Field Sprayer Calibration

Agdex#:	744
Publication Date:	12/88
Order#:	88-129
Last Reviewed:	12/88
History:	Replaces Factsheet "Weed Sprayer Calibration", February 1983
Written by:	OMAFRA Staff

Table of Contents

1. Introduction
2. Pre-Calibration Check
3. Calibrating for Broadcast Application
4. Calibrating for Band Spraying
5. Calibrating for Liquid Fertilizer
6. Adding Herbicide to the Tank
7. Nozzle Height
8. Safety
9. Useful Formulas and Conversions

Introduction

Proper field sprayer calibration is one of the most important steps in a chemical weed control program. The correct herbicide applied incorrectly, may be worse than applying no herbicide at all. An improperly calibrated sprayer can result in reduced crop profits and/or negative environmental impact. Reduced profits may be the result of either over or under application of the intended herbicide. Crop injury can result from over application, while under application may result in insufficient weed control, excessive crop competition, and potentially reduced crop quality. Environmental concerns are of no less significance, with the potential of increased soil residues, ground and surface water contamination, and off-target drift.

At the very minimum, field sprayers should be calibrated at the beginning of the spraying season. Sprayers should be recalibrated if you are changing water volumes, carrier solutions, field speed, operating pressure or any other variable. Calibration should also be more frequent when abrasive materials, such as wettable powders, are used. Once you become familiar with the basics of sprayer calibration, you should incorporate the exercise into a routine as regular (and as important) as changing the oil in your tractor.

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Pre-Calibration Check

Before you can start the calibration exercise, you need to check that the sprayer is in good operating condition. Check the pump, hoses, pressure gauge, nozzles and screens. Run the sprayer with clean water to check for any leaks or malfunctions. At this time, check that all nozzles are of the same type and size, and free from clogs or obvious signs of wear. Use only a soft brush (such as an old toothbrush) to clean nozzles and screens. Discard any that are defective.

Select the desired application volume. You will find this on the herbicide label. Most herbicides are applied at volumes between 100 and 300 litres per hectare (L/ha). Using manufacturer's recommendations, select the appropriate nozzle type and size to apply the required volume at anticipated field speed within a reasonable pressure range. For herbicide application the pressure range is usually between 135 and 345 kilopascals (20 to 50 pounds per square inch). To avoid off-target drift, use the lowest pressure possible which still maintains a good spray pattern.

Whether calibrating your sprayer for broadcast or band spraying, the principles of proper calibration are the same. The amount of spray solution applied per unit area of land is influenced by: (a) the forward speed of the sprayer, (b) the size and spacing of nozzles; (c) the pressure at the nozzles; and (d) the density or viscosity of the liquid.

Various calibration kits are available, and can be a useful tool. These systems are built on the basic principles outlined below. An understanding of the principles of sprayer calibration will help in improved sprayer operation.

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Calibrating for Broadcast Application

With these factors in mind you can begin to calibrate your sprayer. You will need the following tools to carry out this calibration exercise: a graduated cylinder calibrated in mL (used only for sprayer calibration), two stakes, a measuring tape or a pre-measured 50 m length of rope, a stopwatch or wristwatch with a second hand, a note pad and pen, and a pocket calculator.

Checking Nozzle Flow Rate

Before you begin the actual calibration, you must be certain that each one of the nozzles on the boom is delivering the correct volume of liquid in the appropriate spray pattern. In order to do this, refer back to the manufacturer's nozzle catalogue. At the suggested pressure, run the sprayer with clean water. Collect the output from EACH nozzle for 30 seconds. Record the output of each nozzle in Litres (1,000 mL = 1 L). If you multiply the output of each nozzle by 2, this will give you the nozzle flow rate in L/min. Now check back to the nozzle catalogue. If the flow rate in the catalogue is given in gallons, use the conversion at the end of this Factsheet.

How does the output collected compare with the intended for the particular nozzle you're using? If the output of an individual nozzle is more than 10% higher or lower than the suggested volume, remove the nozzle and clean it with a soft bristle brush (never blow out a plugged nozzle with your mouth). If cleaning the nozzle does not return the output to within the accepted 10% variance, discard the nozzle and replace with a new one. Recheck the output. As most nozzles are manufactured within an accuracy of +/- 5%, a variance of 10% is realistic.

