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Nozzle size: spray mix/water volume rate is known

You know which type of nozzle to use. Now find the right size of nozzle that will produce the right spread of spray drops for this spraying task.

Find a nozzle size to test

Your pesticide label or local expert has advised a spray mix/water volume rate for an area (e.g. 200 L/ha). Refer to your nozzle chart to find which nozzle size (e.g. 03 Red) will deliver:

  • the advised spray mix/water volume rate (e.g. 200 L/ha) at
  • a spraying speed that you can manage (e.g. 2.4 km/hour) with
  • a pressure at the nozzle that your knapsack sprayer can achieve (e.g. 3 bar) with
  • the right swath width for your spraying task (e.g. 0.75 meters).

(Make sure that the volume rate (L/ha) given by the chart is for single nozzle use: often the volume rates shown are for when a set of nozzles is used on a boom).

​Fit this nozzle and adjust your sprayer.

Adjust your knapsack sprayer

Adjust your knapsack sprayer to produce the right spread of spray drops. To do this you need to balance:

  • the nozzle type and size
  • the spraying distance
  • the nozzle spacing
  • the nozzle spraying pressure
  • the spraying speed.
Spraying distance

This is the distance of the nozzle from the target surface: nozzle charts often state this as 50 cm. It can be lower if a more narrow swath is needed.

Nozzle spacing

For a single nozzle, this is the distance between the midpoint of one spray width and the midpoint of the next spray width. On a miniboom it is the distance between adjacent nozzles mounted on the miniboom.

Note the nozzle spacing on your manufacturer's chart. Where there is an even spread of drops across the spray width, then the nozzle spacing, the spray width and swath width are all the same. This means that if you are broadcast spraying or spraying a vertical row laterally, each spray width will need to abut its neighbor. However, if you are broadcast spraying or spraying a vertical row laterally using a standard flat fan nozzle, you will need to overlap the outer edges of the spray widths as there are fewer spray drops at these margins. The nozzle spacing and the swath width will be the same and will be narrower than the spray width.

Spraying pressure

Adjust your pressure regulator if you have one. Use just enough pressure to produce the spray cover and size of drops you want. Pump slower for lower pressures and bigger drops. Pump faster for higher pressures and smaller drops.

Spraying speed

This is the speed at which the nozzle moves over the target surface.

Adjust your spraying speed so that you get the right cover of spray on the target surface with just one pass of the nozzle. Practice spraying with water until you get an even coverage on every leaf with no run off. Visit spraying downwards, laterally or upwards for more help on how to spray.

If changes to your spraying pressure and spraying speed do not give the right cover of spray drops, change the size of the nozzle. Remember to check the nozzle spacings of the new nozzle.

When you can spray an even coverage on every leaf with no run off, move on to the next step.

Downward spraying: Find your actual spraying speed and required nozzle flow rate (L/minute)

Next find your actual downward spraying speed. Then find the nozzle flow rate (L/minute) that will deliver the spray mix/water volume rate that you need at this speed.

To find your actual spraying speed and nozzle flow rate (L/minute), spray water over a test area. Spray in the same way as when you adjusted your sprayer. Then calculate your actual spraying speed, water volume rate and nozzle flow rate. Compare these actual rates with your planned rates.

Typical knapsack spraying speeds over level ground are about 1 meter/second or 3.6 Km/hour. If spraying conditions are difficult, the spraying speed can be much slower. The act of spraying will also affect the time taken, so practice spraying with water over the test area first.

1. Decide on the size of your test area for downward spraying If your treatment area is measured in;

  • metres use a test area of at least 25 m2
  • hectares use a test area of at least 50 m2.

For ease, make the width of the test area the width of your crop-row, or inter-row gap, or swath width/nozzle spacing as is best for your spraying task.

If you are using a single standard flat fan nozzle to broadcast spray, note the nozzle spacing given on your nozzle chart. For this type of nozzle, each spray width must overlap at the edges. The amount of overlap is set by the nozzle spacing. Overlapping the swaths means that the spray width and the swath width are different. As the nozzle spacing will be the same size as your swath width, use the width of your nozzle spacing for the width of your test area and not the spray width.

This table may help.

To spray using a single nozzle Broadcast spraying downwards Over-row spraying downwards Inter-row spraying downwards

Even flat fan Hollow cone
Solid cone
Deflector nozzles

Use swath width
(= spray width)

Use swath width

(= spray width

= row width )

Use swath width

(=spray width

= inter-row width)

Standard flat fan nozzles Use nozzle spacing Not advised Not advised

When you know the width of your test area, use this calculator to find the length of the area.

2. Mark out the test area in the treatment area

  • Mark a start point in the area to be sprayed.
  • Mark out the width of the test area.
  • Measure and mark the end point to make your known area.

