22 FOR SPRAY OPERATORS to reduce the potential for pesticide drift, they must be situationally aware, willing to make operational changes, and they must possess a basic understanding of why drift happens — and that means physics. Sir Isaac Newton was 23 years old when he postulated, “An object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.” He was telling us that the more we can control or compensate for the forces that influence a droplet’s movement, the more likely we can direct where it will end up. Here’s a run-down on some of those forces and what operators can do about them. DROPLET SIZE Coarser droplets have more mass, which means they have more inertia; they are less prone to deflection by wind, more prone to deflection by gravity, and therefore most likely to travel ballistically (e.g. like a cannonball). Coarser droplets also have smaller surface-to-volume ratios. This means they resist evaporation on hot and dry days, staying on course and remaining wet longer on target surfaces (often associated with better product performance). Switching to nozzles that produce a coarser spray will reduce drift potential but may also negatively affect coverage by reducing droplet count, canopy penetration and retention. Operators should understand how much coverage is required for whatever they are spraying and be prepared to switch to a coarser spray as the situation dictates. DISTANCE TO TARGET The longer a droplet remains in the air, the more opportunity for outside forces to affect its trajectory. Increasing pressure can increase droplet velocity, but it also increases spray volume (on non-PWM sprayers), reduces the average droplet size, and does not have an appreciable influence on the finer, more drift-prone droplets. A far better approach is to bring nozzles as close to the target as practicably possible. In other words, horizontal boom sprayer operators should lower the boom and airblast sprayer operators should invest in towers, Spray drift A LESSON IN PHYSICS FROM ISAAC NEWTON Jason Deveau adjust deflectors, and consider pruning or hedging practices. TRAVEL SPEED Consider the following metaphor. If you dropped a ball from the window of a moving car, it would follow a downward/forward vector. The faster the car, the greater the forward component. How would wind play a role in this? Windspeed and travel speed are additive, which means that driving into a headwind adds travel speed to wind speed, increasing the chance of deflecting the ball — or in the case of a sprayer, displacing droplets fromthe swath and leaving them hanging in the air. Those suspended droplets are also subject to turbulent eddies created by the sprayer chassis and wheels that swirl in unpredictable ways and can propel them upwards, increasing their distance to target. Increased travel speed also tends to amplify boom yaw, necessitating higher booms to prevent impacts and increasing a droplet’s distance to target. Operators sometimes resist slowing down because of the negative effect on their work rate. However, this can be offset by more efficient filling and cleaning practices and/or from an improved yield arising from better coverage uniformity. Slowing down works. WIND AND ATMOSPHERIC STABILITY Spray droplets can be displaced from the swath by the wind. The distance they drift depends on apparent wind speed and droplet size. On moderately (not excessively) windy days, operators can predict the direction spray might drift and plan accordingly when there are sensitive areas to protect. If the options are available, wind can be blocked by shrouds or outcompeted by air assist. However, simply reducing the distance to target and increasing droplet size can be effective practices. In any case, Agronomy
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