Calibrating Chemical Application Equipment
Used to Treat Lawns and Ornamentals

Richard P. Cromwell
Agricultural Engineering Department
University of Florida
Gainesville, Florida 32611


Calibration of chemical application equipment enables the applicator of the equipment to determine the area (acres or square feet) treated by a given amount of spray. Knowing the area treated by a volume of spray, the applicator can determine from the pesticide label how much pesticide formulation should be added to the spray mixture.

Accurate calibration of application equipment (primarily sprayers) requires a constant output, speed, and swath spacing. Equipment used to treat lawns and ornamentals often cannot be calibrated as accurately as tractor powered equipment used in row crops because the output, the speed, and the swath cannot be precisely controlled. However, anyone applying chemicals with the hand-carried equipment commonly used in small landscaped areas (pump-up and backpack sprayers, handguns attached to long hoses, and small dry material spreaders) can and should calibrate as accurately as possible.

Calibration Procedures

Listed below are two equations for determining the amount of spray applied per unit of area:

     (1) Gallons per acre = GPA  = gallons per minute = GPM
                                   -----------------    ----
                                    acres per minute    ac/min


     (2) Gallons per 1000 sq. ft. = GPT = gallons per minute
                                           1000's sq. ft./min
Equation #1 is used when the area treated is relatively large and the label recommendation is given in rate/acre. Equation #2 is used when treating small areas like lawn and ornamental pest control workers often treat. The recommendations of many pesticides used in lawn and ornamentals work are often given in the amount per 1000 sq. ft. of area.

When applying dry materials the equation is:

       Pounds per acre = PPA =  Rounds per minute  =  PPM
                                ----------------     -----
                                 acres per minute    ac/min

Determining GPM

The gallons per minute (GPM) delivered by a sprayer can be determined by spraying for a timed period and measuring the amount of water needed to restore the initial level in the tank. GPM can also be determined by catching the flow from the nozzles on the sprayer for a timed period (this is the preferred method when a multi-nozzle boom is used because it indicates whether the nozzle output is uniform). The equation for GPM is:

           GPM = gallons
Example 1:A pump-up sprayer is filled to a readily identifiable level and the sprayer is run for 1/2 minute. It takes 26 ounces to refill the sprayer to the initial level. What is the GPM delivered by the sprayer.

The amount caught for the timed period must be converted from ounces to gallons.

            Gallons  =  26  = 0.203
where: 128 is the ounces in a gallon

           GPM  =  0.203  =  0.406
Example 2:A backpack sprayer has four nozzles on a short boom. The flow from each nozzle is caught for 30 seconds and the amounts caught are recorded. What is the GPM and is the nozzle uniformity acceptable, if + 10% from the average amount is allowed?

             Nozzle #       ounces/30 sec

               1                 18
               2                 22
               3                 18
               4                 17
                                 75  Total

              Average  =  75  =  18.75
              10% of the average = 0.10 x 18.75 = 1.87
Subtract 1.87 and add 1.87 to the average of 18.75 to determine the allowable range of nozzle flow rates.

      Range = 18.75 - 1.87 to 18.75 + 1.87 = 16.88 to 20.62
The only nozzle that should be rechecked and possibly changed is nozzle number 2. It delivered 22 ounces and the high end of the range was 20.62.

Assume that nozzle number 2 is changed and the uniformity test rerun and all the nozzles fell within the allowable range. If the total amount caught during 30 seconds is 72 ounces, what is the GPM?

          Ounces per minute  =  72  =  144

       72 is divided by 0.5 because 30 seconds is 30/60 = 0.5 minutes.

          GPM =  144  = 1.125

Determining The Acres Or Square Feet Treated Per Minute

The area treated per minute of time depends on the speed and swath of the sprayer. The equations that are most useful for determining acres and thousands of square feet treated per minute for landscape spraying are:

        (3) Acres per minute = swath (ft) x speed (fpm)

Where: swath is the width,treated per pass in feet
               fpm = feet per minute

               43560 = square feet in one acre

        (4) 1000's of sq. ft./min = swath (ft) x speed (fpm)
Example 3:An applicator is treating a turf area with a handgun on the end of a long hose. Determine the GPA applied by the applicator, if he swings the hose through a swath of 10 feet?

Determine GPM by catching the flow in a bucket for one minute.

Assume that 380 ounces were caught in a minute:

          380 ounces = 380/128 = 2.96 gallons

          GPM = 2.96, since the flow was caught for 1 minute
Determine acres per minute treated by the sprayer:

Acres per minute depends on the swath treated and the speed. The swath was given as 10 feet. The speed in feet per minute is determined by timing how long it takes the applicator to travel a known distance at a steady pace.

Assume it takes 28 seconds to travel 100 feet:

        fpm = feet traveled

        28 seconds is 28/60 = 0.467 minutes

        fpm  =  100  =  214

        Acres per minute = swath x speed = 10 x 214 = 0.049
                           -------------   --------
                               43560         43560
Determine gallons per acre applied by the hand gun:

          GPA =   GPM   =   2.96  =   60.4
                ------     ------
                ac/min      0.049
Once the gallons of spray being applied to each acre of area are known, the applicator can determine the number of acres that will be treated by the amount of spray in the sprayer tank and the amount of pesticide that must be added to the spray mixture to apply the amount recommended on the pesticide label.

The acres covered by a given volume of spray can be found by dividing the volume in gallons by the gallons per acre being applied.

           Acres treated = gallons of spray
                           gallons per acre
Assume the applicator above that was applying 60.4 GPA has a truck mounted sprayer and is mixing 400 gallons of spray mixture. How many acres would the mixture treat?

           Acres treated  =  400  =  6.6 acres

If the pesticide used is recommended on the label at 3 pints per acre, how much pesticide should be added to the 400 gallons of spray mixture?

    Amount of pesticide =  acres treated x amount per acre

                        = 6.6 x 3 = 19.8 pints

Label Recommendations per 1000 Square Feet

Chemicals used to treat small landscaped areas often have recommendations given in the amount per 1000 square feet rather than the amount per acre. The gallons per acre applied in Example 3 was 60.4.

Since there are 43560 square feet in an acre, there are 43560/1000 or 43.56 areas containing 1000 square feet in an acre.

Example 4:Determine the amount of chemical to add to 200 gallons of spray mixture, if the GPA is 60.4 as in Example 3. Consider that the chemical is recommended on the label at 1.5 ounces per 1000 square feet.

Determine the gallons applied per 1000 square feet by dividing the gallons per acre by 43.56, the number of 1000 square feet areas in an acre.

        Gallons per 1000 square feet =  60.4  =  1.38

Determine the number of 1000 square feet areas that can be treated by 200 gallons of spray mixture by dividing the gallons applied per 1000 square feet into the amount of spray mixture or 200 gallons.

        Number of 1000 sq. ft. areas treated =   200  = 145

The amount of pesticide in the 200 gallons of spray is determined by multiplying the number of 1000 square feet areas treated by the 200 gallons times the amount recommended on the label per 1000 square feet.

        Amount to add = 145 x 1.5 = 217 ounces

To convert pesticide label recommendations from rate per 1000 sq. ft. to rate per acre or from rate per acre to rate per 1000 sq. ft. use the formulas below:

        Rate/1000 sq. ft.  = rate/acre

        Rate/acre = rate/1000 sq. ft. x 43.56

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