If the output of two or more nozzles is outside the acceptable range, it is better to replace all the nozzles on the boom, as this is a potential indicator of nozzle wear across the boom. As nozzles wear, there is an increase in flow rate followed by a distortion in the spray pattern. By replacing a few nozzles with new ones, there is a risk of uneven distribution of the spray.

Nozzles are likely the most important, yet the most often neglected, component on the sprayer. Check your nozzles often to be certain that they are delivering the correct amount of liquid in a uniform pattern.

Calibrating Field Speed

The first factor that must be determined is the actual ground speed of the sprayer. Tractor speedometer readings can be inaccurate because of wheel slippage, so actual speed must be calculated. To do this place two stakes exactly 50 metres (or 164 feet) apart in the field or in an adjacent fence row. It is important to carry out this exercise in a field as opposed to on a hard surface such as a driveway. Select the gear at which you wish to spray and note the throttle position. Consult the manufacturer's recommendations for correct r.p.m. setting for the pump. Allow for varying topography and field conditions, and select a speed which can be easily maintained across the entire field. Field speed is important in ensuring proper application and spraying speed should be between 6 and 10 km/h (4 to 6 mph).

Figure 1. Check your actual speed in the field by clocking the time to travel 50 m.

At spraying speed, with the sprayer tank half filled with clean water, clock the time (in seconds) taken to travel between the two stakes. Be certain that the tractor is at the desired speed when you reach the first stake, maintain a constant ground speed, and do not slow down before passing the second stake. Repeat this two more times, taking care not to travel in your previous wheel tracks. Take an average of these 3 runs and record this time for use later. If you want to know your travel speed in km/h, use the formula at the end of this Factsheet.

If the spraying operation is to be combined with either cultivation or planting, carry out the speed check with this implement. The cultivator or planter should be set to operating depth during the test run as this will influence your field speed. Use undisturbed soil for each test run.

Calculating Nozzle Output

Figure 2. For calibration, collect water from a nozzle for the same time as to travel 50 m.

Park the sprayer in the field with the PTO engaged and adjust the throttle to the position you noted in your speed check. Adjust the pressure regulator to obtain the desired working pressure with the boom turned on. (Pressure is usually less at the boom, and a pressure gauge at the boom is a better indicator of actual pressure.) Confident that all nozzles are delivering the same rate of liquid in a uniform pattern (as tested above), collect the liquid from one nozzle with the graduated cylinder for the same length of time it took you to travel 50 m as calculated above. Record the volume in mL. Repeat this for at least 2 more nozzles. Calculate the average nozzle output by adding the individual outputs and then dividing by the number of nozzles checked.

Nozzle Spacing

Satisfied that all your nozzles are operating properly, measure the distance between nozzles on the boom. Record this value in metres. A common nozzle spacing is 0.5 m (20 inches).

Calculating Application Rate

With the values we have determined, we are now going to calculate the rate of spray being applied. This simply is the volume sprayed over a given area. The following formula will allow us to calculate the application rate.

Application Rate (L/ha) = (Average Output per Nozzle* (mL) ) ÷ (Nozzle Spacing (m) ) x 0.2 (a constant)

*Average output per nozzle is for the same time taken to travel 50 m as calculated previously.

Checking the Applied Rate

Compare the actual application rate calculated above to the recommended or intended rate. If the actual rate is more than 5% higher or lower than the intended rate, you will need to make necessary adjustments. Begin the adjustments with minor changes in pressure. Lowering the spray pressure will reduce the volume of spray delivered, and raising the pressure will increase the rate of spray. Be certain to maintain a pressure within the acceptable range for the nozzles being used. Depending on the situation and the herbicide being sprayed, increasing pressure may not be desirable as this increases the opportunity for spray drift.

You can also correct the application error by changing the actual travel speed. At slower speeds more spray is applied and at faster speeds, less spray is applied on a given area. Recalibrate after any of these adjustments. If you are still unable to obtain the recommended application rate with these changes, you may have to select a new set of nozzles of either a smaller or larger size, depending on the adjustment required.

It is important to realize that all of the above exercises are carried out with water, and without pesticide added to the spray tank.

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Calibrating for Band Spraying

Pesticides may sometimes be applied in a band over the crop row. The application rate on the treated band is the same as the broadcast rate, but less total pesticide is applied as only a portion of the field is treated. As herbicide recommendations are given for broadcast rates, we will need to calculate the band rate. The following formula will convert a broadcast application rate to a band rate:

Band Rate = (Band Width) ÷ (Row Spacing) x Broadcast Rate

As long as the units to measure band width and row spacing are the same, it doesn't matter what units are used.

