Calibrating Sprayers

Accurate calibration of spray equipment is very important because with only slight changes the application rate may cause chemical damage to the crop or the environment, be wasteful of materials, or be ineffective. There is one important difference between the design of fertilizer spreaders, grain drills, row crop planters and sprayers. The method used to control the flow rate of the material for a sprayer is not normally ground driven. The application rate (gal/ac) is a function of the flow rate of the nozzles (gal/min) and the speed of the sprayer (mi/hr). The flow rate from the nozzles does not change as the ground speed changes. The exception to this statement occurs when an application rate controller is added to the sprayer. Most application rate controllers vary application rate to compensate for changes in applicator speed. This is generally done by changing the operating pressure of the sprayer. When a rate controller is not used, the speed of the sprayer must be considered in calculating the application rate.

Figure 8.7 illustrates the common parts for the typical overlapping boom sprayer. Overlapping boom sprayers consist of tank, filter, pump, means to control the pressure, multiple sections of nozzles, and boom selection valve. The mixture in the tank flows through the filter to remove any particles that might plug the orifice in the nozzles, and then to the pump. One of several types of pumps is used, depending on the flow (gal/min) and the pressure (psi) needed by the system. From the pump the mixture goes to the pressure regulating valve. In some sprayer designs, a portion of the flow from the pump is returned to the tank for agitation to prevent the spray materials from separating. Any excess flow produced by the pump also is returned to the tank by the pressure regulating valve. From this valve the fluid is pumped to the boom selector valve. At some point in this line a connection is made for a pressure gage. The boom selector valve directs the flow of the mixture to the different sections of the boom. In some types of valves provision is made for a handgun. A handgun is very useful for spraying skips or along fencerows and other obstructions.

Boom Sprayer Parts
FIGURE 8.7. Parts of a typical boom sprayer.

One popular selector valve design has seven positions:

1. All outlets off

2. All booms on

3. Left boom on

4. Center boom on

5. Right boom on

6. Handgun on

7. Booms and handgun on.

In the typical design, the pump can be engine, power take off (PTO), or hydraulic motor driven. As long as the pump is operating and the selector valve is in one of the on positions, fluid will flow. It is important to understand the effect ground speed has on the spray application rate (gal/ac). When a sprayer operating in the field slows down, the application rate increases. This occurs because the nozzle flow rate (gal/min) is constant. When the travel speed is reduced the same amount of material is applied to a smaller area.

gal gal min ac min ac

When the travel speed is reduced, the acres per minute (ac/min) are reduced. Conversely, when the sprayer travel speed increases, the application rate decreases. Therefore, precise control of the sprayer speed is very important.

The design shown in Figure 8.7 often is modified to meet the demands of different types of plants or application methods. Two additional examples are shown in Figure 8.8 and Figure 8.9. The application rate (gal/ac) of a field sprayer is controlled by three factors:

2. The rate of discharge from the nozzle (gal/min).

3. The width covered by one nozzle (in).

Arranging these variables into one equation produces the standard sprayer equation:

V x w where R = application rate (gal/ac); 5940 = units conversion constant; Q = flow rate per nozzle (gal/min); V = travel speed (mi/hr); w = nozzle spacing (in).

Note: Use either the flow rate (gal/min) from one nozzle and the spacing between two adjacent nozzles, or the flow from all nozzles (gal/min x number of nozzles) and the total width of the sprayer (w x number of nozzle). In either case, the application rate (gal/ac) will be the same. Do not interchange these values.

The calibration of a sprayer is a multiple-step process. In addition, the process can be started at different points, depending on which one of the three variables (speed, nozzle flow rate, or application rate) is selected first.

Problem: What size of nozzles (gal/min) is required for a boom type sprayer to apply 20.0 gal of spray per acre? The sprayer has 24 nozzles spaced 18.0 inches apart.

