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Knapsack Sprayers
A knapsack sprayer is suitable for small plantings up to an acre in size. This sprayer is entirely manual and is carried on an operator's back. Also called a backpack sprayer, the knapsack sprayer is designed to be as light as practical. A typical empty weight is about 12 pounds. With 4 gallons of water, the weight is approximately 45 pounds. The parts of the knapsack sprayer are the same as those found on most sprayers: a tank to hold the spray mix; a pump to produce pressure and flow; a regulator to control the flow; and at least one nozzle to atomize the spray mix (Figure 2.2). Tanks on the knapsack sprayer typically are made of plastic or steel and hold three to five gallons. Some knapsack sprayers have a mechanical agitator that moves when the pump is used. Others have jet agitation. Before spraying, shake the entire sprayer to ensure a uniform mix.
The sprayers have a built-in piston or diaphragm pump that is operated by hand. Some models can be adapted to either right- or left-hand pumping; the other hand is needed to operate the flow-control valve and the wand. The pumps are capable of relatively high pressures (80 to 180 psi). Some backpack sprayers are equipped with pressure gauges as well as pressure regulators. The chamber that pressurizes the chemical liquid is very small, so the operator must pump while walking and spraying. At least one company manufactures a knapsack sprayer with an engine-powered pump. This eliminates hand pumping but increases the empty weight of the sprayer by about 50 percent.
The distribution system includes an on-off valve, usually with a pistol-grip handle, and one or more nozzles on a wand. The nozzle is often mounted at an angle on a 16- to -20-inch-long wand to aid spray placement on the trees. Some designs allow for interchangeable nozzle tips so that the nozzle can be better matched to the job. Others use an adjustable nozzle that can be varied from a wide discharge angle to a solid stream. Although the solid stream setting will throw the liquid farther, it should not be used on fruit trees because there is little or no atomization. Poor coverage and pest control results.
The time and energy needed to use a knapsack sprayer on fruit trees limits the device to small plantings. When labor is of minimal consideration, such as with homeowners and hobbyists, the knapsack sprayer can be effective. Its size, however, is not practical for applying high rates of water per acre. Considerable practice is required to obtain thorough coverage of trees without overspraying, which creates wasteful runoff and may increase the risk of phytotoxicity.
A great deal of skill is needed to obtain a uniform application. The rate of spray will be less reliable by hand than if a tractor sprayer is used. Application rates, walking speeds, and coverages also will vary with operator fatigue caused by temperature conditions, the time of day, the slope of the terrain, and the walking surface. In addition, the risk of overspraying and underspraying is increased because the knapsack sprayer uses a small volume of water. Extra care should be given to coverage and uniformity.
