In Ontario, most airblast sprayer operators use disc & core (or disc & whirl) combination nozzles. Depending on the manufacturer, the disc plate is defined by it’s diameter in 64th’s of an inch. The core or whirl plate might be described by the number of holes (e.g. 2-hole, 3-hole, etc.), or some other manufacturer-specific nomenclature (e.g. 45’s, 25’s etc.). The rates emitted by combinations of disc & core are determined using water sprayed over a range of pressures (read more about selecting nozzles for the sprayer here). Sprayer operators have access to all this information in the form of nozzle tables.
Most nozzle catalogues have tables similar to the example below. If you don’t have a current catalogue (or indeed, any catalogue), ask your sprayer parts supplier for one. They’re free. The higher the pressure (shown along the column headings), the higher the rate emitted by the disc & core combination (shown along the row headings). If you want to know how much your nozzle is emitting at a given pressure, find your nozzle on the left, find your operating pressure along the top, and the rate is where they intersect. Alternately, if you have a rate in mind, and want to know which nozzle is the best choice, find your operating pressure along the top, and look down the column until you find your rate. Then, look left to the nozzle combination.
It’s a rare and wonderful thing to find exactly the rate you’re looking for – you may have to make concessions to operating pressure to achieve your desired rate. Alternately, you may have to accept a rate that’s close to what you want, but not exact.
You might notice that different disc & core combinations can create the same rate. The rate may be the same, but the median droplet diameter and the angle of the spray cone are different. Most nozzle tables clearly indicate the different spray angles, as shown in the last three columns in the example table.
The angle of the spray cone can have a big impact on spray coverage when the target is very close to the sprayer, such as in a vineyard or when spraying canes or berry bushes. If nozzle angles are too small, the spray may throw farther, but not overlap sufficiently before it reaches the target.
To see if this is happening to you, park the clean sprayer in the alley between crops and start spraying water. Have someone stand behind the sprayer and look for gaps in the swath. It might look very different compared to what the operator thinks they are seeing from a shoulder check. Creating a full, overlapping spray swath that spans the entire canopy is a function of nozzle spacing, distance-to-target, and sprayer air-settings. It can be affected by humidity, wind speed and wind direction at the time of spraying.