Solution: Because the application rate of field sprayers is speed-dependent, begin by selecting a reasonable speed that can be maintained in the field, and then determine the size of nozzles needed. For this problem we will use the typical speed (Appendix IV) of 6.5 mi/hr. The required flow rate for the nozzles can be determined by rearranging the standard sprayer equation to solve for the flow rate (gal/min).

2340 gal

5940 min

For this application rate and nozzle spacing, nozzles with a capacity of 0.39 gal/min should be installed on the sprayer. Before it is used, the sprayer should be calibrated to ensure that the application rate is correct because small variations in the construction of the nozzles or in the pressure at the nozzles can cause an unacceptable error in the application rate.

Assume the operator installed the 0.39 gal/min nozzles on the sprayer and proceeded with the calibration.

Problem: A container placed under all 24 nozzles of the sprayer collected 14.40 gal of spray in 2.0 min of operation. The desired application rate was 20.0 gal/ac. Is the sprayer accurate?

Solution: In this example only the total volume is known. One alternative is to determine the average flow rate per nozzle and then use the sprayer equation, but this process will not be as accurate as using the units cancellation method and the total flow rate:

/gal\ 14.40 gal 60.0 min 1 hr 1 mi 43,560 ft2 12.0 in R — = -x -x -x -x -x ac / 2.0 min 1 hr 6.5 mi 5,280 ft 1 ac 1 ft

1 nozzle 1

18.0 in 24 nozzles

451630080 gal

29652480 ac

The desired application rate was 20 gal/ac, but the calibration indicates that the application rate is 5 gal/ac less than this (20 gal/ac -15 gal/ac). It is important to check the label of the chemical to determine if this is an acceptable application rate. If the error is unacceptable, how do we reduce it? The first step is to check the filters and nozzles of the sprayer to make sure that one or more were not slightly restricted. If all of the nozzles are in proper working order, the sprayer must be adjusted to apply the correct rate.

Adjustments can be made in the speed and/or the system pressure. To adjust the speed the sprayer equation can be rearranged to calculate a new speed of travel. Adjusting the pressure is not as effective because only a small amount of change in the application rate can be made by adjusting the pressure. The pressure must be doubled to increase the flow rate by 41%, and modern boom type sprayers operate within a narrow pressure range. For this problem we will adjust the speed of travel.

Units cancellation could be used, but this time we will rearrange the sprayer equation to solve for the speed of travel. Because the sprayer equation requires the flow rate per nozzle, we will determine the average flow rate per nozzle. This average provides an acceptable level of accuracy.

Q gal 14.4 gal gal nozzle No. nozzle 24 nozzles and because the spray was collected for 2 min:


2.0 min

For convenience, the unit "nozzle" usually is not used; gpm = 0.30. Then:

gal min



gal 1 min gal

1782 160

If the speed of the sprayer is changed from 6.5 to 5.0 mi/hr, the sprayer will apply the correct rate.

Although the preferred way of calibrating a sprayer is to determine the nozzle size first, this method requires the purchase of a new set of nozzles if the correct size is not available. In some situations the nozzle size is selected first (the best available), and then the required speed of travel is determined. When this method is used, the calculated speed of travel may be unrealistic.

Problem: We need to apply 15.0 gal of spray per acre. Only one set of nozzles is available, and they have a capacity of 0.25 gal/min. The sprayer has 35 nozzles spaced 24.0 inches apart. What speed will be required to apply the correct rate?

Solution: The sprayer equation could be rearranged to solve for speed. Instead we will use the units cancellation method.

12 in 1

1,900,800 hr

In this problem, if the sprayer is operated at 4.1 mi/hr, the correct rate will be applied.

Other types of sprayers can be calibrated by using these methods if the appropriate adjustments are made for differences in how the area and the application rate are determined. For example, to calibrate the row crop sprayer in Figure 8.9, the width becomes the distance between the rows, and each nozzle should apply one third of the required flow (gal/min) per row.

Sprayers used for banding also can be calibrated. In checking a sprayer used for banding, the nozzle spacing (w) becomes the width of the band, Figure 8.8.

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