Best methods for best results
Early in the season and in busy times, growers or applicators hook up their sprayer, fill their tanks with water, measure the chemical needed and drop it in the tank, hop on their tractor and begin spraying. They feel compelled to quickly treat for insects before feeding causes damage. What they may forget is to calibrate their sprayer properly before they begin spraying. However, what is the best method? That depends on the sprayer and what crop you are spraying.
One of the easiest methods of calibrating broadcast boom or directed boom sprayers for vegetable applications is called the 1/128th or ounce method. “You begin by determining your nozzle spacing in feet [15 inches = 1.25 feet],” says Allen Straw, an area specialist with Virginia Tech’s Cooperative Extension Service in Glade Spring, Va. “Divide 340 [1/128th of an acre] by the nozzle spacing in feet. That gives you the linear feet required to cover 1/128th of an acre.”
To assist growers with proper calibration, he says universities and cooperative extension offices offer wallet-sized cards with common nozzle spacings and distance required covering 1/128th of an acre. For example, see the accompanying chart:
Using the chart, Straw says to measure out a course corresponding to the distance needed. “It is best to do this in the field, but at least on ground similar to your normal application conditions,” he says.
Next, growers should fill their sprayer half full with water so they get an average of the weight carried during application. Then, select the proper gear and revolutions per minute (RPM) on the tachometer.
“Of course, many new tractors have speedometers that can be used, but still keep track of your RPM setting, as you will need it directly,” Straw says. “Drive the course and record the number of seconds required to travel the course. Make sure to be at the correct speed when beginning and ending your time measurement. In other words, don’t start timing from a dead stop. Repeat this process at least three times and average the time required to drive the course.”
After these steps are completed, he says to stop the tractor and put it in park or set the parking brake. The next step is to set the throttle to the same RPM setting as when driving the course. The grower should then turn the sprayer on and collect the output from a nozzle to get the average time calculated to drive the course. Straw says every ounce of liquid that is collected equals 1 gallon per acre (GPA). For broadcast and banded applications, he advises to collect from one nozzle. For a single-banded nozzle, the nozzle spacing is the row width. For directed applications, he says to collect from all nozzles per row, keeping in mind the spacing in the row width.
Straw recommends to examine all nozzles to be sure all the strainers and nozzles are the same type and size. He cautions to watch spray patterns, checking for nozzle wear and blockages. For the best calibration, he says to collect water from each nozzle and average the amounts to determine the GPA. If the output of any nozzle varies more than 10 percent from the average, he advises to replace the nozzle.
A different approach
This calibration is for vegetables, but does calibration need to be different for fruits? “It doesn’t have to be different,” Straw says, “but different sprayers require different calibration methods. The above ‘ounce’ or ‘1/128th’ method is great for boom sprayers, but is much more difficult for cannon and air-blast sprayers.”
A more universal but complicated calibration technique to collect and measure in ounces: GPA = output (ounces/second) / speed (feet/second) / nozzle spacing in feet x 43,560 (feet2/acre) / 128 (ounces/gallon). To collect and measure in milliliters (ml), use GPA = output (ml/second) / speed (feet/second) / nozzle spacing (feet) x 43,560 (feet2/acre) / 3785 (ml/gallon).
When performing this calibration method, Straw says to set the throttle on the tractor to the same RPM as when spraying. Then, collect water from the nozzles as described in the ounce method above. “However, you can just set a time; I prefer 30 seconds,” he says. “This is long enough to minimize error, while still minimizing time. If you collect 15 ounces of water in 30 seconds, the output equals 15 ounces divided by 30 seconds equals 0.5 ounce per second.”
For cannon and air-blast sprayers with large directed booms, Straw says to determine output by putting a few gallons of water in the sprayer and running it until it just runs out and shuts off the sprayer immediately. Next, add back a known amount of water (5 or 10 gallons) and time how long it takes to spray out the water. He says 5 gallons = 640 ounces. “If it takes five minutes to spray out the 5 gallons, then 640 ounces divided by 300 seconds [five minutes] equals 2.13 ounces per second.”
To determine speed with the more universal calibration technique, growers can apply the same course direction as used in the ounce method. “However, I generally measure a 100-foot-long course,” Straw says. “Drive three times collecting your average time, divide the length of the course by the average seconds required to drive the course—100 feet divided by 23 seconds equals 4.35 feet per second, which is your speed.”
With this method, nozzle spacing for a broadcast boom is the actual nozzle spacing, which for a single-nozzle, band application is the row spacing, he says. Nozzle spacing for a directed sprayer is the row spacing.
“Remember to collect the output from all of the nozzles directed on that row,” Straw stresses. “Nozzle spacing for cannon and air-blast sprayers is the swath width. This is often 25 to 50 feet.” For instance, when calibrating a cannon sprayer, it is output equals 32 ounces per second [10 gallons (1,280 ounces), which is sprayed out in 40 seconds]. Speed of 1.5 miles per hour equals 2.2 feet per second (45.5 seconds to drive 100 feet). Nozzle or swath equals 50 feet, so 32 (ounces per second) / 2.2 (feet per second) / 50 (feet) x 43,560 (feet2 per acre) / 128 (ounces per gallon) = 99 GPA.
Peanut sprayer calibration
For ground spraying of peanuts, growers use the 1/128th calibration method, says Mike Parrish, unit coordinator and extension agent in Dinwiddie County, Va. He explains the method in this way: it is based on the number of ounces (128) in a gallon of liquid. The grower or applicator selects a known length for a timed test run. He says the grower should conduct the test run in the field on the same soil conditions that he experiences in the field, such as if it is tilled or non-tilled soil. Then, he selects his distances so he can adjust the tractor or sprayer unit for the desired ground speed.
“Try to maintain minimal boom movement during operation,” Parrish says. “We do not want to operate the sprayer at speeds that make it look like a goose flapping its wings across the field. This creates a number of issues for poor chemical application like increased drift, poor allocation of products and damage of the sprayer.”
Next, he recommends setting the pump for the ideal pressure needed for good coverage of the chemical applied. He says the grower then must time his speed to travel, the known distance recommended for nozzle spacing. The space of the nozzles and distance will measure 1/128th of an acre. “If your spacing is 20 inches between nozzles, use a distance of 408 feet,” he suggests. “If you take the width of the nozzle space and multiply the distance you will travel, you will come up with an area that is 1/128th of an acre.”
He says growers should make at least three passes so they can obtain a good average time to work with.
Once the grower figures out the time to travel the required distance, he can go to the collection step. To do this, Parrish says to park the tractor or sprayer, then let it run at the same RPM. Next, the grower can pump pressure that he traveled at earlier.
“Use a stop and collect your sample from a nozzle or a group of nozzles fixed to cover a row,” he says. “Hold a measuring cup with ounce units on it under a nozzle to collect water for the same amount of time that you recorded to travel the desired distance. Each ounce you collect now is equal to a gallon per acre. So, if you collect 10 ounces for 22 seconds, you now know you are applying 20 gallons of liquid per acre.”
Parrish says if the grower fails to collect the desired amount, he can easily change the pressure or tips so he can obtain the desired amount. “If tips or pressures don’t work, you may need to change speed and rerun the distance to get a new travel time to use during collection,” he says.
Rocky Womack has written about agriculture and business for more than 25 years and currently serves as a contributing writer and correspondent for agriculture and business magazines, domestically and internationally. In the past, he has worked as a magazine editor and daily newspaper writer. Womack has won numerous awards for his interviewing, writing and in-depth reporting.