Proper application of pesticides and fertilizers, which protects water quality, is possible only with a sprayer or spreader that is calibrated accurately. Pesticides applied with equipment that has not been calibrated may be misapplied by more than 10%. This may lead to repeat applications, damaged plants, excessive costs, and contamination of the environment.
Sprayers should be calibrated after every fourth application. The sprayer system and nozzles should be checked before every use. The time it takes to gather the information needed to calibrate a sprayer will offset any losses from an improperly calibrated sprayer.
There are three main calculations used in calibrating a sprayer:
- Miles per hour — speed of the sprayer
- Gallons per acre — amount of output over a given area
- Gallons per minute — rate of output
These calculations are used to determine or change the appropriate output for a given area.
Miles Per Hour (MPH)
Before you can make any other calibration calculations, you need to know the speed of the sprayer.
- Mark off a 200 ft course on terrain that is typical of your area.
- Fill the spray tank to 1⁄2 full.
- Extend the booms of the sprayer.
- Test to determine which gear and RPM will allow the sprayer to maintain 40 PSI pressure on the nozzles, while maintaining a constant speed of between 3 and 5 MPH.
Drive the sprayer through the 200 ft course at least three times, and record the length of time of each run (in seconds). Start far enough away from the course so that the sprayer can attain the desired speed before reaching the first marker and maintain it through the run.
Average the three test run times.
Average time of sprayer to cross 200 ft = (Time 1 + Time 2 + Time 3) ÷ 3.
Fill in the appropriate course distance and average time (in seconds) into the following formula to calculate the miles per hour (MPH) of your sprayer at the selected gear and RPM setting.
- Calculate miles per hour (MPH) = (distance (ft) × 60) ÷ (average time (s) × 88).
You mark your 200 ft distance, made three test runs, and logged the following
times: Time 1 = 31.5 seconds, Time 2 = 30.3 seconds, Time 3 = 29.2 seconds.
Determine the average time as follows:
Average time = (31.5 + 30.3 + 29.2) ÷ 3 = 30.3.
Fill in the average time to determine the MPH.
MPH = (distance (200 ft) × 60) ÷ (average time (30.3) × 88) = 4.5.
Gallons per Acre (GPA)
Now that you have calculated the speed of your sprayer, you need to determine the output. The number of gallons per acre can be computed from the output of a single nozzle.
- Determine the gallons per minute (GPM) of the test nozzle, measured in tenths of a gallon. Each type of nozzle has a specified GPM. For more information on GPM, refer to the next section.
- Measure the width in inches between two nozzles on the boom.
Calculate the gallons per acre.
GPA = (GPM × 5,940) ÷ (MPH × width)
Assume the following information has been gathered from your sprayer:
- GPM = 0.5 (output of a single nozzle is 1⁄2 gallon per minute)
- MPH = 4.5 (as calculated from the MPH formula)
- Width = 18 in. (measures distance between two nozzles on the boom of the sprayer:
GPA = (0.5 × 5,940) ÷ (4.5 × 18) = 2,970 ÷ 81 = 36.67
To determine the equivalent gallons per 1,000 sq ft, divide the GPA by 43.56.
Gallons per 1,000 sq ft = 36.67 GPA ÷ 43.56 = 0.84
Gallons per Minute (GPM)
If the GPA that is calculated according to the formula does not meet the requirements of a specific application, the rate of output can be adjusted by switching the nozzle. To determine the nozzle with the correct output, use the GPM formula.
- Determine the rate of output required for a specific application.
- Calculate the gallons per minute.
GPM = (GPA × MPH × width) ÷ 5,940
Since you have a pesticide that must be applied in 22 gallons of water per acre, 33.88 GPA from the calculation is too much. You can do one of two things: increase the speed of the sprayer, or change the output of the nozzles. You should not increase the speed of the sprayer because you are already close to the maximum recommended speed of 5 MPH. However, you can easily change the output of the nozzles just by switching to a nozzle with a lower output.
Using the same information from the GPA example, but by substituting 22 GPA for the 33.88 GPA, we calculated for the sprayer:
GPM = (22 GPA × 4.5 MPH × 18) ÷ 5,940 = 0.30
- You can then change to nozzles that release 0.30 GPM to apply 22 gallons of mixed material per acre.
As mentioned in the calculation of the GPA, each type of nozzle has a specific output rate. This specification includes the spray angle and the output rate of the nozzle in gallons per minute.
A flat fan nozzle, which has a specification number of 8003, sprays at an angle of 80 degrees and has an output rate of 0.3 gallons per minute.
