SPRAYER CALIBRATION FUNDAMENTALS

Sprayer Calibration Fundamentals

 Facts.

• Inaccurate pesticide application rates, spray patterns and droplet size can lead to the movement of pesticides from the targeted area and reduce pesticide effectiveness.
• The first step in sprayer calibration is to determine the correct nozzle type and size.
• Nozzle material is important in reducing inaccurate applications due to nozzle wear.

Due to timeliness and effectiveness, chemical pesticide application has become a leading method of weed and insect control in  agricultural production. The continued use of pesticides in the agricultural industry has led to concerns of chemical trespassing by groundwater contamination or drift.

 Although inaccurate tank mixing causes some of these errors, a majority of the problems result from improper spray equipment calibration and worn nozzles.

Nozzle Selection

The first step in sprayer calibration is to determine the correct nozzle type and size (flow rate). Flat-fan nozzles are used for broadcast spraying of most herbicides and some insecticides where a medium droplet size is needed. Flat-fan nozzles are used for banding herbicides. Flooding type and full cone nozzles used for pre-plant herbicides produce drift-resistant large droplets, and wide nozzle spacing can be used. Hollow cone nozzles produce smaller droplets and are used to apply insecticides and contact herbicides that need to penetrate the canopy.

Inaccurate applications can be due to nozzle wear. Therefore it is important to select the correct nozzle material. Wear-resistant materials such as tungsten, carbide, ceramic and hardened stainless steel help nozzles maintain a constant flow rate after a long period of use. Nozzles made from less durable materials (plastic, brass) demonstrate increased 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 percent, whereas a hardened stainless steel nozzle will increase only about 2 percent. The increased flow rates result from an increased nozzle orifice area. The added cost to purchase a more durable nozzle can pay for itself many times over by reducing the over-application that results from nozzle wear.

Nozzle size depends on the desired application rate, ground speed and nozzle spacing. For each nozzle type and spray angle, the manufacturer recommends spray height and nozzle spacing. Nozzle spacings of 50 and 70 CM are most common. The desired flow rate from the nozzle can be determined from the following equation;

Application rateL/Ha*Swath(M)*Speed(Km/Hr) =Flow rateL/Min

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Calibration Procedure

Spray Rig Preparation

1. Thoroughly clean the spray rig. Check for signs of rust, leaks or other problems.
2. Determine the litres needed per hectre based on the recommended rate from the pesticide label, tank size, pesticide container size, and rate of pesticide application per Ha..
3. Calculate a rough estimate of nozzle application rates based on the planned application speed and boom pressure.
4. Check all nozzles on the spray boom for signs of wear and nozzle size. Replace worn nozzles and nozzles of the wrong size for the desired application.
5. Half-fill the spray tank with water and go to the prepared field.

One Way to Calibrate a Sprayer

1. Measure the ground speed of the rig with the sprayer implement in place. (Average the travel time of the tractor in seconds over 100M in the field for two separate passes.)
2. Calculate the ground speed.
3. Measure the distance in metres between spray nozzles on the boom.
4. Calculate the desired nozzle output (l/min).
5. Catch one minute's worth of water from one or two nozzles at the operating pressure.
6. Adjust the pump pressure or ground speed until the desired output is reached.
7. Calculate the acreage covered on one tank of spray mixture.
8. Finish filling the spray tank with pesticide and carrier (usually water). Apply about one-half tankful of spray and determine if the correct amount of acreage has been covered.
9. Continue spray application; recalibrate if the first half tankful didn't cover the correct acreage.

Use small adjustments in pressure to obtain the desired nozzle flow rate within the recommended operating pressure. Operating a nozzle at excessively high pressures will produce small spray droplets susceptible to drift. Operating at excessively low pressures produces larger, less-effective spray droplets and poor spray pattern uniformity down the length of the boom.

Spray System Checks

After all the adjustments are made, fill the sprayer with water and measure the nozzle flow rates by catching the nozzle output for 1 minute. Maintaining the desired application rate is essential. Over-application results in wasted pesticide, potential groundwater contamination, and possible crop injury. Under-application can produce ineffective pest control.

Erroneous flow rates can result from damaged, worn or plugged nozzles or strainers, and spray hose restrictions between the pressure gauge and the nozzle. Clean nozzles with a toothbrush, not a pocket knife. Never blow out a nozzle with the mouth.

Check the pressure along the length of the boom. If a large pressure difference is found, look for restrictions or install a larger diameter spray hose. An accurate pressure gauge is worth the extra cost.

Field Checking

Conduct field calibration when spraying the pesticide. Start with the tank full of solution, spray a known distance in the field (at least 100m2) and determine the number of litres needed to refill the tankMultiply the used volume by 100 to determine l/Ha.

Spray Distribution Uniformity

Spray distribution uniformity is important for broadcast spraying. Uniform spray coverage eliminates weed streaking and crop injury. Concentrations up to four times the recommended amount can result from non-uniform applications. To obtain even coverage, make sure all the nozzles are the same and are equally spaced along the boom. Check each nozzle to make sure the flow rates are correct. Replace nozzles if the flow rates are 10 percent or more in error. The boom height should be adjusted to the recommended height .

Check spray uniformity by spraying water on a concrete surface and observing the amount of streaking that occurs when the water dries. Spray patterns that result in excessive accumulation below the nozzle are produced by:

1. Nozzle wear
2. Low boom height
3. Low operating pressure
4. Large nozzle spacing

Irregular spray patterns result from damaged nozzle tips, mismatched nozzles and uneven booms.

Pesticide drift is a major concern. In addition to reducing effectiveness, pesticide drift can damage non-target areas. One method to decrease drift is to use a low volatile formulation that is less likely to volatize and drift.

Pesticide drift also can be controlled by reducing the number of small droplets emitted from the sprayer. Nozzle type, angle and orientation, boom height, and operating pressure can influence the production of driftable drops. Spray thickeners can reduce drift, as can spraying at low temperatures and high humidity.