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Determining the Amount of Chemical
The main reason for calibrating a sprayer is to determine the amount of formulated chemical to add to a tankful of water. The following must be known:
- The calibrated output of the sprayer.
- The measured capacity of the sprayer tank.
- The recommended rate of application.
The operator needs to know the application rate so the area per tankful can be calculated. With a knapsack sprayer, the area likely will be less than an acre. Check the capacity of the sprayer tank; decals or molded marks often are in error, and foreign-manufactured tanks may be marked in metric units. The operator must know the percentage of an acre covered by a tankful. By multiplying this percentage by the recommended application rate per acre, the operator can find the amount of chemical required for each tankful.
With a handgun, a tankful may cover several acres with an herbicide; perhaps less with insecticides and fungicides.
Example: If a knapsack sprayer has a 4-gallon tank and is applying 16 gallons per acre, then a tankful will cover 0.25 acre. If an herbicide is recommended at 3 pounds per acre, then 0.75 pound should be added to each tankful of water.