Calibrating Airblast Sprayers
Table of Contents
Calibration is one of the most important aspects of spray application and yet it remains neglected and misunderstood by most operators. Common reasons for not calibrating are:
This Factsheet describes a simple and effective method for calibrating an airblast sprayer, with clear terminology and equally clear reasons for calibrating regularly.
It is very important that any calibration technique be followed exactly. Do not mix and match parts of different calibration techniques. It is important that protective safety clothing is used while calibrating and adjusting spray equipment. Protective clothing should protect against both contamination and physical injuries, and be tight-fitting when worn close to moving parts such as power take-off (PTO) shafts, fans and belts.
"Application rate" is a general term used to describe the amount of material that is placed on a treatment area. Depending on the publication, this term may refer to the total volume of spray mixture, the amount of formulated product or the amount of active ingredient. Therefore, for the purposes of explaining the calibration procedure, the term "Application Rate" will not be used in this Factsheet.
"Sprayer output" describes the total amount of material that is delivered by the sprayer to the treatment area (e.g., 500 L/ha). This includes the formulated product as it is supplied from the manufacturer and the carrier used to make up the total spray mixture.
"Product rate" describes the label-recommended amount of formulated pesticide product, as it is supplied by the manufacturer, placed on the treatment area (e.g., 1 kg/ha).
Calibration is essential because it:
That final point is very important. A boom that has both worn and plugged nozzles can still deliver the calculated sprayer output, so every nozzle must be tested. Even brand new nozzles must be tested. It is not unheard of for new nozzles to deviate from the ideal rate by as much as 15%.
Product rates and sprayer output requirements change, depending on the crop type, the plant spacing and the crop and pest staging. Therefore, calibrate for each significantly different situation. Calibrate airblast sprayers:
Figure 1. Typical calibration tools, including a stopwatch, a tape measure and a collection vessel of known volume.
When calibrating an airblast sprayer, wear coveralls, gloves, boots and possibly a face shield. You will also need (see Figure 1):
Calibrate sprayers in a vineyard, nursery or orchard that is representative of the vineyard, nursery or orchard to be sprayed. Calibrating a sprayer on a hard surface (such as pavement) can induce errors as high as 15% compared to calibrating in a tilled field. Calibrate away from buildings and wells.
There is more than one way to calibrate an airblast sprayer. Essentially, every method requires you to determine:
Use the checklist below to help with your calibration.
Perform a pre-calibration inspection
Fill the decontaminated sprayer half-full with clean water and:
Start the pump and set the tractor engine speed to the desired rpm. Open the manifold valve to fill the lines and begin spraying. Adjust the pressure regulator, or set the main by-pass, to obtain the desired operating pressure. Perform the following steps:
Adjust the air stream
The air stream created by the sprayer fan carries the spray mixture into the trees or vines and distributes it throughout the foliage. To reach all leaf surfaces and achieve adequate pesticide coverage, all the air around the foliage must be replaced by the spray-laden air stream.
Early-season airblast spraying generally requires very little air, unless competing with wind. Later in the season, larger, fuller canopies require a greater volume of air. In some cases, air volume can be varied by increasing or decreasing the power take-off speed (PTO rpm), engine rpm or the fan gear.
The air direction or angle of attack to the foliage is equally important:
Figure 2. Ribbons show where air and spray will go.
Confirm sprayer pressure
To confirm that the main pressure gauge is accurate, temporarily install a second oil-filled gauge in-line beside the main pressure gauge.
Pressure in the booms is often less than the desired operating pressure.
Piston-operated sprayers and sprayers equipped with diaphragm pumps generally do not maintain the desired output when spraying from a single boom. Most units will experience a pressure increase, thereby increasing the sprayer output. Some sprayers compensate through an electric bypass valve that shunts extra volume and pressure back to the tank. The rate can be adjusted using a throttling valve to match the volume that would be spraying out through the closed boom. Ideally, the pressure should remain the same when a boom is shut off. To set the bypass:
Figure 3. Three methods to attach a pressure gauge to an airblast boom:
A. Use an elbow and a quick-connect cap for
sprayers with booms with quick-connect nozzle bodies.
