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Fine Tuning a Sprayer with "Ounce" Calibration Method



Authors as Published

Robert Grisso, Extension Engineer, Biological Systems Engineering Department; Mike Weaver, Coordinator of PAT, Entomology Department; Kevin Bradley, Postdoctoral Associate, Plant Pathology, Physiology and Weed Science Department; Scott Hagood, Professor, Plant Pathology, Physiology and Weed Science Department; and Henry Wilson, Professor, Eastern Shore Research and Extension Center; Virginia Tech

This extension publication discusses guidelines to quickly evaluate the performance of a sprayer. Sprayer calibration, nozzle discharge, spray pattern uniformity, speed checks, pump performance and plumbing arrangements are evaluated with minimal calculations.

Tractor-mounted, pull-type, pick-up-mounted and self-propelled sprayers are available from numerous sources. Rising chemical costs and new low rate chemicals are making accurate application more important than ever before. Proper calibration must be a primary management consideration whether one is a farmer or a custom applicator. Since most pesticides are applied with hydraulic sprayers, users should also know proper application methods, chemical effects on equipment, and correct cleaning and storage methods for hydraulic sprayers.

Proper sprayer application depends on the combination of six basic properties. These are:

  • sprayer design,
  • nozzle type,
  • boom height,
  • boom pressure,
  • agitation, and
  • ground speed.

Chemicals will be applied correctly when these six components are used in the right combination and chemicals have been properly mixed. Properly applied pesticides are expected to return a profit. Improper application can result in wasted chemical, marginal weed, insect, or disease control, excessive carryover, groundwater contamination, and/or crop damage. Thus, inaccurate application is usually very expensive in terms of cost and decreased income.

Preseason visual checks are not adequate for accurate application, nor is the fact that the equipment and nozzles are new. In several surveys, only one of three applicators was applying within +/- 5 percent of estimated rate. Thus, checking all nozzles for correct flow rates and uniform application, and calibrating the sprayer cannot be eliminated. Manufacturers' catalogs are guidelines, but fine-tuning of a sprayer is the operator's responsibility. Sprayers should be calibrated every time a different pesticide is applied. In addition, a sprayer should be recalibrated at least every other day when in continuous use. Since these checks may have to be performed often, evaluating a sprayer quickly and without excessive investment or calculation is very important. There are a number of calibration techniques. The following outlines a method for quick sprayer calibration.

Determining Gallons per Acre (gpa)

The purpose of any calibration method is to determine the number of gallons of solution being applied per acre. Subsequently, the gallons of solution applied per acre can be used to determine the quantity of active ingredients to add in the spray tank. The following method requires: a stopwatch, a container for collection, a tape measure, marking flags, and a scale or container graduated in ounces. The procedure is as follows:

STEP 1. Select the distance according to the nozzle spacing on the broadcast boom shown in Table 1. When using a directed or banding rig, use the row spacing. Measure the correct distance in a level field. The distance measurement may be marked off in a permanent fashion. The travel area should be typical of the surface and soil conditions of the area to be sprayed. Many tractors and sprayers will gain or lose in excess of 10% of desired travel speed while moving up and down slopes. If field variations are wide, several areas may be needed. Remember, this traveled distance will give a clue to the actual speed the sprayer is going under field conditions, therefore the measured distance and timing must be exact.

STEP 2. Drive and time the sprayer in seconds (Figure 1) at the throttle setting and load that normally occurs during spraying (spray tank should be 1/2 to 2/3 full) over the marked area. Engage the incorporation equipment (disk, planter, etc.) or other devices used while spraying. Do not change the gear or throttle setting after you have chosen a spraying speed. A change in ground speed will change the sprayer application rate and this will require recalibration.



tractor Figure 1. Measure the appropriate length for the nozzle spacing, then measure the time to run the course.


