Uneven pressure throughout the sprayer boom can mean uneven application of pesticides. The first step in sprayer calibration is to determine that the spray pressure is equal throughout the spray boom.

Bob Klein, University of Nebraska sprayer specialist, offers the following several items to check for before you take the sprayer into the field.

  • Check pressure all along the boom first because there might be a restriction in the line and uneven pressure. If the pressure is not even through the boom, you’ll be wasting your time trying to calibrate the sprayer.
  • Check and compare the cab pressure with boom pressure. If there’s a difference, make adjustments so you know the sprayer’s exact pressure.
  • Set up the pressure gauge with quick-attach fittings that can snap in place on the nozzle fitting.
  • Select glycerin-filled, stainless-steel gauges, as lesser quality gauges won’t last.

When it comes to your applicatino rates, Klein says there are three basic things that affect the number of gallons applied per acre.

Speed. If you run the tractor twice as fast, you’ll apply half the rate, if nothing else is changed.

Nozzle Spacing. The typical nozzle spacing is 20 inches, but 30-inch nozzle spacing is just as effective if 110-degree nozzles are used. Most new air-induction nozzles are only available in 110 degrees. Another reason for 30-inch nozzle spacing is that most farmers are still planting 30-inch rows. If you want to use drop nozzles, add extensions to create drop nozzles.

Thirty-inch nozzle spacing also allows for 50% larger nozzles. That, in turn, provides for using lower application rates without going to extremely small nozzles. Avoid nozzles that require less than a 50-mesh nozzle screen because smaller screens tend to plug.

Nozzle Flow Rate. This is determined by the nozzle orifice size, pressure and solution density. Remember that anything heavier than water will come out more slowly than water and anything lighter than water may come out faster. Adjust the sprayer accordingly.

Sprayer calibration provides the information you need to determine the amount of spray solution — both pesticide and carrier — being applied per acre. This information can help ensure you're applying the amount of solution stated on the product label.

The ounce calibration method involves collecting the spray from an application of water to 128th of an acre, measuring the amount delivered and using that information to calibrate your sprayer, Klein says. This method does not require any calculations, but does require a stopwatch, container to collect nozzle discharge, a tape measure, marking flags and a container graduated in ounces.  (For more information, see NebGuide G1756.)

Step 1. Select the travel distance according to the nozzle spacing on the sprayer using Table 1. Measure the travel distance in a level field. The travel area should be typical of the surface and soil conditions of the area to be sprayed. Many tractors and sprayers will gain or lose more than 10% of desired travel speed while moving up and down slopes.

If field variations exist, several speed check areas may be needed. Remember, the time required to drive the travel distance will give the speed of the sprayer, so the measured distance and timing must be exact.

Step 2. Drive and time the sprayer in seconds at the throttle setting, pressure setting and load used during spraying (spray tank should be one-half to two-thirds full). Engage incorporation equipment (discs, planter, etc.) or other devices used while spraying. Do not change the gear or throttle setting after you have chosen a spraying speed. A change in ground speed will change the sprayer application rate and will require recalibration.

Step 3. While in a stationary position, bring the power unit to the proper throttle setting and sprayer to the boom pressure used in Step 2. Catch the nozzle discharge for the time recorded in Step 2. Measure the discharge in ounces with a graduated container. For an accurate assessment of the sprayer, measure all nozzles and average the results.

Remember, from a safety point, the collection of discharge should be done using water only! Even when collecting water, use the proper personal safety clothing and protection.

Step 4. The measured ounces from a nozzle are equal to gallons per acre that will be applied. Since this calibration was based on water, conversion factors (Table 2) must be used when spraying solution heavier or lighter than water. Multiply the observed rates of water by the conversion factors to attain the rate of other spray solutions.