Nozzle selection

While the nozzle is one of the smallest components in the spraying operation, it has a massive influence on the application. Understanding more about nozzles and droplet sizes will help you to improve performance and control drift, both of which will increase application efficacy, while protecting the environment.

In ‘The Art and Science of Spray Application’, Tom Robinson, an independent spray application specialist, explains the importance of droplet sizes:

Generally speaking small droplets are well retained on plants, particularly shiny surfaces from which larger drops tend to bounce off. Fine sprays are important for retention on difficult weed leaves – like blackgrass, wild oats as well as for pea crops.

Large droplets are, however, surprisingly well retained on easy to wet leaves, such as mature cereal plants, beans, potatoes and many broad-leaved weeds.

There are two further ways of making large droplets stick to a target:

  • Slow them down
  • Put a shock absorber (air bubble) in them.

These are the main principles behind the air-induction nozzles.

Size matters

spray fan

For a given volume of liquid, if the droplet size is halved, the number of droplets produced goes up by a factor of eight. So, if you halve the size a single 200-micron droplet it will create a total of 8, 100-micron droplets, because the number of droplets produced goes up by a factor of 8.

Did you know? Droplet sizes are measured in microns. One micron = 1/1,000mm. To put that in context a human hair is about 100 microns in diameter.

Quick Quiz: Droplet sizes

And now for some serious mathematics. Test your knowledge of droplet sizes:

CONTINUE LESSON:

Averaging it out

All nozzles produce a spectrum of droplet sizes so the classifications, such as fine, medium and coarse don’t specify an single size. Instead they use the Volume Median Diameter (VMD). This refers to the midpoint droplet size (median), where half of the volume of spray is in droplets smaller than the median, and half of the volume is in droplets larger than the median.

For example – If a nozzle has a VMD 400 this indicates half of the volume is in droplet sizes smaller than 400 microns and the other half is in droplet sizes larger than 400 microns.

Hitting the target

Droplet size also influences the spray application’s retention, penetration and drift. Your job as an operator is to select the appropriate droplet spectrum to hit the target with minimum drift.

Fine droplets tend to provide good coverage, because there are lots of them, which gives good retention. While they provide good coverage, they are small which means they tend to land at the top and don’t penetrate too far down a crop. Their size (VMD 100 microns) and speed mean they are also more prone to drift.


Medium droplets (VMD 250) will have a higher proportion of small and large droplets, which means they don’t provide the coverage of the fine, but will penetrate better and are less prone to drift.


Coarse droplets (VMD 340), which are about one and half times the size of fine droplets, provide the least retention, but the best penetration. Their size and higher velocity make them less susceptible to drift.

The following table shows the size and characteristics of droplets:


Spray Quality

Droplet size (microns)
Relative coverage/litre
(% of fine drops/cm2)

Retention

Penetration

Drift potential
Fine
F110/0.85/3.5
VMD 200100Very goodPoorHigh
Medium
F110/1.44/2.5
VMD 25050GoodGoodModerate
Coarse
F110/2.58/2.0
VMD 34020Moderate/poorHighLow

Quick Quiz: Droplet characteristics

Droplet size influences the spray application’s retention, penetration and drift. So, can you select the appropriate droplet spectrum to hit the target with minimum drift?

CONTINUE LESSON:

App helps make decisions

There is now a wide choice of Apps that provide a broad range of information on products, weather and application advice.

One example is Syngenta’s ‘Spray Assist’ that helps operators select the most appropriate application techniques for different crops, situations and timings.

The App links to live local weather data, to analyse the factors that influence accurate application and potential risk of spray drift, including wind, rain or frost. It then suggests techniques to enable sprayer operators to mitigate risks or alter practices. The following video gives you an insight as to what these types of apps can offer.

You have now covered ‘Nozzles’ and should move on to the next topic in this lesson. Please click ‘mark complete’ to take you to the next topic, Clean Regime.