STEP 3. While in a stationary position, bring the power unit to the proper throttle setting and sprayer to the correct boom pressure used in Step 2. Catch the nozzle discharge for the time recorded in Step 2. Measure the discharge in ounces (Figure 2) with a graduated container or scale. If more than one nozzle per row is used (direct, insecticide or fungicide rigs), catch the discharge of all nozzles within the row spacing selected in STEP 1. Then combine the amount of discharge from all nozzles spraying in that row.



boom pressure Figure 2. In stationary position and proper boom pressure, measure the output over the same time period to make the run. The number of ounces collected is equal to the gallons per acre.


Remember, from a safety point of view, the collection of discharge should be done using clean water only! Even while collecting water, use the proper personal safety clothing and protection.

STEP 4. The total discharge measured in ounces is equal to gallons per acre (gpa) applied. Since this calibration was based on water, conversion factors (Table 2) must be used when spraying solutions that are heavier or lighter than water. Multiply the observed rates by the conversion factors to attain accurate rates using the spray solutions.

Example of "Ounce" Calibration Suppose a sprayer was set-up with 30 inch nozzle spacing. Step 1. Using Table 1, 136 feet was marked off and the sprayer was driven through and timed. Step 2. The time recorded was 21 seconds. According to Table 1, the travel speed was about 4.5 mph. Step 3. In a stationary position, the sprayer was brought to the proper pressure. A nozzle was collected for 21 seconds as found in Step 2. The discharge was measured to be 15 fluid ounces. Step 4. The calibration was 15 gallons per acre. Several nozzles should be measured and averaged from different sections on the boom (ie., left, right, and center) for a more reliable measurement.

Suppose the actual carrier will be 28% fertilizer and not water. The carrier rate should be adjusted. Using Table 2, the carrier rate would be: [15 gpa * .885 (Table 2)] = 13.3 gpa. This would be the value used to determine the proper amount of active ingredient to add to the spray tank.

Calibrate Frequently... The "ounce" calibration method describes a procedure without calculations in order to evaluate a hydraulic sprayer. Most farmers use the "Known Area" calibration method. This method requires the operator to know a given area and determine the number of gallons of liquid applied to that area by the sprayer. The gallons per acre can be calculated by dividing the number of gallons applied by the known acres covered. The method is a useful field check but should not be the primary method because an error of misapplication can occur before it can be detected.


Table 1. Calibration distances and speeds for varying nozzle or row spacing.

Nozzle or Row Spacing (in)Calibration Distance (ft)Time in Seconds for Various Ground Speeds (mph)*

* 1 mph = 88 feet per minute
+ Note: for times less than 20 seconds (shaded area) improved accuracy can be attained by doubling the collection time (STEP 3.) and dividing the output collected by two.


Table 2. Conversion factors for solutions with densities different than water.
Density of Solution (lb/gal)Specific GravityConversion Factors
8.34 - WATER1.001.000
10.65 - 28% nitrogen solution1.28.885
11.0 - 7-27-7 fertilizer1.32.871
11.06 - 32% nitrogen solution1.33.868
11.40 - 10-34-0 fertilizer1.37.855
11.50 - 12-0-26 fertilizer1.38.852
11.60 - 11-37-0 fertilizer1.43.848

Band Applicator Calibration

The same calibration methods can be used to calibrate band applicators as were used for broadcast spraying. The only difference is the amount of area being covered. Total acres refers to the entire acreage in the field. This would include the sprayed band and the area between the bands. A treated acre refers only to the treated area in the band. The spray that would be discharged by the broadcast rate is concentrated in a narrow band by the ratio of row spacing to band width (Table 3). In band spraying, the row spacing and the nozzle spacing are the same. After performing the Steps 1-4 of the above procedure, multiply the answer by the appropriate conversion factor to attain band rate.

Unless otherwise specified, chemical application rates are given on a broadcast basis. For band applications, the rate per treated area is the same as the broadcast rate, but the total amount of pesticide used on a field is less because only a portion of the field is treated.

