Declared plant calibration of spray equipment

Introduction

The actual volume of spray applied will vary with the type of equipment, chemical and the density and height of the vegetation. For instance, low volume (30-50 litres per hectare, L/ha, or less) is adequate for chemicals that are translocated by the plant (for example, phenoxy herbicides, glyphosate), but higher volumes may be necessary for contact herbicides such as paraquat or diquat which must adequately cover the foliage. While 50L/ha is adequate to apply from boom sprayers on closely grazed pastures, and on young weed growth, 100L/ha or more may be necessary for dense and tall growth. Once handlead or spot spraying is used, volumes can be from 500-1000L/ha. In some cases when treating tall clumps of blackberry volumes in excess of 2000L/ha have been applied. Coverage is largely determined by droplet size and the number of droplets reaching the target.

Poor results from weed spraying are rarely the fault of the chemical. The usual reasons are incorrect application rate or unfavourable conditions at the time of application.

Most modern boom sprayers now have their speed and flow rates computer controlled. However, at some time a check should be made to determine whether the correct volume of pesticide is being sprayed.

Power-driven boom spray

Factors controlling the volume of spray applied per hectare are:

• nozzle size
• nozzle spacings
• pressure
• speed of travel. 

Every spray unit should be serviced, cleaned and calibrated at the start of a spraying program. Check calibrations during the program.

There are a number of ways of calibrating a unit but only one is shown here. Because of the variation in nozzle output, it is not satisfactory to just read the output from the manufacturer’s chart, particularly if the machine has previously done a lot of spraying.

Checks, maintenance and calibration

1. Remove all nozzles and strainers, soak in water and clean them with compressed air, a soft brush or an old toothbrush. Do not use wire or pins to clean the nozzle tip orifice.
2. Flush out the tank and boom with clean water while nozzles are out.
3. Replace nozzles but do not over-tighten. Offset each nozzle about 10-15° to prevent adjacent fan patterns colliding.
4. Start pump and motor and set the pressure to operate between 150-275 kilopascals (kPa; 22-40 pound force per square inch, psi) depending on the manufacturer's recommendations for the nozzles used.
5. Run the pump, collect and measure the amount of spray delivered in one minute by each nozzle. Record the outputs and find the average.
6. Discard any nozzle that differs from the average by more than 5%. Replace and measure the output of the new nozzle. It is also important to check the fan pattern of each nozzle for even spray distribution. There should be no streaks or gaps in the fan pattern.
7. When a reasonably uniform set of nozzles has been selected, add up the total output of all nozzles on the boom which is a value in litres used per minute (L/min).
8. Measure the swath width of the boom. This is the distance between the first and the last nozzles plus the distance between any two nozzles.
9. Level the boom and set at correct height. Allow a double overlap, that is, each nozzle overlaps into half the neighbouring nozzle. With 500 millimetre (mm) nozzle spacings and 80° flat fan nozzles the boom height should be about 45-55 centimetres (cm) and 35-50cm for 110° flat fan nozzles. At the lower pressures fan angle tends to decrease so height of boom needs to be raised to maintain correct overlap.
10. Speed of spraying: maintain a constant and accurate speed of travel whilst spraying. This is possibly the most important factor in the accurate application of herbicides.

The following method can be used to determine speed:

1. Measure out 100 metres (m). Set throttle and select a gear setting to give approximately the speed shown on the speedometer.
2. Time sprayer in seconds over 100m.
3. Divide number of seconds taken over 100m into 360. This is equal to speed in kilometres per hour (km/h).

Note: A tachometer is essential for accurate reproduction of the calibrated speed under working conditions. Alternatively the throttle must be set in a fixed position.

Calculate the volume applied in litres per hectare

Given the desired speed in km/h, swath width in m and boom output rate in L/min apply this formula:

Sprayer output = 600 x boom output (L/min) / swath width (m) x speed (km/h)

This formula is based on the fact that a 1m wide sprayer travelling at 1km/h will cover 1ha in 600min.

Example:

Boomspray has 30 nozzles at 500mm spacings and each nozzle discharges an average of 425 millilitres per minute (mL/min). Total boom discharge is 0.425 x 30 = 12.75L/min.

Swath width is 15m; speed selected is 10km/h.

Sprayer output = 600 x 12.75L/ha / 15 x 10

= 51L/ha

Calculate the required speed

Given the discharge rates of individual nozzles in L/min and the volume required per hectare, the required speed can be calculated as follows:

Speed = 600 x boom output (L/min) spray volume required/ha x swath width

Example:

Boomspray has 30 nozzles at 500mm spacings and each nozzle discharges an average of 425mL/min at operational pressure. Therefore the boom output is 30 x 0.425 = 12.75L/min.

You want to apply the herbicide in 50L of water/ha. How fast do we need to travel to obtain this volume?

Speed  = 600 x boom output  / (rate/ha x swath width)

= 600 x 12.75km/h / 50 x 15

= 10.7km/h

Time to cover 100m = 360 / 10.7

= 33.6 seconds (s)

To spray 50L/ha the vehicle needs to travel at 10.7km/h and at this speed it would take approximately 34s to cover 100m distance.