3. Find the amount of water needed to spray this area

  • Half fill your knapsack sprayer with water.
  • Mark the water level on your knapsack.
  • With your knapsack sprayer on your back, walk towards the start point at a comfortable speed. This should be the speed you used when you adjusted your knapsack sprayer.
  • As your nozzle passes the start point, start spraying and start the timer.
  • Keep your spraying speed constant and spray the test area to get a good cover of drops with (no runoff).
  • Stop the timer and stop spraying as your nozzle passes the end point.
  • Mark the new water level and calculate the difference to find the amount sprayed.

4. Calculate your actual water volume rate, spraying speed and nozzle flow rate.

What is the length of your test area? [meters] (a)
What is the width of your test area? [meters] (b)
What was the swath width/nozzle spacing? [meters] (d)
What was the time taken to spray this test area? [seconds] (e)
How many liters of water did you spray over your test area? [liters] (g)
What is your planned spray mix/water volume rate? [(L/ha] (i)

The size of your test area is ....(c) m2
Your walking spraying speed is ... (f) km/hour
To spray your planned spray mix water volume rate of ..... (i) L/ha at .... (f) km/hour you need a planned nozzle flow rate of ... (j) L/minute

Your actual spray mix water volume rate was .... (h) L/ha at ... (f) km/hour and your actual nozzle flow rate was ....(k) L/minute

There is a …. (m) % difference between your actual and planned nozzle flow rate. And thus also between your actual and planned spray mix/water volume rate.

Next, confirm your nozzle size.

 

Lateral/Upward spraying: Find your actual spraying speed and required nozzle flow rate (L/minute)

Next find your actual spraying speed when spraying rows laterally or upwards. Then find the nozzle flow rate (L/minute) that will deliver the spray mix/water volume rate that you need at this speed.

To find your actual spraying speed and nozzle flow rate (L/minute), spray water over a test area. Spray in the same way as when you adjusted your sprayer. Remember that spray widths of standard flat fan nozzles must overlap at the edges. Spray widths of nozzles with an even spread of drops across the swath must adjoin but not overlap.

Next calculate your actual spraying speed, water volume rate and nozzle flow rate. Compare these actual rates with your planned rates.

Typical knapsack spraying speeds over level ground are about 1 meter/second or 3.6 Km/hour. If spraying conditions are difficult or if you are laterally spraying tall plants, then the spraying speed can be much slower. The act of spraying will also affect the time taken so practice spraying with water over the test area first.

1. Set a test area for spraying rows of crops laterally or upwards

  • Mark a start point in a row to be sprayed.
  • Measure the row width or row spacing (as guided by your pesticide label) of the plants to be sprayed.
  • Calculate the length of the test area
  • Mark an end point so that your test area is 25 or 50 square metres as needed.

2. Find the amount of water needed to spray this area

  • Half fill your knapsack sprayer with water.
  • Mark the water level on your knapsack.
  • With your knapsack sprayer on your back, walk towards the start point at a comfortable speed.
  • As your nozzle passes the start point, start spraying and start the timer.
  • Keep your spraying speed constant as you spray up and down the plants, as you walk along the row. Make sure that you get a good cover of drops with (no run-off).
  • Stop the timer and stop spraying as your nozzle passes the end point.
  • Walk back to the start point to spray the other side of the row.
  • As your nozzle passes the start point, start spraying water and start the timer.
  • Keep your spraying speed constant as you spray up and down the plants, as you walk along the row. Make sure that you get a good cover of drops with (no run-off).
  • Stop the timer and stop spraying as your nozzle passes the end point.
  • Mark the new water level and calculate the difference to find the amount used.

3. Calculate your actual water volume rate, spraying speed and nozzle flow rate (L/minute)

What is the length of your test area [meters] (a)
What is the width of your test area [meters] (b)
What was the time taken to spray this test area? [seconds] (d)
How many liters of water did you spray over your test area? [Liters] (f)
What is your planned spray mix/water volume rate? [L/ha] (h)

The size of your test area is …..m2 (c)
Your walking spraying speed was …. (e) km/hour
To spray your planned spray mix/water volume rate of ...(h) L/ha at (e) km/hour, you need a planned nozzle flow rate of ... (i) L/minute.
Your actual water volume rate was …. (g) L/ha at ... (e) km/hour and your actual nozzle flow rate was … (j) L/ha
There is an …(k) % difference between your actual and planned nozzle flow rate. And thus also between your actual and planned spray mix/water volume rate.

Next, confirm your nozzle size.

 

 

Confirm your nozzle size

Compare your actual spray mix/water volume rate (calculated above) with your planned/advised volume rate.

  • If they are the same and you are getting the right spread of spray drops on your target surface, you have properly calibrated your sprayer.
  • If your volume rate is +/- 10% or more, you may need to change your nozzle size.

Take care when using nozzle charts. Nozzle sizes are often given in L/minute at a given pressure, but other units may be used. Make sure that your nozzle chart uses units that make it easy for you to calculate your spraying needs.