Calibration Procedures

Daniel Stephenson, Webster, Eric P., Price, Randy R., Strahan, Ronald E., Orgeron, Albert, Sexton, Mary, Mudge, Christopher, Fontenot, Kathryn  |  12/20/2018 7:26:29 PM

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Sprayer calibration is the process of determining the correct vehicle speed, tip and pressure to create a desired application rate (usually in gallons per acre or GPA) and most manufacturers have tables to help you determine these values for their equipment and tips. Still, differences can occur between your spray vehicle and the manufacturer’s rates because of tip wear, inaccurate pressure gauges and flow meters, varying liquid viscosities (created by multiple chemical mixtures and additives) and hose losses. To properly calibrate a sprayer you should select the correct tip size, pressure, speed and type to obtain the desired GPA rate, and then use sprayer calibration to make sure that your sprayer is applying this rate. Note that most farmers can quickly determine correct flow rate in the field from liquid usage in the tank, but this type of monitoring does not indicate plugged or non-uniformity nozzle flow, or flow inconsistencies in the boom sections. When selecting a nozzle tip, keep in mind that slower speeds (5 to 12 MPH) typically require less power and pressures to achieve that flow rate and have a larger pressure envelope in which to operate (i.e. - the spray controller has a larger pressure range in which to operate), so tip selection should be designed for middle of the spray range. After selection, Sprayer calibration can be achieved using the following steps:

1) Measuring the flow rate of individual tips

Measuring the flow rate from individual tips can help determine when tips need to be changed and the actual flow rate Individual nozzle tests are performed by either using a calibration container (Teejet, etc.) or a Spot-on calibrator ( Spot-on calibrators can greatly reduce your time in performing this operation and make the test less labor and time intensive and more practical on booms with multiple nozzle tips. To check nozzles, place the calibrator under the tip of the nozzle for the set time period and read the amount of GPM for that nozzle (note: to perform this operation successfully, you may have to re-program the flow controller (or by-pass) to operate while the machine is not moving for the rated pressure you will use in the field. Always perform a visual spot check to insure that all nozzles are operating and that filters are not clogged (filters and anti-drip caps should be removed and flushed with water to remove chemicals and debris). The GPM of each nozzle should be within 10% of target GPM. If any tip varies by more than 10%, a new tip maybe necessary. If the average GPM of all nozzles is above or below the target rate, slight adjustments in the pressure, speed, or even re-calibration of the flow rate sensor maybe necessary (check manufacturer’s manual on how to clean and re-calibrate the flow sensor). Flow rate test should be performed on nozzles at least once a year.

The general equation for calculating individual nozzle flow rate is given by:



GPM is the gallons per minute of liquid used per application width

GPA is the gallons per acre

MPH is the average speed of the vehicle through the field

W is the nozzle application width in inches (typically the distance between the nozzles for broadcast sprayers or banding width for banding sprayers)

This equation can also be rearranged to indicate application rate (GPA) for the entire boom width by adding all the individual GPM nozzles together and using the entire boom width in inches. 2017 Louisiana Suggested Weed Management Guide CALIBRATION PROCEDURES.

2) Insure that the sprayer speed is correct

Speeds must been correctly reported to the rate controller (or operator) to obtain the correct GPA rates and this value should be held as consistent as possible when travelling through the field. Typically on new sprayers, speed is inputted with GPS directly into the flowrate controller and no calibrations are required. Older machines that use radar (and some GPS unit that output radar type readings) are based on pulsed frequencies and this value must be calibrated in the rate controller to indicate the correct speed. Typically this procedure is performed by travelling a prescribed distance with the controller set in a speed calibration mode. Consult the manufacturer’s manual for this procedure and the travel distance needed for calibration. Calibration of speed sensors should be performed once a year. In some cases, the radar unit may slip down in its mounting creating the wrong angle between the radar’s emitter unit and the ground (typically this angle is 30 to 45 degrees) which can cause sluggish, noisy, or non-existent return signals. In this case, check that the radar unit is securely affixed to the vehicle frame and that the correct angle is created between the unit and ground. Consult the manufacturer’s manual for the correct orientation as some radar units contain angled emitters which allow the unit to be mounted horizontal, while still allowing for the correct angle for radar readings. Wheel speed sensors should not be used for general row crop purposes, but may be acceptable in pasture and other situations where terrain conditions are more consistent. To check the travelling distance versus speed for these type systems use the equation given below with at least a 500 feet or larger travel distance (to insure 0.5 MPH speed accuracy).


*Use distances of at least 500 feet or more for 0.5 MPH or better accuracy in the 5 to 20 MPH speed range.

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