Whenever I give a presentation to growers about the need to calibrate a sprayer and how to do it, there is always someone asking this question:

“I have a rate controller in the cab that regulates the flow rate of the sprayer regardless of the changes in sprayer ground speed. I just enter the gallons per acre application rate, and the controller does the rest, just like cruise control in a car. So should I still calibrate the sprayer?”

The answer is: Yes, a calibration should be done. Although rate controllers do an excellent job with regulating the flow rate of nozzles to keep the application rate constant regardless of the changes in travel speed, a manual calibration at least once a year is needed to ensure the rate controller is functioning properly and that the rate controller is not forced to operate outside the pressure operating range for the nozzles on the sprayer boom. 

Why Sprayer Calibration is Critical

Let me elaborate on both points and share with you the reasons why a manual calibration of a sprayer is a good idea.

  1. If you are stopped by a police officer for speeding, telling the police officer that the car was in cruise control and set to the speed limit will not get you out of getting a ticket. Cruise controls go bad and so will the sprayer rate controllers. That is why it’s best to manually check the flow rate of nozzles to make sure the gallons per acre application rate you enter on the controller matches the gallons per acre rate provided by the nozzles.
  2. Your controller may be in good shape, but if the ground speed sensor is giving inaccurate data to the controller, it will not work accurately. For example, if the speed sensor is based on the number of revolutions of the tractor wheels, the ground speed determined may not be accurate, because of the tire slippage that may occur under some ground conditions. Even having the tire pressure being off by just a few pounds per square inch may change the number of tire revolutions per minute, leading to erroneous travel speed readings. 
  3. Unfortunately, most standard electronic controllers can’t detect flow rate changes on each nozzle on the boom. So, if a nozzle is plugged or extremely worn, the rate controller can’t warn you about these problems that happen all the time. It will try to maintain the constant application rate by changing the system pressure and force other nozzles on the boom to spray less or more to overcome the problems with one or several nozzles on the spray boom.
  4. Rate controllers don’t show changes in spray patterns that may happen when a nozzle is defective, plugged or worn-out. If several nozzles are not maintaining the proper spray angle, the proper overlap between adjacent spray patterns can’t be established. This will result in untreated areas and streaks under the boom, or some areas may receive excessive deposition of the pesticides applied. As a result, you will have to continue manually checking the flow rate of the nozzles and visually observing the changes in spray patterns until the technology is developed to do these observations remotely and on-the-go.
  5. To achieve the best results when spraying pesticides, the application rate, as well as the droplet size must remain relatively unchanged during the entire spraying period  When sprayer speed goes up to maintain the pre-set application rate, the controller requires the system pressure to go up in order to increase the nozzle flow rate. Unfortunately, this results in more drift-prone droplets coming out of the nozzle, especially if the nozzle is designed for low application rates within the recommended pressure ranges.
  6. Conversely, when the sprayer slows down, the opposite happens and the controller forces the system to lower the pressure in order to reduce the flow rate of the nozzles. This will result in production of larger than the desired size of droplets, leading to inadequate coverage.
  7. If you are spraying dicamba or 2,4-D, you need to pay even more attention to the operation of your rate controllers. As you know, only a small number of nozzles at specific ranges of pressure can be used to spray these herbicides. Significant changes in ground speed may force your rate controller to make significant changes in spray pressure that may be outside the allowable legal pressure range that is required to spray these herbicides.

Without realizing it, you may find yourself in violation of the label. Make sure the nozzle size selected will allow the controllers to make the necessary changes in the flow rates while still staying within a safe, applicable, and allowable pressure range.

Calibrating the Sprayer

It usually doesn’t take more than 30 minutes to calibrate a sprayer. Only three things are needed: a watch or smart phone to record the time when measuring the nozzle flow rate or the travel speed, a measuring tape and a jar graduated in ounces.

Please look at the Ohio State University Extension publication FABE-520 for an easy method to calibrate a boom-type sprayer.

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