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Choosing the proper spraying equipment can make pesticide application either a simple or a difficult task. The following set of guidelines will help you decide which sprayer features best match your needs. As you make your decisions, keep in mind your or your operator's skill level in using the equipment. Some sprayers require additional training depending on the features you select.

When considering the purchase of spray equipment, several variables are important. You need to look at tank aspects, pumps, pump capacity, agitation method, strainers and filters, booms, nozzle bodies, sprayer monitors and controls, nozzle monitors, pressure gauges and safety aspects. Let's consider each here.

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Spray-tank aspects Your first consideration when purchasing spray equipment is determining the size tank you need, as well as what materials you prefer for its construction. Common tank-construction materials include fiberglass, molded plastic and stainless steel. Most importantly, the tank material should be corrosion-resistant and compatible with the chemicals you apply.

To ensure ease in cleaning, look for a tank design that minimizes leftover spray mix in the bottom. This will help you avoid disposal problems. Some tanks have a built-in sump that helps empty them completely. Also look for a drain plug at the tank's lowest point to ease cleaning. Finally, the liquid level in the tank should be clearly marked, or the tank should have a sight gauge (visible from your seated position) that shows liquid levels.

Choosing a pump The pump on your sprayer must deliver adequate flow and pressure for all applications. It also should handle the desired chemicals with minimal corrosion and wear. Pumps generally fall into two categories: positive-displacement pumps and non-positive-displacement pumps. Positive-displacement pumps include roller pumps, diaphragm pumps and piston pumps. Non-positive include centrifugal pumps.

* Roller pumps produce moderate flows and pressures. A slotted rotor, revolving in an eccentric case, holds the rollers. As the rollers pass the pump inlet, the cavities between and under them enlarge and draw in liquid. When nearing the outlet, the cavities contract-due to the eccentric housing-and force the liquid out of the pump.

Roller pumps handle a variety of pesticides and have low initial and maintenance costs. These pumps operate efficiently but, as pressure increases, their volume (output) decreases. If you primarily use wettable powders, a roller pump probably isn't your best choice. Roller pumps are not well-suited to abrasive materials because they rapidly wear the pump housing, the rotor slots and the rollers. Replacing the rollers is easy but-depending on housing wear-may not restore the pump to its previously satisfactory working condition. Roller pumps with nylon rollers work well with most chemicals, but rubber rollers are slightly better if you must use abrasive materials with these units.

* Centrifugal pumps create flow and pressure from an impeller's centrifugal force. Liquid enters through the impeller's center. As it spins, centrifugal force throws the liquid into a spiral passage leading to the outlet. The only moving parts in a centrifugal pump are the shafts and impellers. The impellers operate at a high rpm to give rated performance. PTO-powered centrifugal pumps require speed-up drives and high engine rpms, which can waste fuel when spraying. An alternative is a pump powered by a hydraulic motor connected to the sprayers' hydraulic system. Centrifugal pumps last a long time-even with wettable powders-and produce a high flow volume that is ideal for hydraulic agitation in the sprayer tank.

* Diaphragm pumps have at least one chamber sealed at one end by a membrane or diaphragm. The other end has an inlet and outlet valve. The diaphragm connects to a piston. As the piston moves, suction draws the liquid through the inlet valve by moving the diaphragm, which enlarges the chamber. The piston's return forces the diaphragm inward, shrinking the chamber and propelling the liquid out. A compression chamber smoothes line pulses. Ask to add one if it isn't already incorporated into the design of a pump you like.

Diaphragm pumps require minimal maintenance because less contact takes place between the spray material and moving parts. Nevertheless, you'll need to periodically check the diaphragm for pinhole leaks that can cause problems and lower pressure. Ask whether the diaphragm pump you are considering will resist the chemicals you use. Abrasive materials are less likely to damage this type of pump.

* Piston pumps propel liquid by a piston moving in a cylinder, similar to a combustion engine. The intake stroke draws the liquid in through one valve, and the output stroke forces the liquid through another valve. Piston pumps have either an internal or external air chamber (surge tank) to dampen pulsations in the liquid flow associated with each stroke. Without a surge tank, the spray will pulse rather than being applied in a steady flow.

Piston pumps develop high pressures, which can increase a sprayer's versatility. However, the relative capacity of piston pumps is often low. Because higher-volume piston pumps are expensive, they normally use mechanical agitation instead of hydraulic agitation. As a professional applicator, look for larger-capacity pumps driven by an auxiliary engine. The large pumps-with 2 to 8 cylinders-achieve higher flow rates, and the multi-cylinder design produces more even flow.

