The droplet spectrum
All agricultural nozzles produce a range of drop sizes. This is a useful feature as the crops to be sprayed always present a 3-dimestional target that have contrasting leaf surfaces and angles. Hence, in a crop canopy, for example, finer droplets are likely to be deposited in the top and larger droplets lower down.
Drop sizes are measured in micron. 1 micron = 1/1,000,000 metre.
To describe the median droplet sizes produced from a specific nozzle, the term VMD is used.
VMD = Volume Median Diameter
VMD is the mid-way drop size that is reached when the accumulated volume of smaller drops accounts for 50% of the sprayed liquid leaving the nozzle; half the volume is atomised into droplets smaller – and the other half of the volume is larger - than the VMD.
Wind drift has in the past been regraded as being mostly caused by droplets below 150 micron in diameter.