Section 5.3: Calibration Calculations and Application Rates (GPA formula)

Calibration Calculations and Application Rates (GPA formula)

Calibration is the process of adjusting and measuring application equipment to ensure that the correct amount of pesticide is applied to a target area. Applying too little pesticide (under-application) can result in poor pest control, wasted time, and the development of pest resistance. Applying too much pesticide (over-application) is illegal under federal and Massachusetts laws, wastes money, and can lead to crop injury, groundwater contamination, and severe regulatory penalties. This section provides a detailed breakdown of the standard Gallons Per Acre (GPA) calibration formula, worked step-by-step examples, the 1/128th of an acre calibration method, and dry granular calibration.

The Standard GPA Formula

For boom sprayers and broadcast liquid application equipment, the application rate is measured in Gallons Per Acre (GPA). The rate is determined by the nozzle flow rate, the travel speed of the sprayer, and the nozzle spacing on the boom. The standard formula is:

$\text{GPA} = \frac{\text{GPM} \times 5940}{\text{MPH} \times W}$

Where:

• GPA: Gallons Per Acre (overall spray application rate).
• GPM: Gallons Per Minute (the output flow rate from a single nozzle on the boom).
• MPH: Miles Per Hour (the travel speed of the sprayer).
• W: Nozzle spacing in inches (for boom sprayers) or the spray width in inches (for a single nozzle or broadcast system).
• 5,940: A constant value that converts minutes, miles, inches, and acres into matching units.

Worked Examples

Example 1: Finding the Application Rate (GPA)

An applicator is using a boom sprayer with nozzles spaced 20 inches apart. The measured output of each nozzle is 0.4 Gallons Per Minute (GPM) at 40 psi. The travel speed of the tractor is 5 Miles Per Hour (MPH). What is the application rate in Gallons Per Acre (GPA)?

1. Identify the variables:
   • $\text{GPM} = 0.4$
   • $\text{MPH} = 5$
   • $W = 20$
2. Plug the variables into the formula:
   • $\text{GPA} = \frac{0.4 \times 5940}{5 \times 20}$
3. Perform the calculations:
   • $\text{GPA} = \frac{2376}{100} = 23.76$
4. Conclusion: The sprayer is applying 23.76 Gallons Per Acre.

Example 2: Selecting the Correct Nozzle Size (GPM)

An applicator wants to apply a herbicide at a rate of 20 GPA. The tractor will travel at a speed of 6 MPH, and the nozzle spacing on the boom is 20 inches. What nozzle flow rate (GPM) is required?

1. Rearrange the formula to solve for GPM:
   • $\text{GPM} = \frac{\text{GPA} \times \text{MPH} \times W}{5940}$
2. Identify the variables:
   • $\text{GPA} = 20$
   • $\text{MPH} = 6$
   • $W = 20$
3. Plug the variables into the formula:
   • $\text{GPM} = \frac{20 \times 6 \times 20}{5940}$
4. Perform the calculations:
   • $\text{GPM} = \frac{2400}{5940} \approx 0.404$
5. Conclusion: The applicator must install nozzles that deliver 0.40 Gallons Per Minute at their selected operating pressure.

Example 3: Adjusting Speed to Meet a Target Rate

A sprayer is currently applying 15 GPA when traveling at 6 MPH. The applicator needs to increase the rate to 18 GPA to comply with a new product label. If the pressure and nozzles remain unchanged, what should the new speed be?

1. Understand the relationship: Spray rate (GPA) is inversely proportional to travel speed (MPH). If you slow down, the GPA increases.
2. Use the inverse ratio formula:
   • $\text{MPH}_2 = \text{MPH}_1 \times \frac{\text{GPA}_1}{\text{GPA}_2}$
3. Plug in the values:
   • $\text{MPH}_2 = 6 \times \frac{15}{18}$
4. Perform the calculations:
   • $\text{MPH}_2 = 6 \times 0.833 = 5$
5. Conclusion: The applicator must decrease the travel speed to 5 Miles Per Hour to achieve the target rate of 18 GPA.

Example 4: Calculating Pesticide to Add to the Tank

An applicator has a sprayer with a 300-gallon tank. The sprayer is calibrated to apply 15 GPA. The pesticide label recommends applying 2 pints of product per acre. How much pesticide product must be added to a full tank?

1. Calculate the acres treated per full tank:
   • $\text{Acres per tank} = \frac{\text{Tank Capacity}}{\text{GPA}} = \frac{300 \text{ gallons}}{15 \text{ GPA}} = 20 \text{ acres}$
2. Calculate the total product needed:
   • $\text{Product needed} = 20 \text{ acres} \times 2 \text{ pints per acre} = 40 \text{ pints}$
3. Convert product to gallons (8 pints = 1 gallon):
   • $\text{Gallons of product} = \frac{40 \text{ pints}}{8 \text{ pints/gallon}} = 5 \text{ gallons}$
4. Conclusion: The applicator must add 5 gallons of pesticide product to the tank, then fill the remainder with water to reach 300 gallons.

The Ounce Calibration Method (1/128th of an Acre Method)

The 1/128th of an acre calibration method (often called the Ounce Method) is highly popular because it simplifies calibration math. Because there are 128 fluid ounces in a gallon, and the test area represents 1/128th of an acre, the number of fluid ounces of water collected from a single nozzle over the test course is directly equal to the Gallons Per Acre (GPA) application rate.

Steps for the Ounce Method:

1. Determine the calibration distance: Measure the nozzle spacing in inches. Find the corresponding calibration distance in the field using the formula: $\text{Distance (feet)} = \frac{4080}{\text{Nozzle Spacing (inches)}}$. For example, if nozzle spacing is 20 inches, the distance is $4080 / 20 = 204$ feet.
2. Drive the calibration course: Drive the sprayer through the measured distance (e.g., 204 feet) at the gear and engine RPM you plan to spray at. Record the time in seconds (e.g., 28 seconds).
3. Collect nozzle output: Park the sprayer, set the pressure to the target level, and catch the discharge from one nozzle in a container graduated in ounces for the exact time recorded in Step 2 (e.g., 28 seconds).
4. Read the GPA: The number of fluid ounces collected is equal to the GPA. If you collected 16 ounces, the sprayer is applying 16 Gallons Per Acre.

Granular Applicator Calibration

Dry granular applicators must also be calibrated. Because granules vary in weight and size, you must calibrate the spreader for each specific product:

1. Measure a test area of 1,000 square feet (e.g., a path 10 feet wide by 100 feet long).
2. Place a known weight of granules in the hopper, set the dial to the manufacturer's starting point, and apply the product over the test area.
3. Weigh the granules remaining in the hopper. Subtract this from the starting weight to find the weight applied (e.g., 2.5 pounds applied over 1,000 sq ft).
4. Calculate the rate per acre:
   • $\text{Rate per Acre} = \frac{\text{Weight Applied}}{\text{Test Area}} \times 43560 \text{ sq ft/acre}$
   • $\text{Rate per Acre} = \frac{2.5 \text{ lbs}}{1000 \text{ sq ft}} \times 43560 = 0.0025 \times 43560 = 108.9 \text{ lbs per acre}$