Herbicides available through the County are typically low use rate products. This means that a little bit goes a long way and small differences in calibration make a huge difference in the product's performance. Even if you are spot spraying, calibration is critical to ensuring that your spray is effective without damaging desirable plants (including grasses).
When calibrating, only use water. Do not add herbicides for this procedure.
How much chemical do you add to the tank?
An herbicide label specifies the amount to be applied per acre. What is added to your tank depends on the Gallons Per Acre (GPA) your sprayer puts out. GPA depends on:
The speed at which the pesticide is applied.
The output of your nozzle(s).
The pressure of the sprayer.
Product Mixing Rate (oz per gallon, lbs per gallon, etc.) = Application Rate (oz, fl oz, lbs) /GPA
Backpack sprayer calibration
Measure and mark an area that is 18.5' by 18.5' or equivalent (area is 1/128th of an acre).
Use a stopwatch to determine how long it takes to spray the area uniformly using a steady sweeping motion, continuous spray, and constant pressure.
Spray into a container for the time calculated in step 2.
The fluid ounces collected is the GPA for your sprayer. Volume sprayed in fluid ounces = Gallons Per Acre
Handheld/Pump Sprayer Calibration
Measure and mark an area 18.5 feet by 18.5 feet (1/128 acre).
Measure out a volume of water (example: 72 oz) into your sprayer using a measuring cup* for accuracy.
Spray the measured area as evenly as possible.
Measure out the remaining water using the measuring cup.
Subtract the remaining volume from the starting volume to determine the volume sprayed. Volume sprayed in fluid ounces = Gallons per Acre.
*Do not use the measuring cup for any other activity, such as food preparation. This procedure may contaminate it with herbicides.
Nozzle Volume Method for ATVs, Tractors, etc. with booms or boomless nozzles. Use This Form
Partially fill your tank with water.
Turn on your pump and allow the nozzles to spray for a few seconds. You may want to be moving while spraying to prevent puddles from forming.
Measure the width of the wetted area covered by your boom or boomless nozzles. This is your spray width (Step 1).
Collect nozzle output for 1 minute. The volume of liquid collected is your ounces per minute (Step 2).
You will now need to determine your ground speed in Miles per Hour (MPH). It is best to follow the steps in the form below for accuracy.
Use the equation given on the form to calculate GPA.