In preparation for Spring, it is a good idea to make sure your equipment is calibrated and in good working condition. This article discusses a quick and easy method for calibrating a hand-powered, backpack sprayer. Calibration is important so that you understand the amount of product that is being delivering by your sprayer. Applying too little product may result in poor performance and applying too much product is not only a waste of money, but can lead to non-target effects in the environment.
The 10 steps below outline the 1/128th rule for calibrating your backpack sprayer.
- Mark off a calibration plot that measures 1/128th of an acre: (example: 8.5 ft. by 18.5 ft).
- Fill the sprayer to normal capacity with water.
- Pump the sprayer to the pressure normally used for herbicide applications. (Hopefully, your sprayer has a pressure gauge! If not, do your best to maintain a consistent pressure).
- Spray water over the plot area while maintaining normal (and constant) operating pressure. Coverage will depend on what is stated on the label. Some pesticides may require that the product be sprayed lightly on the target area while others may need to be sprayed until runoff.
- Record the time in seconds it takes to spray the plot area: (example: time = 30 sec).
- At the same constant pressure, by pressure gauge or constant pressure on the handle, spray into a bucket for the same time (number of seconds) it took to spray the plot area.
- Measure the volume of water in fluid ounces: (example: volume collected = 25 fl. oz.). The number of fluid ounces collected is equal to number of gallons that would be applied to one acre, if the sprayer is operated at the same pressure and walking speed used in the plot area. In this case the sprayer is applying 25 gallons of water per acre. This quick conversion from fluid ounces per 1/128th acre to gallons per acre (gpa) works since there are 128 fluid ounces in one gallon of water.
- Determine the total volume of your tank: (example: 4 gallons).
- Divide tank volume by output = number of acres treated per tank: (example: 4 gal ÷ 25 gpa = 0.16 acres).
- To find out how much pesticide product to add to your sprayer, multiply the herbicide application rate (given on product label) times number of acres treated per tank: (example: 2.0 oz of product x 0.16 acres = 0.32 oz of product should be added to 4 gallons of carrier).
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