Ground-driven boom sprayers require regular maintenance like all farm equipment as well as calibration checks.

Further checks and maintenance

1. Check the tyre pressure. Tyres must be kept at the manufacturers recommended pressure to ensure the correct number of wheel revolutions per hectare. (For the ‘Computor’ sprayer the pressure is 124kPa (18psi).
2. Check the agitator jet in the bottom of the spray tank regularly for blockages, excessive wear or corrosion. Fit a trickle irrigation filter in the agitator line to reduce the chance of blockage. Blockage causes an increase in the volume of spray applied by the nozzles and prevents agitation during spraying. If the jet is enlarged, the output of the boom decreases. Fit a liquid-filled pressure gauge (0-600kPa) to the agitator line. Maintain a pressure of between 200-350kPa while working using standard nozzles. Any pressure rise indicates the agitator jet, tip filter or nozzle is blocked. For Computor sprays, standard wire gauge drill bits may be used to clear blockages, but check the correct size for the model with the manufacturer. A small sliver of wood such as a toothpick would be safer. Any increase in size of the agitator jet will reduce the amount of liquid being delivered to the booms.
3. The pump should be maintained regularly. Fluid discharging from the cover plate on the rear of the pump is a sign of severe wear. A low reading on the pressure gauge also indicates a worn pump.
4. Take out all nozzles and filters and soak in water. Use a nylon brush such as a toothbrush for cleaning. Never use a wire brush as this will damage the holes. Wear may produce an uneven fan pattern for which the only solution is a new nozzle. A complete new set of brass nozzles will be needed about every two years for average farm use, more frequently if flowable powder chemicals are used extensively. Other materials such as sintered alumina or hardened stainless steel, acetal plastic or a hard coated nozzle are better than brass.
5. Flush out the tank and boom with clean water while the nozzles are out.
6. Replace the nozzles, taking care not to over-tighten. Offset nozzles 10° to the boom so that adjacent fans do not interfere with each other. Set them at opposite angles on each wing so the pattern can be seen from the tractor seat. That is turn the outside edge of the fan forward on both sides so that the driver is looking almost directly into the spray fans.
7. Supply water to the boom under pressure from a fire fighter pump or the water mains and check the nozzles in operation. Replace any which differ from the average output by more than 5% or show streaks in the fan pattern.
8. If the agitator line feeds from the boom line, re-plumb and fit a flowmeter to measure output to the boom.

If the agitator line arises before the boom, the flowmeter can be placed to read the boom output directly.

Record the flow over a measured run (at least 400m). Drive around in a circle in the paddock before starting the run to ensure that all lines are full.

The flowmeter measures the spray output used in the formula below.

Now calculate the sprayer output

Spray output (L/ha) = output / area

Example:

Swath width = 12.2m
Length of run = 400m
Output of all nozzles during run = 23L

Area covered = 12.2 x 400

= 4880m² 

= 0.48ha

Output in L/ha = 23L / 0.48ha     

= 47.9L/ha

= say 48L/ha

Next, work out the amount of chemical to add to the tank:

Amount = tank capacity x chemical rate/ha / sprayer output (L/ha)

Example:

Tank capacity = 1000L
Recommended rate = 750mL/ha = 0.75L/h

Output calculated (from above) = 48L/ha
Amount of chemical to be added to tank = 1000 x 0.75L / 48 = 15.6L                                                                                   

Set the boom at the correct height

One final check: the speed

Keep it between eight and 15km/h. At higher speeds too many fine droplets are reduced. If the speed is excessive the pressure gauge will show a high reading. Small droplets may drift off target, wasting chemical and perhaps damaging susceptible crops.

Keep the spraying pressure at 200-300kPa.

Speed may be calculated by timing travel over a known distance

Speed (km/h) = distance (m) x 3.6 / time (s)

Boom spray storage

After the finish of the spraying program for the season flush out the boomspray with clean water, removing nozzles and strainers for thorough cleaning. Run a mixture of water and soluble oil (for example, crop oils) through the pump relief valve and nozzles. Soluble oil is available from fuel depots.

Support the boom arms so that they will not be damaged by other machinery.

Wide angle and side delivery nozzles calibration 

Measure the volume of water delivered in one minute, then use the formula:

Output (L/ha) = water volume in 1min x 600 / spray swath width (m) x speed (km/h)

Mister calibration

Method 1

Fill the tank with water and run the mister at operational speed for exactly one minute. Then measure the quantity of water needed to refill the tank to exactly the same point.

Output (L/ha) = water volume in 1min x 600 / distance within runs (m) x speed (km/h)

The most important factor is the calculation of swath width. Some manufacturers have special cards for collecting spray droplets to determine if a uniform deposition is occurring. For best results use these droplet capture cards if they are available. Although the swath may appear to drift for a considerable distance the very fine droplets may not be landing. It is much better to work on a fixed swath width and calculate the amount of chemical needed consistently regardless of how far it appears to be going on a windy day.

