Working Adjustments

Application rate Three factors affect the appli­cation rate of crop sprayers:

• Tractor forward speed A low forward speed gives a high application rate. By changing gear to double the forward speed, the appli­cation rate will be halved.

• Spraying pressure Higher application rates are obtained by adjusting the relief valve control to increase working pressure. High pressure reduces droplet size and increases the risk of the chemical being blown away by the wind (drift). This should be avoided, as drift can damage nearby crops and gardens. Spray drift is also a serious threat to the environment as it will harm trees, hedge­rows, wild flowers, etc.

• Nozzle size Nozzles with a large hole will give a high application rate. Smaller ones must be fitted when a reduction in applica­tion rate is required.

Once set, the application rate can be kept con­stant with an automatic control system. This electronic device is found on the more expen­sive sprayers. Some sprayers have a system of pressure compensation which avoids variations in working pressure when a section of the spraybar is turned off.

Figure 13.8 Setting the height of the spray boom.

Spray boom height. The spray boom must be level when in work. Figure 13.8 illustrates how the spray patterns from adjacent nozzles should meet above the target for even distribution of liquid across the width of the boom. When spraying weeds which are taller than the crop, the weeds must be considered the target. On most sprayers, the nozzles are spaced at 500 mm intervals along the spraybar. The height of the nozzles above the target is determined by the angle of the spray pattern. For example, nozzles with an 80 degree fan pattern and spaced at 500 mm on the boom should be about 550 mm above the target. Nozzles with 110 degree fan pattern will need to be set lower at about 350 to 400 mm above the target. When weather conditions allow, the nozzles can be set a little higher to avoid missed strips when uneven field surfaces cause spraybar bounce. The height of the spray­bar is adjusted by moving it up or down on the frame. This adjustment is important because incorrect nozzle height will cause overdosing or missed strips.

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