How airblast spray droplets behave, or misbehave

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Some pesticide labels require or prohibit certain droplet sizes to reduce the potential for drift. But, even when labels are silent about size restrictions, operators should be aware of the potential for droplet size to affect coverage. In the case of airblast, droplets should be:

  • large enough to survive evaporation between nozzle and target.
  • small enough to adhere without drifting off course.
  • plentiful enough to provide uniform coverage without compromising productivity (e.g. affecting refills and travel speed).

Once spray leaves the nozzle, the operator has no more control over the application, so it’s important to plan for as many contributing factors as possible. Deciding which nozzles to use (and yes, you have alternatives beyond disc-core), requires an understanding spray quality symbols and basic droplet behaviour.

Spray Quality

Droplet diameter is measured in microns (µm). For a given pressure, a nozzle creates a range of droplet sizes which are described by the American Society of Agricultural and Biological Engineers (ASABE) standard S572.1. In North America, these spray quality ratings range from “Extremely Fine – XF” to “Ultra Coarse – UC”. For interest, the scale is based on the British Crop Protection Council (BCPC) system, which is slightly different.

The name, symbol and range of droplet sizes used to describe the median droplet diameter produced by nozzles.
The name, symbol and range of droplet diameters used to describe nozzle spray quality. Ignore the colours and remember the symbols.

To make sense of the spray quality rating, we must first understand that not every droplet produced by a hydraulic nozzle is the same size. We noted that a single nozzle produces a range of droplet sizes. Spray quality captures that span using a few key metrics. The first is the Volume Median Diameter (VMD) or DV0.5. Think of it this way: Let’s say you have a hollow cone nozzle that breaks a volume of liquid up into droplets. Let’s arrange them from finest to coarsest as in the following graph.

The DV0.5 refers to the droplet size where half the spray volume is comprised droplets smaller than the DV0.5, and the other half is comprised of larger droplets. But we need more to understand the variation in the population. In other words, are they all the same size, or do they vary a great deal?

That’s why we also assign a DV0.1 which tells us the droplet size where 10% of the spray volume is comprised of smaller droplets, and a DV0.9 which indicates that 10% of the spray volume is comprised of larger droplets. Let’s add them to the graph:

With all three numbers, we can calculate the Relative Span (RS) by subtracting the DV0.1 from the DV0.9 and dividing by the DV0.5. The smaller the resulting number, the less variation there is in the spray quality. Two nozzles might produce a range of droplets with the same DV0.5, but the one with the larger RS is more variable, and is more likely to drift. Since we don’t typically have access to the RS of each nozzle, we rely on the spray quality symbols in nozzle catalogues to alert us to potential drift issues.

Relative Droplet Size

Did you notice in the graph that there are a lot of Fine droplets compared to Coarse?  Disc-core (or disc-whirl) nozzles do not have spray quality ratings, and moulded hollow cones may or may not. This is, in part, because the standard was developed for flat fan nozzles, but mostly it arises from the nature of airblast spraying. No matter the original droplet diameter, the air shear from the sprayer and the distance-to-target reduce the DV0.5 considerably by the time spray reaches the target. It is safe to assume that the final spray quality will be much finer than the nozzle’s rating.

Incidentally, this is a big difference between boom sprayers and airblast: Where the boom sprayer operator should be aware of how pressure affects droplet size, it’s of little consequence to an airblast operator. On an airblast sprayer, pressure really only affects nozzle rate.

So, while shear and evaporation raise drift potential, shear also increases droplet count. Imagine the volume a nozzle emits as a cake. No matter how many slices you cut the cake into, you still have the same amount of cake. The finer the slices, the more people can have a slice, albeit not very much. Similarly, a single Coarse droplet can contain the same volume as many finer droplets. Mathematically, a droplet with diameter X represents the same volume as eight droplets with diameters of 1/2X. See the illustration below:

The one to eight rule: Every time the median diameter of spray is doubled, there are eight times fewer droplets. Conversely, every time the median diameter of spray is halved, there are eight times more.
The eight to one rule: Every time the diameter of a droplet spray is doubled, there are eight times fewer droplets. Conversely, every time the diameter of a droplet is halved, there are eight times more.

Droplet Behaviour

The droplets that comprise the spray behave differently from one another. Finer droplets have a low settling velocity, which means they take a long time to fall out of the air. Conversely, coarser droplets fall out of the air more quickly. Think of how a ping pong ball (the finer droplet) has much less mass than a golf ball (the coarser droplet). When thrown into the wind, the golf ball follows a simple trajectory before falling. The ping-pong ball behaves erratically, like a soap bubble. Wind, thermals, humidity and many other factors will change where it goes because it is too light to resist them. It may even land behind the thrower, blown by the prevailing wind.

It is because of the behaviour of finer droplets, and the airblast sprayer’s inclination to create them, that we must be so diligent when we adjust the air settings.

We once explored this at a nursery workshop. The operator was spraying whips, which are young trees with very few lateral branches. He used a cannon sprayer to cover 30 rows (15 from each side) and felt he would incur less drift if he just used pressure, not air, to propel the spray. Water sensitive paper exposed the erratic coverage that resulted. Coverage uniformity was greatly improved when air was used, even when only spraying from one side of the 30 row block. Of course, this was only to demonstrate a principle; we don’t recommend alternate-row-middle-spraying.

Air-induction nozzles can be used to increase the median droplet size on an airblast sprayer. When used in the top nozzles positions, the coarser droplets that miss the top of tall targets will ultimately fall (reducing drift). They can also be used in positions that correspond to restricted airflow. In this case the operator relies on pressure to propel the coarser droplets where there is limited air to carry finer droplets.

Conclusion

The net result of all this is that the sprayer operator must choose a nozzle, pressure, and travel speed while considering the effect of distance-to-target and the weather. The resultant range of droplets should be fine enough to increase droplet count and be carried by sprayer air to deposit uniformly throughout the canopy. However, droplets should also be coarse enough to reduce drift if they miss.

Hey, if it was easy, anyone could do it!

How droplets behave (2013).

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