Nozzle Discharge and Uniformity Check
Nozzle condition is very important for uniform application. Observe the spray pattern from each nozzle. If the spray pattern is not uniform from visual observations, large output variations exist. Often poor spray patterns are due to clogged nozzles and strainer components. To clean a metal nozzle or strainer use an old toothbrush or a wooden toothpick. For plastic tip nozzles use only a nylon (rather than brass fibers) toothbrush (special nozzle cleaning brushes are available). Never use a metal object when cleaning a nozzle or strainer.

Check nozzle discharge uniformity (Figure 3) by repeating STEPS 3 and 4 above for all nozzles. 


tractor with sprayer Figure 3. Check the discharge from each nozzle across the boom.


If variation of output is not within 10 percent of the average output of the nozzles - replace all the nozzles. After adjustment or correction, recalibrate. To calculate the gallons per minute (gpm) discharge of a nozzle, use:

gpm  =  ounces collected x 0.47 / time (in seconds)

The flow rate (gpm) can be adjusted slightly by changing pressure. However, to double the output, the pressure will need to be increased four times. High pressures (>45 psi) may exert excessive strain on sprayer components, increase wear on the nozzles, and produce drift-susceptible spray. Low pressures (<10 psi) will not develop a full width spray pattern for each nozzle and coverage may not be adequate for contact pesticides. If more discharge is required, the best methods are to increase the nozzle size or to slow down the travel speed. Remember the limits because most nozzles perform best over a limited pressure range.

The relationship between gallons per acre (gpa) and nozzle output in gallons per minute (gpm) can be determined by using the sprayer speed in miles per hour (mph) and spray width in inches (w) using the following equation:

gpa = (5940 x gpm) / (mph x w)

where: w = nozzle spacing in broadcast or band spraying, spray swath of boomless spraying or row width of directed rigs.

Do not mix nozzles of different materials, types, discharge angles, or gallon capacity on the same sprayer. Mixing of nozzles will produce uneven spray patterns. Select the correct type of nozzle spray pattern for the intended job. Know the specific use of a nozzle tip. One nozzle type will not meet all spraying needs, so plan ahead.

Another factor influencing spray uniformity is boom height. Boom height controls the amount of nozzle pattern overlap. Best performance of a broadcast sprayer is when the nozzle spacing, height, and angle provide for 100% overlap at the target height. Booms should be relatively rigid in all directions. Swinging back and forth or up and down is undesirable. Gauge wheels mounted near the boom ends should help to maintain a uniform boom height.

The primary way to check uniformity at the ground surface or target height of broadcast and banded rigs is the use of corrugated spray tables or spray tapes. Due to the expense, these devices are probably not practical for individual ownership. However, a quick check of uniformity can be performed by spraying over a hot slab of concrete. Then watch the dry patterns. Those areas that dry faster may indicate that proper overlap has not been achieved.

Table 3. Conversion factor to convert broadcast rate (gallons per
total acre) to band rate (gallons per treated acre).
Band width (in)Row Spacing (in)

Ground Speed Check

A survey of field sprayers indicated 65 percent of the operators had errors of greater than 5 percent in estimated travel speed. STEP 2 of the described sprayer evaluation method provides a clue to the actual speed the sprayer was traveling under field conditions. Accurate application requires that the operator know exactly how fast the sprayer moves over the ground surface. Because of wheel slippage and rough surface conditions, the actual rig velocity is often different from the tachometer and speedometer readings. For a more accurate measurement of travel speed mark off a distance of 220 feet. Drive and time the operation as in STEP 2. The speed is calculated as:

mph = 150 / seconds timed

Pump Pressure Check
The pump output should have at least 20 percent more capacity than the largest volume required by the nozzles. A pump with reserve capacity will allow for proper agitation and deliver the necessary volume as the pump wears (see VCE Publication 442-452 - Plumbing Systems of Agricultural Sprayers).

The pump should be evaluated if the system is not receiving adequate pressure. In order to check pump capacity, disconnect the outlet side of the pump and put the hose in a 55 gallon drum. Run the pump at the throttle setting normally used during spraying. Measure the time (in minutes) required to fill the drum. Calculate the pump capacity by dividing 55 gallons by the minutes measured. If the pump's capacity is too small or excessively worn, replace it.

