Ground Cover Supplement : GC Supplement - Spray application 2016
3 Spraying speed Water-sensitive paper and the SnapCard app (see pages 18 to 21) can be used to evaluate the effects of spraying speed and coverage where droplets land. FAST FACTS PHOTO: BRAD COLLIS Increasing spraying speed can influence more than pressure at the nozzle. Before you put the ‘pedal to the metal’ it is important to understand how increasing speed can affect sprayers aerodynamically and change how droplets are produced and where they land However, the temptation to spray more hectares per hour by increasing spraying speed is not necessarily a valid strategy for improving the overall level of control. When application speed is too fast for good spray deposits, it can start to erode efficacy and increase the potential for drift. The impacts of higher spraying speeds include the following: INCREASED PRESSURE AT THE NOZZLE The most obvious effect of changing spraying speed is on the pressure at the nozzle (the main exceptions are for spray application with manual pressure or pulse width modulation). With conventional hydraulic nozzles, increased pressure produces smaller droplet sizes. Regardless of the system used, spraying speed can also influence how droplets behave close to the sprayer. Increasing speed affects spray droplets at all stages from droplet formation to drop deposition on the target, including: ¢ at the nozzle, by increasing small droplet escape from the spray pattern; ¢ air movement around the sprayer (aerodynamically), which affects droplet movement close to the chassis (known as the wake effect) and adjacent to the wheels and tyres; and ¢ target coverage by influencing how the droplets may deposit and penetrate a canopy. 1Increased escape of small droplets from the spray pattern Air movement affects the nozzle and can change how the spray pattern is formed and the ability of droplets to remain within the spray pattern. When the airspeed coming into contact with the spray pattern is fast enough, it can: ¢ change the shape of the spray pattern, causing it to narrow and wrap backwards -- this can affect the overlap of the spray patterns and the evenness of the spray deposits onto the target; ¢ cause a loss in downward velocity Increasing speed can reduce coverage By Bill Gordon TARGETING THE MOST susceptible stage in the life cycle of a pest or weed is a critical factor in achieving effective control. Many growers prioritise the ability to get over as much country as possible with their sprayer while weeds are still fresh, grubs are small or before a fungal pathogen spreads.
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