Backpack Sprayer Calibration for Broadcast Applications

- Backpack sprayer calibration for broadcast applications.

With broadcast herbicide applications, a large contiguous area of vegetation is treated.

In forestry, broadcast applications are commonly used for spraying areas of interfering plants such as hay-scented and New York fern, as well as infestations of some invasive plants, including Japanese stilt grass and mile-a-minute vine.

Additionally, these applications can be used for site preparation prior to a tree or meadow planting or to apply pre-emergence herbicide to an area.

This video covers the calibration process for a broadcast application using a backpack sprayer and an adjustable cone nozzle which requires sweeping the nozzle from side to side as you spray.

A large area can also be treated using a nozzle that produces a fixed width pattern which would not be moved from side to side while spraying.

If you are using a fixed width setup, see the calibration video for band applications.

To properly conduct a broadcast application, you must practice your spray technique.

Choose an even surface such as gravel, pavement or a mode grassy area.

Fill your backpack sprayer with clean water and add indicator dye if you're practicing on vegetation.

Pump up the sprayer and adjust the nozzle to a mid-range setting that forms a somewhat solid cone.

practice walking forward and spraying swaths in front of you by moving your spray wand or gun from side to side at a comfortable width.

You must maintain even spray coverage on the ground.

Maintain pressure by pumping the lever steadily and keep the spray nozzle at a consistent height.

Direct the spray outward about six to eight feet in front of you.

Be sure to release the trigger at the end of each sweep of your arm or the edges will receive heavier coverage.

Check the spray pattern on the ground to ensure it's being covered evenly and there are no skips.

The goal is to apply even, but light coverage where individual droplets can be seen, rather than solidly painting the ground.

Applying lightly allows you to cover a larger area with one tank of spray.

Once you feel comfortable maintaining consistent pressure, swath width and coverage, it's time to start calibrating.

The calibration process allows you to measure the amount of spray applied to a known area and then calculate your application rate in gallons per acre.

An efficient method is to use a calibration area of 340 square feet.

This size area equals 1/128th of an acre.

Since there are 128 fluid ounces in one gallon, spraying 1/128th of an acre allows for a direct conversion from ounces sprayed on the calibration area to gallons per acre application rate.

For example, if you spray 25 fluid ounces onto the 340 square foot calibration area, that means your application rate is 25 gallons per acre.

Start by measuring your swath width.

Here we applied heavily for easier measuring.

Our swath width was 10 feet.

Next, divide 340 square feet by your width to determine the length of the calibration area.

In our example, our spray swath is 10 feet wide, so the length of the calibration area would be 34 feet.

After laying out the calibration area and ensuring the spray nozzle is at the desired setting, make sure the sprayer is fully pressurized.

Then check that there is at least one gallon of water in the sprayer tank and mark the level of water on the tank.

The sprayer must be pressurized so the pump and hose are filled with water before you mark the water level.

Fill a measuring cup to a known level with water.

30 to 40 ounces should be sufficient.

Spray the calibration area in the same manner as you practiced.

Then using the measuring cup, refill your sprayer to the original starting level.

Use subtraction to determine how many fluid ounces you applied to the calibration area.

For example, if the cup started with 32 ounces and had seven ounces left after you refilled the sprayer to the starting point, then you sprayed 25 ounces.

This means your application rate was 25 gallons per acre.

Repeat the calibration process several times to get a reliable average of your application rate.

Now that you have your application rate in gallons per acre, you can easily calculate how much herbicide to include in your spray mixture based on the herbicide rate per acre found in the product label.

This is why sprayer calibration is so important.

Calibration takes some time upfront, but it is time well spent to ensure proper herbicide applications.