Inadequate hose size could also cause pressure reductions. If the hose size is unable to carry the volume of fluid needed, large pressure drops will result. Use guidelines in Table 4 to evaluate the suction and discharge lines. Remember, plugged screens and nozzles can also reduce spray output.

Strainers and Screens Check
The sprayer should be equipped with several strainers and screens. The more strainers and screens used, the less chance a nozzle will become plugged. A 10 to 20 mesh basket strainer should be used in the tank manhole. This strainer will stop large items such as label booklets. Sometimes these booklets are loosely attached to the container and can fall into the tank while adding chemicals.

A 50 mesh strainer should be placed on the outlet side of the pump and should be frequently cleaned. Using a large screen on the inlet side of the pump will keep sand, gravel, or debris from damaging the pump. Additional strainers should be placed after the flow control assembly. Cleaning a strainer is easier than cleaning each nozzle screen. Each nozzle should have a 50 to 100 mesh screen to stop any particles that may plug the nozzle. Check the manufacturer's recommendations for the exact screen size. Remember, partly plugged orifices and screens are hard to detect, so periodically clean them.

Spray Monitor and Controller Check
Spray monitors and controllers are becoming more popular to achieve uniform application, but they do not eliminate the good practices of sprayer inspection and calibration. System monitors record the operating conditions such as travel speed, pressure, and/or flow rate. Spray controllers are monitors with the added capability of automatic rate control. The controller goes a step further than a monitor and receives the actual application rate from the monitors and compares it to the desired rate. If a difference exists, the controller will adjust the application rate automatically (usually by adjusting pressure). Since these adjustments are a direct response to various sensors, it is important that these sensors are periodically calibrated. Do not assume that the monitors are fool-proof. Consult the manufacturer's operators manual to properly calibrate and adjust the sensors.

Table 4. Recommended hose sizes.
Pump output (gpm)SuctionDischarge
(inside diameter inches)
< 123/45/8


Spray equipment in good condition will apply chemicals properly if frequently calibrated and correctly operated. Manufacturer's manuals include tables to show rates of application for various nozzles, pressures, and ground speeds. Use this information to initially set up the sprayer, then use the methods described in the publication to evaluate and adjust the sprayer for accurate application.


The authors would like to express their appreciation for the review and comments made by Dennis Duncan, Assistant Professor, Agricultural Technology; Keith Dickinson, Extension Agent, Fauquier County; Samuel M. Johnson, Extension Agent, Westmoreland County; Lloyd Hipkins, Senior Research Associate, Plant Pathology, Physiology and Weed Science Department; and Mike Parrish, County Agent, Dinwiddie County.

Weight and Measures Conversions

16 ounces = 1 pound = 453.6 grams
1 gallon water = 8.34 pounds = 3.78 liters

Liquid Measure

1 fluid ounce = 2 tablespoons = 29.57 milliliters
16 fluid ounces = 1 pint = 2 cups
8 pints = 4 quarts = 1 gallon


3 feet = 1 yard = 91.44 centimeters
16.5 feet = 1 rod
5280 feet = 1 mile = 1.61 kilometers
320 rods = 1 mile


9 square feet = 1 square yard
43560 square feet = 1 acre = 160 square rods
1 acre = .405 hectare
640 acres = 1 square mile


88 feet per minute = 1 mph
1 mph = 1.61 km/h


27 cubic feet = 1 cubic yard
1 cubic foot = 1728 cubic inches = 7.48 gallons
1 gallon = 231 cubic inches
1 cubic foot = 0.028 cubic meters

Common Abbreviations and Terms Used:

gpm = gallons per minute
gpa = gallons per acre
psi = pounds per square inch
mph = miles per hour
rpm = revolutions per minute
gph = gallons per hour
fpm = feet per minute

Reviewed by Bobby Grisso, Extension Specialist, Biological Systems Engineering


Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.


Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.


May 1, 2009