NMSU: A-613: #4: Sprayer Calibration
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Extension Pesticide Applicator Training Series #4: Sprayer Calibration

Guide A-613
R.C. Runyan, Extension Specialist
College of Agriculture, Consumer and Environmental Sciences, New Mexico State University.

Note: This sprayer calibration procedure applies to “over the top” ground equipment only.

Sprayer Constants

There are four variables for which a sprayer is calibrated. The calibrated output of the sprayer will remain constant if each of the four variables remain constant. Any time any one of the variables change, the sprayer must be re-calibrated.

  1. Ground Speed. Speed traveled over the ground will significantly affect sprayer output. Faster speeds result in lower output per surface area. Conversely, slower speeds will increase output. However, adjusting ground speed is not the most economical method for manipulating sprayer output. The speed should be dictated by evenness of the terrain, obstacles to avoid, condition of the equipment, etc. The primary consideration of ground speed is safety. The speed should be set to complete the operation as quickly as possible without risk to people or property.

  2. Spray Pressure. There are several considerations when deciding spray pressure. Some product labels require that the pesticide be applied within a specified pressure range. Several relatively new products are considered low-pressure, low-volume pesticides that are applied with very low pressure. Applicators must also always be aware of drift potential from the operation. High pressures present greater risks from drift. As a result, adjusting spray pressure to change sprayer output is usually not a good choice. And, unless a significant change of 10 or more PSI can be accomplished, adjusting spray pressure will result in only minor changes in output.

  3. Number of Nozzles. The number of nozzles is determined by the design configuration of the sprayer. For sprayers that are equipped with directional boom control valves for partial boom applications, each arrangement of boom selection should be calibrated independently. In most cases, the number of nozzles will remain constant unless the sprayer is rebuilt or retrofitted.

  4. Nozzle Orifice Size. Nozzle size or type is the most practical method for adjusting sprayer output. Several brands of nozzles are available with similar spray patterns (flat fan, cone, etc.) but are made with various orifice sizes for increasing or decreasing output.

Getting Started

Here is a list of basic tools needed to calibrate a sprayer.

  • Personal protective equipment – gloves, goggles, rubber boots, etc.
  • Measuring tape 50 foot or greater
  • Measuring beaker in fluid ounce graduations
  • Stopwatch or sweep second hand
  • Calculator

These figures are needed to make calibration calculations (see below).

  • 43,560 feet per acre
  • 128 fluid ounces per gallon

Calibration Procedure

Step 1: Determine Effective Spray Width (ESW)

ESW (broadcast applications) = Nozzle spacing × the number of nozzles

Step1: Example.

Step 2: Measure off a test area (TA) and determine the square footage. Measure and mark a test area between 50 feet to 100 feet. Any convenient length, such as a fence line or driveway will work. The width of the TA will be the ESW as determined in the previous step.

Square feet of TA = length of TA × ESW

Step2: Example.

Step 3: Determine number of TAs per acre.

Step3: Equation and Example.

Lawn and garden pesticide rates are often given per 1,000 ft2. To determine the number of TAs in 1,000 ft2, simply substitute 43,560 with 1,000 in Step 3.

Step 4: Determine the time required to cover the TA. Use water only in the tank during calibration.

  • Operate the sprayer with speed and pressure at field operating conditions.
  • Achieve operating speed before entering the TA course.
  • Mark start and finish time from the same point on the machinery (e.g. driver’s seat).
  • Take an average of three timed trials for shorter TA lengths.
  • Record the average or most accurate time required to cover the TA.

Step 5: Determine the sprayer output for the TA.

  • Set parking brake or otherwise secure the machinery.
  • Operate the sprayer at field operation pressure.
  • Collect spray output from one nozzle for the same period of time required to cover TA (determined in Step 4).

Step5: Equation and Example.

While performing Step 5, it is a good time to compare nozzle wear by testing all nozzles. Nozzle output should be within ± 5% of each other. Replace worn nozzles with those that have equal flow rates.

Step 6: Determine the sprayer output per acre.

Step6: Equation and Example.

To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at aces.nmsu.edu.

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Printed and electronically distributed February 2006, Las Cruces, NM.