Nozzles wear out over time, and this can increase their output and potentially affect plant health and the environment. To ensure that nozzles are applying the correct output, calculate the percent error for a nozzle:
- Set the proper PSI for the nozzles on the boom. For many nozzles, the rated pressure is 40 PSI.
- Start the spray flow for the boom by using only water.
- Collect the water sprayed from a single nozzle in a large measuring cup for 1 minute. The cup should be reserved for use only with pesticides and calibration tests.
- Divide the total ounces in the cup for 1 minute by 128 (number of oz in 1 gallon) to find the GPM.
GPM = oz collected ÷ 128
- Subtract the actual GPM from the GPM on the nozzle number to find the difference in GPM.
- Divide the difference in GPM by the GPM on the nozzle number to determine the percent error in the nozzle. If a nozzle is off by more than 10% (0.10), replace it with a new one. If more than one nozzle is off by more than 10%, replace all nozzles on the boom.
You have 8003 nozzles on your boom. You collect 75 oz in one minute from the first nozzle.
GPM = 45 oz collected ÷ 128 oz per gallon = 0.35
0.35 actual GPM − 0.30 specified GPM = 0.05 difference in GPM
0.05 difference in GPM ÷ 0.30 specified GPM = 17% difference
Because the difference in actual output compared to the specified output is more than 10%, the nozzle must be replaced. Repeat these steps to test a second nozzle. If the second nozzle is off by more than 10%, replace all nozzles on the boom.
- Fill the sprayer tank 1⁄2 full with water. Never add pesticides to a sprayer before it is checked for leaks and operating condition.
- Check the tank, hoses, and fittings for leaks.
- Start the sprayer's engine to begin circulating the water through the system.
- Check all hoses, fittings, and the pump for leaks. Take the time to fix any leaks. A small leak at the beginning of the spraying operation may quickly turn into a break while spraying.
- Turn on the spray boom(s) and check for leaks in the hoses to the boom, the nozzle hoses, and the nozzle assemblies.
- Remove and clean the main in-line filter.
- Remove each nozzle on the boom, and clean and replace the screens, as necessary.
- With the nozzles removed, start the pump to flush any debris from the system.
- Return the nozzles to their assemblies, making sure that the spray pattern is in line with the boom. However, if the nozzles are the flat fan type, the spray pattern should be slightly off line to allow for a 30% overlap.
- Check the spray pattern from each nozzle. If a pattern is inconsistent, replace the nozzle with a new one of the same style and output volume as the others on the boom.
- Adjust the distance between nozzles and their height above the ground to the recommended measurements. There should be a 30% overlap for flat fan nozzles and a 100% overlap for hollow cone nozzles.
Proper maintenance, operation, and calibration of spreaders are essential to optimize the effectiveness of fertilizers, pesticides, and other materials applied to turf. Calibration involves determining the distribution pattern and the application rate of the material being applied for a particular spreader. Techniques for measuring the distribution pattern and the application rate depend on the type of spreader.
Rotary spreaders cover a wider area more quickly and are preferred for larger areas. Rotary spreaders have distribution patterns that are more forgiving of operator error, require a lower push effort, have better ground clearance, and have less delicate rate mechanisms, which allow them to maintain their calibration longer over time. However, proper calibration will require a determination of optimum overlap to achieve a uniform application of material.
Drop spreaders have uniform and consistent patterns, low drift potential, and precise control of the edges of the patterns, which is useful for edging around small areas such as driveways and flower beds, and near environmentally sensitive areas such as ponds or streams.
Spreader Calibration Factors
Four factors that affect the distribution patterns of a rotary spreader (and to some extent, drop spreaders) are the spreader design and product, as well as environmental and operator factors.
The manufacturer determines impeller (paddle or spinner) characteristics such as diameter, speed (gear ratio), concavity (flat or concave), height above the ground, fin shape and angle, and surface characteristics. The radial drop point of the material onto the impeller may be adjusted on some spreaders that are equipped with an adjustable port or a pattern adjustment.
Characteristics of the application product (fertilizer or pesticide) such as particle shape, size, size uniformity, density, critical relative humidity (the humidity at which the product becomes sticky), and surface friction affect distribution.
Wind speed and direction are two obvious factors that affect distribution. Temperature and relative humidity are also influential. Applicators must decide if applications should be made under current weather conditions or postponed until the conditions become acceptable.
The operator controls the walking speed, handle height, and pattern of travel.
To determine a valid indication of spreader performance, conduct the calibration under conditions similar to the actual operation. Ground speed, rate and pattern settings, operator, wind speed and direction, terrain, temperature, humidity, and the application product should all be similar.
For the calibration of all types of spreaders, select a walking speed suitable for the applicator. Three miles per hour is considered a reasonable walking speed. Pacing devices are available that can help achieve a consistent walking pace. For the purposes of calibration, use the setting suggested by the spreader’s manufacturer.
