Sprayer calibration is an important step when applying pesticides and should not be overlooked. A lot of different sprayers can be used in agriculture, including boom sprayers, boomless sprayers, air and blast sprayers. The following information outlines the process for calibrating handgun sprayers, which are common on large sprayers powered by a tractor’s power take-off, battery-powered sprayers used on all-terrain vehicles (ATV), and smaller hand pump sprayers.

Constant speed and pressure are needed to accurately calibrate sprayers. Maintaining these are hard to do with a hand pump sprayer while walking on uneven ground. Some hand pump sprayers have a pressure gauge, which makes maintaining constant pressure easier. However, maintaining a constant speed while walking is difficult. It is easier to maintain constant speed and pressure with a power take-off or battery-powered sprayer and driving some type of equipment.

Determining Pesticides Per Acre

It is extremely important to understand sprayer calibration before applying pesticides. For farmers and other professionals, the pesticide label will explain how much of the pesticide to apply per acre. Years ago, it was common for home garden products to recommend a certain amount of pesticide to be applied per gallon of water. With this method, one person may apply much more pesticide on a certain area than another, even if the pesticides were mixed the same. For this reason, it is now common for the label to recommend a certain amount of pesticide on a certain area. An example of this may be a recommendation to apply 2 ounces of pesticide per 500 square feet or per 1,500 square feet. So, how much pesticide should be put in a tank? First, you need to determine how many gallons of water you are applying per acre. Then, you calculate how much pesticide to add to the tank.

Calibration Steps

The following are the steps for the 1/128 calibration method, but other methods can be used. The method below is for handgun or wand applicators, regardless if the sprayer is handheld, a backpack, on an ATV, or mounted on a tractor. While calibration is done only using water in the tank, personal protective equipment should still be worn while calibrating.

First, measure 1/128 of an acre, which is approximately 340 square feet (18.5 feet by 18.5 feet). Use flags, string, spray paint, etc. to mark the area. Then, with only water in the tank, measure the time required to spray the area. The goal is to apply the water consistently, so try it several times until you determine your average time. Wetting the area more or less will change the calibration rate. The goal is to spray the same way in the field as you did while calibrating. Then, spray the water in a container for the length of time it took to spray the 340-square-feet area and measure the water in ounces. The ounces measured equals the gallons of water the applicator is applying per acre. For example, if it took 23 seconds to spray a 340-square-foot area, you would spray and collect water for 23 seconds and measure it in ounces. If 50 ounces were caught, you would be applying 50 gallons per acre. If 15 ounces were caught, you would be applying 15 gallons per acre. To adjust the gallons of water per acre, change pressure at the pump, change or adjust spray tips, or adjust the speed. Then, repeat the calibration process until you are applying the desired amount per acre. Once you are applying the desired volume of water per acre, you do not need to adjust the pressure, tips, or speed.

Once you calculate the area that can be covered with one tank, you can determine the amount of pesticide needed per tank. The pesticide label will give a range of desired gallons of water per acre that is needed to be applied along with the recommended rate of pesticide.

If you are applying 30 gallons per acre and the label recommends 16 ounces of pesticide per acre, how much pesticide would go into a 4-gallon tank? First, divide 4 by 30 to get .133, which is the amount of acres covered with 4 gallons of spray. Then, multiply .133 and 16 to get 2.1, which is the number of ounces of pesticide to add to a 4-gallon tank.

If you are applying 30 gallons per acre and the label recommends 2 ounces of pesticide per 1,500 square feet, how much pesticide would be added to a 4-gallon tank? First, calculate how much water you are applying per square foot then multiply that number by 1,500. At 30 gallons per acre, you are applying .00068 gallons of water per square foot. Multiply that number by 1,500 to get 1.0, which is the gallons of water that is being applied per 1,500 square feet. For this example, for every gallon of water in the tank, you should add 2 ounces of pesticide.

More Information

Remember to always read the label before purchasing and applying pesticides, and always wear the recommended personal protective equipment. For more information on sprayer calibration, contact your county Extension office or visit the Alabama Extension website at www.aces.edu. There, there is also information on pesticides, herbicides, and fungicides that are labeled for certain crops—such as turf, ornamentals, vegetables, fruit, and forages—and other areas, such as insects in wood structures.