Use 30-40m swath width with ULV (ultra-low-volume) formulations and 20-30m with EC (emulsifiable concentrate) formulations. For most applications a deposition of 20 to 30 droplets/cm² is necessary to achieve control of the target weeds.

Method 2

Place a tube over the delivery nozzle and collect the output for 1min. If using ULV chemicals, first check using distillate. Do the final calculations wearing full safety equipment with the neat chemical or chemical distillate mix. The output will vary slightly depending on the viscosity of the chemical.

Handwand/handlead and spraygun calibration

Step 1. Spray volume applied per hectare

1. To measure sprayer output, mark off an area 10m x 5m (that is, 1/200ha) in the weed infestation which is typical of the area to be sprayed.
2. Select pressure: usually 500-700kPa.
3. Record the time it takes to spray the above area with water to point of run off.
4. Discharge spray into a bucket for the same period of time. Record volume in L. This is the sprayer output on 1/200ha. Multiply this value by 200 to give volume/ha.

Step 2. Amount of chemical to add to tank

1. Find tank capacity in L.
2. Find recommended pesticide application rate in L/ha.
3. Measure sprayer output in L/ha (above).
4. Calculate the amount of chemical you need to add to the tank.

Amount = tank capacity x chemical rate / sprayer output

Calculation of pesticide in spray tanks

Having first calibrated the equipment to be used for the spray operation, the next step is to calculate the amount of pesticide to add to each tank.

1. Record calibration results.
2. Calculate the volume of spray tank (if you don’t know it from the handbook):

For square or rectangular tanks:
Multiply length (m) x breadth (m) x height (m) to give volume in m³.

For cylindrical tanks:
Multiply π (3.14) x radius squared x length to give volume in m³.

For spherical tanks:
Multiply 4/3 x π (3.14) x radius cubed to give volume in m³.
Multiply resulting volume of the tank in m³ by 1000 to give capacity of tanks in litres. One m³ = 1000L

3. Find the recommended application rate of product.
   This may be expressed as volume or mass, for example L, mL or kg, grams per hectare (g/ha).
4. Calculate the amount required per tank as follows:

Amount required (L/tank)  = recommended application rate x spray tank capacity (L) / output of spray unit (L/ha).

Decontamination 

Spraying unit contamination

In order to prevent spraying equipment contamination, it is important to adhere to the cleaning protocols outlined in the previous sections Contamination occurs when a previous herbicide is not thoroughly cleaned from the system. If a crop sensitive to the herbicide contaminant is then sprayed severe damage can be recorded.

Boom spray contamination is most commonly a problem in cereal/canola/legume rotations. It mostly occurs when a sulfonylurea herbicide is not adequately cleaned from the spraying unit after spraying a cereal crop. If the next crop to be sprayed is a canola or legume crop, damage can occur.

Cleaning up after using sulfonylureas (for example, Glean®, Ally®, Brushoff® and Logran®).

Chlorine (at a rate of 300mL of 4% chlorine solution, that is, household bleach, per 100L of water) is the best chemical for cleaning a boom spray after sulfonylurea use. To be effective, it must be agitated then left standing in the sprayer for long enough to break down the sulfonylurea molecules. When cleaning, remember to pay attention to the boom and spray lines. Nozzles and screens may need to be cleaned separately.

Contamination may also occur through other mechanisms. For example, if you have a wash out tank on your boom, it is worthwhile to check the one-way valves and ensure that your wash out water is clean. A modification kit has been made available by some boom spray manufacturers to prevent the siphoning of contaminants through one-way valves into the wash out tank.

Herbicide labels detail the appropriate method of decontamination for that product.

Chemicals other than sulfonylureas

When the spraying operation have been completed, dispose of any unused spray mix in a suitable location, well away from water catchment areas.

• Remove all nozzles and filters and drop them into a bucket of water containing a cleaning solution (see below).
• Refill the tank with water and discharge it into a sump or soakaway.
• Wash the outside of the spray unit and vehicle to remove chemical. Make sure that run-off discharges into a sump or soakaway.
• Refill the tank and add a cleaning agent:
  oil-based chemicals: use 50mL of liquid detergent/100L of water.
  other sprays: use 100-150g of washing soda/100L of water.
  Alternatively, use household ammonia at 1L to 100L water.
• Agitate the solution in the tank and discharge it through the spray lines into the sump or soakaway.
• Flush the tank with clean water.
• Clean the nozzles and filters with a toothbrush.

A commercially available product, Boom Kleen (150g/L potassium hydroxide, 40g/L available chlorine) is ideal for cleaning booms contaminated with sulphonyl urea residues.

Water quality for spraying herbicides

When spraying herbicides, always use the best quality water available and apply the mixed herbicide without delay. Factors which can reduce the effectiveness or change the physical compatibility are hardness, salinity, muddiness, alkalinity or acidity.