When we return to the calibration formula, we can substitute our values as follows:

Band Rate (L/ha) = (Average Output per Nozzle* (mL) ) ÷ (Band Width (m) ) x 0.2

*Average output per nozzle is for same time to travel 50 m.

With the band rate known (from our previous calculation) as well as the row spacing, we can calculate the output required per nozzle for the length of time to travel 50 m. (The calculation of field speed is the same as for broadcast application.) This is the volume of water we need to collect from each nozzle. Where more than one nozzle is used per spray band, this is the total volume of all nozzles applying on one band. As with broadcast calibration, adjustments should be made to correct to within 5 percent of the intended rate.

To better explain calibrating band applications, follow through the following example.

1. Band Rate = (0.25 m) ÷ (0.76 m) x 200 L/ha = 66 L/ha (equivalent to 1/3 ha sprayed at a rate of 200 L/ha)
   
   
2. Band Rate = (Avg Nozzle Output) ÷ (Row Spacing) x 0.2
   
   
3. Avg. Nozzle Output
   = (Band Rate x Row Spacing) ÷ 0.2
   = (66 x 0.76) ÷ 0.2
   = 251 mL (this is the volume that should be collected from each nozzle for the specified time.)

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Calibrating for Liquid Fertilizer

Some herbicides are registered to be applied in a carrier of liquid fertilizer (most commonly 28% UAN). Be certain to check the herbicide label for this specific use. Calibration with a fertilizer carrier is similar to the procedure with water. The major difference is a reduced flow rate with the fertilizer due to a higher specific gravity or a thicker, more viscous liquid. The calibration exercise will still be performed with water in the sprayer, but we need to take into account the higher specific gravity of the fertilizer solution. This can be done by converting to the equivalent water application rate using the following formula:

(Water) Application Rate = (Fertilizer Application Rate) ÷ (Conversion Factor)*

* Conversion factors for liquids with different specific gravities are given below. The specific gravity of water is 1.0 and of 28% UAN is 1.28. Ask your fertilizer dealer for the specific gravity of the fertilizer solution to be used.

e.g. Actual N required = 90 kg/ha Volume 28% required = 251 L/ha (your fertilizer dealer can help calculate this)
Conversion Factor = 0.88 of 28% (S.G. = 1.28)
L/ha of water = (251 ÷ 0.88) = 284 

Knowing the volume of water to apply, we can continue with the calibration as described in the broadcast calibration section.

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Adding Herbicide to the Tank

With the sprayer now calibrated, it is important to add the correct quantity of herbicide. You don't even need to know what a hectare looks like to apply pesticides in litres per hectare. First, accurately determine the capacity of the spray tank. If you are uncertain of the capacity, use a carefully calibrated pail to fill the tank and mark the side of the tank in fixed increments.

Hectares Per Tank = (Tank Capacity (in litres) ) ÷ (Application Rate (L/ha) )

To determine the amount of herbicide needed per tank, simply multiply the number of hectares treated per tank by the amount of actual chemical to be applied per hectare. Follow the label for instructions on mixing herbicides.

Nozzle Height

Proper spraying begins with good calibration, but certainly doesn't end there. To ensure a uniform spray pattern, adjust boom height according to manufacturer's recommendations for the specific nozzles being used. Flat fan and flood jet nozzles of different angles require different overlap patterns for uniform coverage. Height of the boom is measured from the top of the spray target. If you are applying a post emergent herbicide, the height of the boom is measured from the top of the emerged weeds.

Safety

Minimize your exposure when working with chemicals. Wear gloves and protective clothing both when handling chemicals and calibrating the sprayer (as trace residues may be left in the sprayer). Keep a supply of fresh water with you while you are spraying. This will be handy in case of a spill or should you need to clean a clogged nozzle. Finally, read pesticide labels!

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Useful Formulas and Conversions

(1) To determine nozzle output in L/min (useful in nozzle selection)

Nozzle Output (L/min) = (Nozzle Output * (mL) ) ÷ (Time to Travel 50 m (sec) ) x 0.06 (a constant)

* nozzle output is for the time to travel 50 m

(2) To determine travel speed in km/h

Travel Speed (km/h) = (Distance (m) ) ÷ (Time (sec) ) x 3.6 (a constant)

(3) Metric Conversion

            a) Litres = Imperial gallons x 4.5 = U.S. gallons x 3.8
            b) metres = inches x 0.025

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