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While there are several reasons you should calibrate your sprayers at least once a year, the most important reason is that it is illegal to apply an unlabeled amount of pesticide, either too little or too much. And applying too much pesticide hurts your bottom line and too little can lead to poor pest control, inadequate results from fertilization and loss of confidence from your clients in your ability to care for their property.

Unfortunately, calibrating your sprayers involves math — but they are the math skills you learned in high school. So sharpen your pencil and get started. Remember that sprayers without a pressure regulator cannot be reliably calibrated.

Whether you are using a flat fan nozzle or shower head gun, you will need the following:

  • Measuring wheel or tape.

  • Graduated measuring jar.

  • Stopwatch or a watch with a second hand. Use this to help ensure consistent walking speeds

  • Non-staining colorant or a large enough paved area for uninterrupted driving. (If you have access to a paved area, you can simply mark where the wet areas stop and start, without having to use the colorant.)

  • Calculator, paper and pencil. Your math will be more accurate if you write it down instead of relying on mental calculations.

Keep in mind that the amounts of active ingredients labeled to apply in an area varies. The trick is to dilute them in an amount of water to cover that area. Whether you need Ω ounce or 4 ounces per 1,000 square feet, the pesticide is usually diluted in one to two gallons of water per 1,000 square feet.

Use strictly water to calibrate liquid sprayers — save the pesticide for the actual application, and don't try to calibrate and apply at the same time. You may save a bit of time, but will probably apply too much (wasting material) or too little (wasting labor). Some liquid fertilizers are thicker than water, in which case you should calibrate with water then use the conversion factors listed in the nozzle manufacturers catalog. Be sure your test area has terrain similar to the application areas. You don't want to calibrate in a flat parking lot to prepare for application in a hilly or rough area. Walk or drive at a similar speed to an actual application and maintain pressure as close to an actual application as possible. Be sure that your equipment is in good working order with no leaks in the hose, no clogged areas in the nozzle and a consistent pressure regulator.

  1. Fill the sprayer at least half full and to a known amount.
  2. Spray a known area (1,000 square feet, for example) at the same pace you would in real application. Do not overlap swaths.
  3. Keeping track of how much water you use, refill the sprayer to the known mark. Write down how much water it took to refill the sprayer. This number equals the amount of spray applied per 1,000 square feet. There are 16 cups per gallon of water. Repeat the previous steps two or three times to get an average.
  4. Plug your averaged numbers into the formula:

Label rate in ounces/1,000 square feet = Number of ounces of pesticide to add per gallon of water./Refill volume in gallons/1,000 square feet

Keep in mind that if your sprayer delivers more than 1 gallon per 1,000 square feet, you will need to reduce the amount of pesticide diluted in the tank. If the sprayer delivers less than 1 gallon per 1,000 square feet, you will have to increase the amount of pesticide diluted in the tank.

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