Spray speak: droplet and drift terminology

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Understanding the terminology used to describe droplet size and nozzle outputs can help operators assess the data supplied by manufacturers when considering using a drift-reducing nozzle or adjuvant

Several terms are used in scientific literature, manufacturers’ technical brochures and on labels to describe the range of droplet sizes that can be produced by a nozzle.

VMD: volume median diameter is the droplet size (diameter in microns, also called micrometres) at which half the volume released from the nozzle will exist as droplets larger than this size and half the volume will exist as droplets smaller than this size (Figure 1).

Dv0.5: used in scientific literature to refer to VMD.

μm: the notation used for micrometres. There are 1000 microns in a millimetre.

Dv0.1: the droplet size (diameter in micrometres) at which 10 per cent of the volume will be smaller than this size. Larger Dv0.1 values indicate fewer drift-prone droplets will be produced.

Dv0.9: the droplet size (diameter in micrometres) at which 90 per cent of the volume will be smaller than this size.

Span: the term used to describe the range of droplet sizes produced, the span is a calculated value, where Span = (Dv0.9 – Dv0.1) ÷ Dv0.5.

graphic of visual representation of VMD (Volume median diameter)

Figure 1 A visual representation of VMD (Volume Mean Diameter)

SOURCE: Bill Gordon

Spray-quality classifications

Spray-quality classifications recognised on Australian product labels are assigned by the American Society for Agricultural and Biological Engineers (ASABE, formerly the ASAE) or the British Crop Production Council (BCPC). The classifications are based on measurements of the droplet sizes produced by a nozzle at a given pressure as compared with the outputs of standard reference nozzles (Figure 2).

graphic of spray quality classifications

Figure 2 Spray quality classifications.

SOURCE: ASABE Standard 572.1

While the ASABE and BCPC systems are not a direct measure of actual drift, they do provide a useful indication of the potential amount of drift-prone droplets that a nozzle could produce at a given pressure.

Generally, moving from a particular spray-quality classification (either the ASABE S572.1 standard or BCPC) to a coarser one should halve the amount of the spray solution that exists as small droplets capable of remaining airborne.For example, a medium spray quality may have up to 20 per cent of the nozzle output existing as droplets less than 150 micrometres, which are capable of moving with the wind. A coarse spectrum will have less than 10 per cent of the total output as droplets less than 150 microns, and a very coarse will have less than five per cent of the total output as droplets less than 150 microns.

Drift potential

Spray operators who are interested in reducing the number of drift-prone droplets, through the addition of an adjuvant or by changing nozzles, should seek information about how much of the nozzle’s output exists as droplets less than a particular size.

Some of the droplet sizes that indicate drift potential include:

  • percentage less than 100 microns: highly drift-prone, difficult to get to the target;
  • percentage less than 150 microns: can move wherever the wind takes them and are susceptible to evaporation;
  • percentage less than 200 microns: droplets larger than 200 microns generally make it to the target and are less susceptible to evaporation; and
  • percentage fines: the percentage of the spray volume shown to remain airborne under test conditions.

Drift-reduction claims should be supported by data that show a reduction in the number of small droplets, (usually) less than 150 microns. Caution needs to be used in evaluating this information. Be sure that the product is not just reducing the percentage less than a given size by increasing the number of droplets that are generally too large for the intended target.   

Products that claim to reduce drift potential by increasing the VMD (Dv0.5) should only be taken seriously if they also provide data to show a significant reduction in the Dv0.1 without adversely increasing the Dv0.9 or span.

More information:

Bill Gordon,
0429 976 565,


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