Calibration of rotary spreaders involves collecting and weighing the material being spread and analyzing the pattern of distribution across the application area.
- Place collection pans in a line perpendicular to the travel direction of the spreader. Use at least 10 pans to cover the full width of the spread. Twenty to thirty pans may be required for large spreaders.
Note: All collection pans must be identical in size. One square foot by 1 to 2 in. deep for small spreaders, and 2 to 4 in. deep for larger spreaders, is recommended. Rectangular pans are preferred.
Pans should be padded or baffled with “ribs” that prevent the contents from coming out of the pans. Material bouncing into or out of the pans can affect results.
- Make several passes in the same direction over the pans. Make sure the spreader is open before reaching the pans and remember to walk at the same speed you will use to distribute the material later.
- Collect and weigh the material in each pan on an accurate scale (grams are the preferred unit). The data collected will be used to determine the distribution pattern and the application rate.
- Empty each collection container into separate identical cylindrical tubes and examine the distribution pattern across the series of tubes. A desirable pattern is one that peaks in the center, descends evenly on each side, and forms a bell-shaped curve. Unlike the drop spreader, some overlap of the pattern will be required to achieve a uniform distribution of the material, due to the bell-shaped nature of the distribution pattern of the rotary spreader. It is important that similar quantities of material are being applied to the left and the right of the spreader. Skewing of the spread may be corrected by repositioning the pattern adjustment control, if the spreader has one, or restricting the discharge ports.
- Determine the width of turf covered by each pass of the spreader, where the tray on the left and right are equal to one-half the amount in the center tray.
- Use the weight of material collected from the pan in the center of the pattern in the equation below to calibrate the spreader’s output:
Grams material per 1,000 sq ft = (1,000 × grams material collected in center pan) ÷ (ft2 pan × # of passes)
Continue this process until the desired rate is achieved, either by increasing or decreasing the spreader setting by the size of the discharge ports.
Calibration of drop spreaders involves collecting and weighing the material being spread.
Push the spreader over a pan(s) and collect and weigh the material that was spread. By knowing the area of the pan and weight of the material, you can determine the application rate by:
Grams of material per 1,000 sq ft = (1,000 × grams material collected) ÷ (ft2 pan × # of pans × # of passes)
Sweep and Weigh Method
Push the spreader over a clean, smooth surface of a known distance, sweep, and weigh the material. The application rate can be determined by:
Grams material per 1,000 sq ft = (1,000 × grams material collected) ÷ (spreader width × distance traveled)
Catch Pan Method
Attach a catch pan to the bottom of the drop spreader. Mark two points of a known distance. Push the spreader over the known distance, opening the hopper at the starting point, and closing at the finish point, while collecting the dropped material in the catch pan. The application rate is determined with the following formula:
Grams of material per 1,000 sq ft = (1,000 × grams material collected) ÷ (spreader width × distance traveled)
Whichever method is used, make enough passes or travel enough distance so that sufficient quantity of collected material can be weighed accurately. If the calculated rate is too high, reduce the setting adjustment. If it is too low, increase the setting adjustment. Continue this process until the desired rate is achieved.
Some general tips on operating a spreader:
- Make sure the distribution lever is closed before filling the hopper.
- Do not overfill the hopper.
- Make sure the screen filter is in place to prevent clogging.
- Push, and do not pull the spreader.
- Push the spreader as close to the calibrated speed as possible.
- Start walking before opening the lever; close the lever before stopping forward motion.
- Hold the handle at the height used in calibration. The impeller should be level.
- Walk in straight lines using reference points such as the spreader wheel marks or footprints.
- Do not spread while turning. Generally, only one wheel drives the impeller. The impeller will either speed up or slow down during a turn, which will affect the distribution pattern.
- Do not make sensitive applications if the wind speed is greater than 5 miles per hour.
- Keep material dry to prevent caking and clogging of the ports.
- Buy products with good application characteristics.
Even a properly calibrated spreader will not perform well if it is not operated or maintained correctly. Here are some operation and maintenance tips.
- Wash the spreader after each day's use to avoid buildup around ports and on the impeller. Cold or hot water is adequate for cleaning water soluble products. When using plastic or waxy products, a solvent or scraping may be required. Be sure to consult the manufacturer for appropriate solvents.
- Consult the operator's manual for the appropriate lubricant and parts that require lubrication.
- Use proper replacement parts and keep frequently used parts on hand.
- Store spreader in a cool, dry place without a load in the hopper.
- Remember that proper maintenance will increase the life of your spreader.
Publication date: July 22, 2022
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