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Calibration and understanding of equipment flow rates promote proper application rates, herbicide efficiency, and herbicide
effectiveness. The following table and worksheet are intended to help calibrate directed herbicide applications using a spray
gun or backpack sprayer. The flow rate (in gallons per minute, GPM) must be known to determine the time it takes to treat
either 1 acre or 1,000 square feet. The recommended application rate of the herbicide solution in gallons per acre (GPA) can
be found on the herbicide product label.
The table shows typical flow rates and calculated spray times. The worksheet below can be used to determine spray times
for specific calculated flow rates. The time to fill a 1-gallon container in seconds (A) can be used to calculate the flow rate in
GPM (B), which can then be used to calculate spray times for 1 acre (C) and 1,000 square feet (D).
Once the worksheet is complete and the time required to spray an area is determined,
the next step is to practice with water and dye. The applicator should practice until the
1,000-square-foot area can be covered in the amount of time calculated in the worksheet
above (line 5). The photographs below demonstrate how to set up a 1,000-square-foot practice area.
CALIBRATION FOR SPRAY GUN OR BACKPACK SPRAYER WORKSHEET
CALCULATE FLOW RATE IN GALLONS PER MINUTE:
1. Determine the seconds it takes to fill a 1-gallon bucket with spray gun or backpack nozzle = (A) seconds
2. 60 (seconds per minute) ÷ (A) seconds to fill a 1-gallon bucket with spray gun or backpack nozzle =
(B) (GPM) flow rate
CALCULATE MINUTES TO SPRAY 1 ACRE:
3. Check the herbicide label for the recommended GPA (GPA) application rate ÷ (B) (GPM) flow rate =
(C) minutes to spray 1 acre
CALCULATE SECONDS TO SPRAY 1,000 SQUARE FEET:
4. 0.02296 (acre per 1,000 square feet) × (C) minutes to spray 1 acre = minutes to spray 1,000 square feet
5. 60 (seconds per minute) × minutes to spray 1,000 square feet = (D) seconds to spray 1,000 square feet
SAMPLE CALIBRATION TIMES
60 seconds
30 seconds
20 seconds
15 seconds
12 seconds
1 GPM
2 GPM
3 GPM
4 GPM
5 GPM
100 minutes
50 minutes
33 minutes
25 minutes
20 minutes
138 seconds
69 seconds
46 seconds
34 seconds
28 seconds
SECONDS TO FILL
1 GALLON
(A)
FLOW RATE IN GALLONS
PER MINUTE (GPM)
(B)
SPRAY TIME
MINUTES TO
SPRAY 1 ACRE
(C)
SECONDS TO SPRAY
1,000 SQUARE FEET
(D)
