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Pesticides drift from the targeted area reduce pesticide effectiveness due to inaccurate application rates, spray patterns and droplet sizes.
Begin sprayer calibration with the correct nozzle type and size. Nozzle material determines the rate of wear. Incorrect tank mixing and nozzle wear, causes error, hence improper spray equipment calibration.
Ensure the correct nozzle type and size as per the required application. Flat-fan nozzles are used for spraying most herbicides and some insecticides where a medium droplet size is needed. Flat-fan nozzles are used for banding herbicides. Full cone nozzles and flood nozzles used for pre-plant herbicides produce drift-resistant large droplets, and wide nozzle spacing used. To apply insecticides and contact herbicides that need to penetrate the canopy, hollow cone nozzles are used because of their small droplets. Select the correct nozzle material. Minimum wear-resistant elements such as tungsten, carbide, ceramic and hardened stainless steel help nozzles maintain a constant flow rate after a long period of use. less durable material nozzles (plastic, brass) increase flow rates after only a short period of spraying. For example, after 50 hours of spraying, a brass nozzle can have an increased flow rate of 10 to 15 per cent, whereas a hardened stainless steel nozzle will increase only about 2 per cent. Widened nozzle orifice area lead to increased flow. Compared to the high costs of over-application due to worn nozzles, the purchasing cost of durable nozzles is negligible.
Desired application rate depends on the nozzle size, ground speed and nozzle spacing, hence the swath. 50cm and 70 CM are the most commonly used nozzle spacing. Nozzle flow rate is determined from the subsequent equation;
Application rateL/Ha x Swath(M) x Speed(Km/Hr) =Flow rateL/Min
Spray Rig Preparation
Thoroughly clean the spray rig. Search for indications of rust, leaks or other issues.
Determine the litres needed per hectare based on the recommended rate from the pesticide label, tank size, pesticide container size, and rate of pesticide application per Ha.
Estimate nozzle application rates based on the planned speed and boom pressure.
Test all spray boom nozzles for signs of wear. Replace worn nozzles and those of the wrong size for the required application.
Calibrate with spray tank half-filled with water.
One Way to Calibrate a Sprayer
Calculate the speed of the rig with the sprayer in place by (Moving along 100M with the tractor and noting the average time in seconds at two different times). Get the distance in metres between spray nozzles on the boom.
Calculate the desired nozzle output (l/min).
Collect water from one or two nozzles in 60 seconds at the operating pressure
Regulate the pressure or speed until the desired output is attained.
Evaluate the acreage covered on one tank of spray mixture.
Then do the actual spray with a pesticide and carrier (usually water) and determine if the correct amount of acreage has been covered.
Recalibrate if the desired results have not been attained.
Adjust pressure to obtain the desired nozzle flow rate within the recommended operating pressure. High pressures will produce small spray droplets susceptible to drift. low pressures produce larger, less-effective spray droplets and poor spray pattern.
Spray System Checks
Fill the sprayer with water after making the necessary changes and measure the nozzle flow rates by catching the nozzle output for 1 minute. Keep to the desired application rate as over-application results in wasted pesticide, potential groundwater contamination, and possible crop injury. Under-application can produce ineffective pest control.
Inaccurate flow rates can result from damaged, worn or clogged nozzles or strainers, and spray hose restrictions between the pressure gauge and the nozzle. Tidy up nozzles with a toothbrush, not a knife or wire. Never blow out a nozzle with the mouth.