The importance of pump capacity Look for a sprayer with a large enough pump to handle your capacity requirements. Due to mechanical inefficiencies, some pumps don't pump at appropriate pressures to move sprayer chemicals. Thus, make sure you choose a pump with a capacity that is 50 to 75 percent higher than your calculated flow requirements. The pump should have sufficient capacity to supply several nozzles, provide hydraulic agitation (if used), return flow for self-cleaning strainers and bypass-type regulating valves, and offset pump wear.

How agitation affects chemical application The type of agitation you choose depends on the chemical formulations you apply. A uniform chemical application depends on a uniform tank mix. If the chemical separates from its carrier, you'll apply unequal chemical concentrations. Therefore, all sprayers benefit from a tank agitator to maintain a uniform mix.

Two common types of agitation are hydraulic agitation and mechanical agitation. * Hydraulic agitation, commonly called jet agitation, uses part of the pump's flow to create a mixing action in the tank. With this type of system, you need a pump large enough to provide the extra flow volume that the jet agitator requires. Typically, you can use 5 to 10 percent of tank volume to determine the necessary flow. The flow can be through a standard agitation nozzle or a specially designed siphon nozzle. The siphon agitation nozzle creates a venturi effect-or vacuum-that increases the nozzle's discharge. This increases the mixing action by two-and-a-half times and is effective when the available flow is marginal.

Hydraulic agitation also can consist of a sparger, pipe or tube with several discharge holes in the tank's bottom. A sparger-a pipe or tube with several discharge holes-is best for large, long tanks. Check that the hydraulic agitator is in the bottom of the tank so it sweeps all areas. Units with hydraulic agitation also should have some type of flow-control device. Too much agitation creates foaming in the tank, while not enough creates unequal chemical concentrations. A flow-control valve in the agitation line allows you to adjust the flow for a given chemical mix.

Mechanical agitation is produced by paddles or propellers on the tank's bottom. The sprayer's power source or a 12-V electrical motor mechanically drives these mixers. Sprayers with piston pumps typically use mechanical agitators, because jet agitation requires a larger flow, which results in a more expensive piston pump.

Any of these agitators, when properly designed and operated, will adequately agitate most pesticides.

Strainers and filters Line strainers are an important part of the sprayer's plumbing system. A properly placed and sized strainer can prevent your worst nightmare: plugged nozzles. Strainers come with different mesh sizes, which indicate the number of screen openings per linear inch. For example, strainers with mesh sizes of 100 and 200 have smaller openings than mesh sizes of 30 or 50.

For most positive-displacement pumps, you need a suction-line strainer-with a mesh size of 30 or 50-between the tank and pump. This type of strainer protects these pumps' small, internal, moving parts.

On a centrifugal pump, you must ensure that the pump's inlet is not restricted. Thus, look for a line strainer on the pressure side of the centrifugal pump. The strainer should have a 50-mesh screen. In this location, it will protect both the nozzles and the agitation system. Using a suction-line strainer, as suggested for positive-displacement pumps, could cause problems on a centrifugal pump because it could plug, which could cause pump cavitation. Pump cavitation causes premature pump wear, leading you to need a new pump sooner rather than later.

A feature that you'll sometimes find on both non-positive- and positive-displacement pumps is a self-cleaning line strainer. This type of strainer directs a high-velocity flow past the screen face, which continuously washes particles into a separate, unrestricted bypass line. Typically, you need a system with 6 to 8 gpm through the bypass line for proper operation.

Some sprayer manufacturers also put a small line strainer-with a 100- or 200-mesh screen-on each boom section. These strainers reduce your need to clean nozzle-tip strainers by catching small particles that larger strainers did not catch.

Nozzle-tip strainers also are important. They are your last line of defense against nozzle plugging. These strainers come in an assortment of sizes and materials. The nozzle's orifice size dictates the necessity for and the mesh size of a nozzle-tip strainer. See your nozzle manufacturer's catalog for recommendations.

The benefit of boom-control valves Sometimes you need to spray an area that's narrower than your boom's full width. As a result, manufacturers offer booms divided into several sections with individual valves. These valves are either manual or electrical and control the flow to either the left, center or right boom section, or any combination.

