This guide outlines the elements needed to calibrate pesticide spray equipment. It is important to calibrate your pesticide application equipment on a regular basis to ensure that the intended application output is consistent. If not properly calibrated under-application of pesticides can lead to a lack of pest control and poor pesticide performance. While over-application increases cost and has the potential to cause injury to desirable crop species. This how to guide will review two of the most common methods for sprayer calibration.

1. The 5940 Method

This method uses an equation to determine sprayer output.

To calculate the gallons per acre (GPA): 5940 is multiplied by the gallons per minute per nozzle (GPM), this product is then divided by the product of miles per hour (MPH) and spacing between two nozzles or spray width of one nozzle in inches (W).

Equation: GPA = (5940 * GPM) / (MPH*W)

Keep in mind the 5940 is a constant value in this equation.

Gallons per Minute (GPM) - This refers to the amount of spray solution discharged from the nozzle per minute. You can measure the GPM using a measuring cup. Turn your sprayer on and collect the output from the nozzle for one minute. Be sure to use a liquid measuring cup to quantify the output in fluid ounces or milliliters. Be sure to convert your GPM output to gallons for the equation. To convert to gallons, divide the amount of water acquired in one minute by 128 or 3,786; the number you need for this conversion depends on whether your container measures the water volume in ounces or milliliters.

It is recommended to collect water from multiple nozzles to ensure that output is similar across the boom. It is common for a nozzle to become plugged, and this practice will help to identify any issues.

Miles per Hour (MPH) - To check the speed of your equipment measure an area of 100 feet and take note of the time it takes to travel that distance. If you are using a truck or tractor you must ensure that your speed are at the same setting as those used for spraying. You will then multiply your distance in feet by 60, the product will be divided by the product of the time in seconds by 88.

Equation: MPH = (distance in feet * 60) / (time in seconds * 88)

Nozzle Spacing/Spray Swath (W) - This is a measurement in inches describing spacing between two nozzles or width covered by one nozzle.

Increasing spray pressure increases GPM which in turn will increase the GPA but only by a small amount. For example, you would need to quadruple the spray pressure in order to double the GPM. Most spray nozzles are not rated to operate over such a large range of pressures, so adjusting pressure is simply a way to fine-tune sprayer output. If you need to make a larger adjustment to the GPM or the GPA you will need to switch to a different sized nozzle and adjust pressure or speed. If you need to operate at a specific GPA, the Equation can be reorganized to instead solve for GPM.

For example: GPM = (GPA*MPH*W) / 5940

2. The 1/128 Method

The 1/128 method is a simplified way to calculate GPA for spray equipment calibration. This method is based on spraying 1/128 acre. Since there are 128 ounces per gallon, the number of ounces sprayed per 1/128 acre is equal to the number of gallons sprayed per acre (GPA). This is a great method for use on boom type sprayers and offers a calibration method that involves little math.

1. For the 1/128 method after you have determined your nozzle spacing or swath width (W) you will need to determine the course length. For this you will divide 4080 by the nozzle spacing (in.)

Distance (in feet) = 4080 / W ; where 4080 is a constant value in the equation.

2. Once you measure and mark the distance, record the time required to drive the length of the calibration course.

3. Collect and measure all spray (from one nozzle) for the amount of time it takes to travel the length of the course. This will be equal to gallons per acre of spray applied.

4. You can collect and measure the ounces caught for several other nozzles to ensure accuracy.

*If you are calibrating a boomless sprayer it is recommended to use the 5940 method.

Useful Conversions

128 fluid ounces = 1 gallon

1 fluid ounce = 1/128th of a gallon

340 sq ft = 1/128th of an acre