altitude. Under a normal “lapse” situation, cool air gently sinks, displacing lower warm air and causing vertical mixing of air. This may cause small droplets to be carried aloft and dispersed. When the “lapse” is stronger, more spray will be carried upward causing a greater chance for spray drift. Research has shown that temperature inversion causes more spray drift than “lapse” conditions at a given wind speed.

Avoid applying herbicides near susceptible crops during temperature inversion conditions. Inversions can often be identified by observing smoke from a fire. Smoke moving horizontally close to the ground would indicate a temperature inversion.

Nozzle type: Droplet sizes produced by various nozzle types at different spray pressures are shows in Table 11. Flat-fan and flood nozzles produce similar sized droplets. The full cone nozzle produces larger droplets than the flat fan, while the hollow cone nozzle produces smaller droplets than the flat fan.

Table 11. Effect of nozzle type on droplet size.
Nozzle Type (.5 GPM flow at 40 psi)	Nozzle Pressure
	15 psi	40 psi	60 psi
	(volume median diameter in microns)
STD. 80° Flat Spray Tip	–	360	330
Ext. Range 80° Flat Spray Tip	460	370	340
Flood Spray Tip	580	450	420
Full Cone Tip	1090	680	–
Hollow Cone Tip	–	260	230

Flow rate: Nozzle flow rate has a large effect on drop size. This is shown Table 12. Nozzles with small orifices produce small drops while large nozzles produce larger drops. Increasing nozzle size to the next step up in size is an excellent way to reduce the number of driftable fines.

Table 12. Effect of flow rate on droplet size.
Nozzle Type (40 psi pressure)	Nozzle Flow Rate
	.2 GPM	.5 GPM	.8 GPM
	(volume median diameter in microns)
STD. Flat Spray Tip	260	360	440
Ext. Range 80° Flat Spray Tip	270	370	450
Flood Spray Tip	370	450	510
Full Cone Spray Tip	–	680	770
Hollow Cone Tip	200	260	–

Spray pressure: Spray pressure influences the formation of droplets from the spray solution. The spray solution emerges from the nozzle in a thin sheet, and droplets form at the edge of the sheet. Higher pressures cause the sheet to be thinner, and this sheet breaks up into smaller droplets. Large nozzles with higher delivery rates produce larger droplets than smaller nozzles. Small droplets are carried farther downwind than larger drops formed at lower pressures. Table 9 shows the percentage of chemical deposited downwind at various distances. It also shows the distance downwind at which the chemical deposition rate decreases to 1 percent of the application rate.

Nozzle spray angle: Spray angle is the interior angle formed between the outer edges of the spray pattern from a single nozzle. Table 10 shows that nozzles with wider spray angles will produce a thinner sheet of spray solution and smaller spray droplets than a nozzle with the same delivery rate but narrower spray angle. However, wide angle nozzles are placed closer to the target than narrow ones, and the benefits of lower nozzle placement outweigh the disadvantage of slightly smaller droplets.

Volume median diameter (VMD) is a term used to describe the droplet size produced from a nozzle. VMD is defined as the diameter at which half the spray volume is in droplets of larger diameter and the other half of the volume is in smaller droplets.

Boom height: Operating the spray boom as close to the sprayed surface as possible is a good way to reduce drift. The closer the boom is to the ground, the wider the angle the spray discharge must be to give uniform coverage. Be sure nozzles are right for the application. Booms that bounce will cause uneven coverage and drift. Wheel-carried booms are a good way to stabilize boom height, which will reduce the drift hazard and produce a better spraying job.

The effect of reducing drift when nozzles are mounted as close to the ground as possible is shown in Table 9. Chemical discharged from the flat-fan nozzle shows a considerable reduction in downwind deposits at both the 4 and 8 foot distances for the lower positioned nozzles. Flood nozzles produce a wide spray pattern and can be operated at low pressures. The wide pattern allows them to be mounted close to the ground keeping drift to a minimum.

Relative humidity and temperature: Low relative humidity and/or high temperature conditions cause faster evaporation of spray droplets between the sprayer and the target. Evaporation reduces droplet size, which in turn increases the potential drift of spray droplets. Spraying during lower temperatures and higher humidity conditions will help reduce drift.

Spray thickeners: Some spray adjuvants act as spray thickeners when added to the spray tank. These materials increase the number of larger droplets and decrease the number of fine drops. They tend to give water-based sprays a somewhat “stringy” quality. Thickeners reduce drift but do not make the spray drift-proof. The reduction in deposits downwind when a thickener is added to the spray tank is shown in Table 9.

Droplets formed from an oil base spray tend to drift farther than droplets from a water carrier because oil droplets are usually smaller, lighter and remain airborne for a longer period. Oils form into smaller droplet sizes than water when the spray is produced with the same hy-draulic nozzle and the same spray pressure. Oil base sprays do not evaporate as soon as water based sprays so drops remain active for a longer time.

Shielded booms: Spray shields have become extremely popular for spraying small grain as studies show that drift is reduced by 50 percent or more. Wind during the spraying season is often a limiting factor to timely spraying in North Dakota. Shields help extend the spraying time when moderate winds blow. Spraying must be stopped when winds are too strong or when susceptible crops are downwind. Shields do not stop all drift, they only reduce it. Major drift problems can result when using shields if applicators are careless by not paying attention to downwind crops.

Drift Control

Because all nozzles produce a range of droplet sizes, the small, drift-prone particles cannot be completely eliminated, but drift can be reduced and kept within reasonable limits.

1. Use adequate amounts of carrier. This means larger nozzles, which in turn usually produces larger droplets. Although this will increase the number of refills, the added carrier improves coverage and usually increases the effectiveness of the chemicals. Smaller droplets will be produced with lower spray volumes, resulting in a greater drift hazard.
2. Avoid using high pressure. Higher pressures create fine droplets; 40 PSI should be considered the maximum for conventional broadcast spraying.
3. Use a drift-reducing nozzle where practical. They produce larger droplets and operate at lower pressure than the equivalent flat-fan nozzle.
4. Many drift-reducing spray additives which can be used with regular spray equipment are available today.
5. Use wide angle nozzles and keep the boom stable and as close to the crop as possible.
6. Spray when wind speeds are less than 10 mph and when wind direction is away from sensitive crops.
7. Do not spray when the air is completely calm or when an inversion exists.
8. Use a shielded spray boom when wind conditions exceed prime pesticide application conditions.

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AE-73 (Revised), September 2004