Centrifugal pumps generally do not have regulators but do have bypasses that offer limited control over operating pressure.
Set forward speed
Forward speed must be slow enough to allow the air stream to completely replace the air in the canopy, but not so slow that excessive blow-through results. Generally, this will be no faster than 5 km/h (~3 mph). Forward speed also impacts the product rate. Measure the time for the sprayer to drive 50 m. This step corrects speedometer errors due to wheel slippage. It is important to perform this step in the field so it accounts for soil type, slope of terrain and the average weight of the sprayer.
Forward speed (km/h) = 50 m x 3.6 / Average drive time in seconds
Forward speed (mph) = 50 m x 2.2 / Average drive time in seconds
Calculate sprayer output for each side
Airblast disc-core, disc-whirl and moulded nozzles are sold based on their output per minute. A set of nozzles on one boom, when added together, should produce the required output per side. Use any of the following formulae, depending on the units used:
Output per side (US gal/min/side) = (Target sprayer output (US gal/acre) x Forward speed (mph) x Row spacing (ft)) / 1000
Output per side (L/min/side) = (Target sprayer output (L/acre) x Forward speed (km/h) x Row spacing (m) / 500
Output per side (L/min/side) = (Target sprayer output (L/ha) x Forward speed (km/h) x Row spacing (m)) / 1,220
Select a set of nozzles to produce the required sprayer output at the selected operating pressure. Choose nozzles that both give a correct total output and produce the desired spray pattern.
Figure 4. A. Suggested spray distribution for airblast sprayers on classic spindle apple trees. B. If the canopy is of uniform depth, such as with vines, spray distribution should also be uniform, where each nozzle sprays the same rate. Fractions represent the portion of the boom. Percentages represent the relative amount of sprayer output. These distributions may change depending on the location of the pest or when there is fruit to protect.
Based on the results of the ribbon test, place nozzles of different outputs in appropriate locations on the sprayer manifold to achieve the desired spray pattern. Usually, most of the spray volume is directed at the thickest foliage (see Figure 4).
Confirm spray distribution and air settings by placing water-and-oil-sensitive paper in the hardest-to-reach portions of the canopy, then spraying the entire canopy with clean water. Make corrections to the set-up to achieve adequate coverage throughout the canopy.
As the canopy grows and fills, reconsider the spray distribution and output volume. For example, some apple growers choose to open another nozzle position lower on the boom to hit low-hanging branches, but this is not the best way to redistribute spray. The better approach is to turn on a lower nozzle position, then distribute a higher sprayer output over the entire boom; this way, the whole canopy gets more spray, not just the bottom of the target.
Figure 5. Example of a nozzle output record.
Measuring actual sprayer output
In theory, the sprayer has been set up to deliver a specific output per side. In fact, the output may be different than expected. This is true both of new nozzles, which can vary in output by as much as 15%, and of old ones, which may be worn through use. To calculate the total delivery rate, measure the actual output per minute of each nozzle. Use a diagram similar to Figure 5 to track nozzle rates:
Pump and Lines
diaphragms and/or plungers checked/replaced
Strainers and Nozzles
strainers from tank opening to nozzle strainers clean and unbroken
Regulators and Gauges
Belts and Power Take-Off (PTO)
belts have proper tension and no wear
Propeller and Agitation
has no nicks or cracks or residue and does not have any lateral
Orientation and AdjustmentAirflow and Direction
or deflectors adjusted to steer air into canopy
pressure set to desired pressure
1.0 L=0.264 US gal (most common in catalogues)
1.0 L=0.22 Imperial gal
1.0 m=39.4 in.
1.0 ha=2.47 acres
1.0 km/h=0.62 mph
1.0 L/ha=0.106 US gal/acre
1.0 L/ha=0.09 Imperial gal/acre
This Factsheet was authored by Dr. Jason S.T. Deveau, Application Technology Specialist, OMAFRA, Simcoe, and reviewed by Helmut Spieser, Engineer, Field Crop Conditioning and Environment, OMAFRA, Ridgetown.
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