Many of these turf sprayers have remote-control, solenoid-operated valves. These typically have about a 10-gpm capacity with a 5-psi pressure drop and about a 100-psi maximum. These systems are relatively inexpensive and, with proper maintenance, can give years of reliable service.

More turf sprayers today offer remote-control-operated, motorized ball valves. Having larger passages, these valves typically have higher flow rates and higher maximum operating pressures than solenoid valves. Ball valves can be slightly more expensive than solenoid valves. However, they tend to be less maintenance-intensive in the long run.

Other sprayers use three-way valves. Available in both solenoid- and ball-valve-type, these units allow the system to maintain constant pressure when any boom sections are shut off. This places less strain on the sprayer system by reducing pressure spikes. A three-way valve system is a good feature if you will constantly turn booms on and off.

Some sprayer manufacturers have turned to installing manifold-type control-valve systems on their sprayers. These make maintenance and repair of the valves easier by allowing you to replace or repair one without completely disassembling the hoses or pipes.

Boom options Booms are available in two basic types: wet and dry. A wet boom is one in which the fluid travels through the boom itself. Dry booms are those that use hoses to supply fluid from one nozzle body to the next. A boom-support member typically supports the nozzle bodies.

Choosing a unit with a sturdy boom made from strong corrosion-resistant material is a good place to start. However some booms have the following extra conveniences you may want to consider: * Automatic boom-height control. This type of unit uses sonar to monitor the distance from the boom to the ground. As the terrain changes, the boom adjusts automatically according to actuators on each boom to maintain a constant application height. This is an important feature to consider because boom height is so important to maintaining good spray distribution.

* Remote-control boom fold-up. This feature allows you to fold the booms without dismounting the sprayer. It is useful if you must drive your sprayer around many obstacles.

* Shielded booms are becoming more popular due to the demand for better drift control. Shielded or hooded sprayers have enclosures over the nozzle bodies or over the entire boom section. These enclosures prevent the wind from carrying the small drops off target. This feature is great for drift control, but it may require the use of a nozzle monitor to inform you of a clogged nozzle because you won't be able to see if they are working properly.

Differences in nozzle bodies Nozzle bodies come in many shapes and sizes. As with boom types, they are available for both wet or dry booms, with or without diaphragm check valves and with multiple- or single-nozzle outlets.

* Diaphragm-check-valve nozzle bodies are a plus because they prevent nozzle dripping after you shut off the booms. This prevents chemical contamination on areas outside the spray zone. * Multiple-nozzle or turret-type nozzle bodies give you greater flexibility when it comes to changing spray tips or unclogging them. All you need is to index the turret to the next nozzle and resume spraying with minimal downtime.

Monitoring and controlling Sprayer-system monitors continuously sense the sprayer's operation, measuring travel speed, pressure and flow. Using this information, the device calculates the application rate and other useful information, such as application swath width and gallons of spray mix in the tank. You can use this information to better control the spraying operation, resulting in more precise pesticide application and better pest control.

Most monitors today come with an automatic rate-control function. This controller calculates the actual application rate and constantly compares it to a pre-set desired rate. If the controller detects a discrepancy (error), it automatically adjusts the application rate (usually by adjusting flow). Some controllers have preset alarms that alert you if the error is too large to correct. These types of errors include speed varying too much for pressure compensation, hose or connection breaks, line-strainer plugs or other serious problems.

A new option: Nozzle monitors Some new sprayers have monitors that sense flow at individual nozzles. When nozzle flow stops, due to plugging or a loss of pressure, a signal-such as a flashing light or a buzzer-alerts you. This helps you detect clogged nozzles you can't see from your driver's seat or nozzles located under hooded or shielded booms.

The presence of pressure gauges Look for a unit with a pressure gauge designed to measure liquid pressure in a range of about one-and-a-half to two times the maximum anticipated pressure. Another beneficial sprayer feature is a dampener between the gauge and the sprayer to smooth pressure pulsations. This makes the gauge easier to read and prolongs the life of moving parts. Liquid-filled gauges are preferable to dry gauges because the liquid dampens needle vibrations.

If you've now detailed the specific features you want your sprayer to have, you probably are ready to shop. Keep in mind that you may not find the perfect sprayer with all of the features described here. Nevertheless, you're bound to find several that meet most of your requirements. At the very least, you know all the advantages and disadvantages to the various features, which can help you better balance which sprayer will be appropriate for your needs.

Dr. Stephen L. Pearson is worldwide technical services director for Spraying Systems Co., Agricultural Division (Wheaton, Ill.).

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