Airblast operators have to know the volume of spray mix their airblast sprayers emit for a given area. Generally, this is referred to in L/ha or US g/ac. This information must be recorded to comply with traceability requirements (e.g. CanadaGAP) and to ensure they are following the pesticide label. There are many ways to determine this value:
Method one: Fill the sprayer to a known volume, spray using typical settings, and then measure off how much area was covered once the sprayer empties. This requires the operator to be able the measure the sprayer area accurately, and while that’s relatively easy for a field sprayer, it’s tricky for an airblast sprayer.
Method two: The inverse of method one is to measure off an area (e.g. a hectare or acre), fill the sprayer, spray using typical settings, and calculate the volume used to complete that area. Once again, the operator must measure both an area and the sprayer volume accurately.
Method three: The preferred method is to use math. It requires the operator to know their average travel speed, row spacing and the current output per sprayer side. Using any of the formulae below (depending on preferred units), they can determine the sprayer output for a given area. Note that these formulae are set up to calculate the output per side. This is because it’s more common that the operator already has a target output in mind (e.g. 550 L/ha), but does not know what nozzles are needed to achieve it. A set of nozzles on one boom, when added together, produce the output per side.
These formulae are for airblast sprayers with two booms. For airblast sprayers with only one boom, such as a cannon sprayer (see below) you must reduce the constant by half: 990 becomes 495, 500 becomes 250 and 1,200 becomes 600.
The formulae will help determine either the output for a given area, or the output for a boom, but how does the operator distribute the output over the boom? In other words, does every nozzle emit the same rate, or do some emit more than others? It depends…
Distribute spray over the boom
The operator should have already decided how many nozzles will be used. This is determined by eliminating any canopy over- and under-shoot using the ribbon test. Often, operators will distribute spray to direct most of the volume at the thickest foliage, or shift higher volumes towards the fruit zone in grapes. A classic practice was to use high-volume full cones in the top boom positions to ensure spray reaches the highest part of the target, but unless the target is exceptionally high and/or dense (e.g. hop yards, nut orchards) my preference is air induction tips and better air management. Part A of the image below shows how many operators distribute the spray as percentages.
Alternately, work at Ohio State showed that if the target is of uniform depth and moderate height, like trellised trees, the operator can safely distribute the spray evenly over the boom (see Part B). Similarly, tower sprayers that make the distance-to-target more consistent for each nozzle may also use the same rate in each position.
Once ideal rates have been determined for each position, the operator uses a nozzle catalogue to select nozzles that most closely match the desired rates for a given pressure. An older (and hard to find) disc-whirl table from John Bean is provided below:
Note that the table goes up to 450 psi! Most orchard, vine, nursery, bush and cane fruit operations do not require pressures above ~120 psi. Only large operations like citrus, nut and other large-canopy crops require such high pressures to achieve high rates.
Remember: Air, not pressure, should propel spray.
Once the sprayer is finally nozzled, the operator should double-check the actual output (which can deviate from intended due to wheel slippage, or pressure issues). Then, check the resultant coverage with water-sensitive paper to ensure your spray distribution matches the target.