Volume median diameter (VMD), also known as D[v,0.5] is where half of the volume of spray contains droplets larger than the VMD in microns(µm) (1000µm = 1 millimetre) and the other half is in smaller droplets.
However VMD alone can be very misleading. Both of the charts below show the composition of a spray with a VMD of 200µm, the first has only 9% of its volume in the 150 to 250µm range the second has 84%.
So we need more information to tell which nozzle to use other than VMD.
the most reliable statistic is probably relative span, which is a dimensionless parameter calculated from the volume distribution only. As with VMD, D[v,0.1] and D[v,0.9] are diameters representing the points at which 10%, and 90% of the volume of liquid sprayed is in droplets of smaller diameter; the relative span is defined as the D[v,0.9] minus the D[v,0.1] and then divided by the VMD (D[v,0.5]). The efficiency of a spray nozzle is inversely proportional to the range of droplets it emits. As shown in the above example where the first has a RS of 2.15 and the second has 1.15 the nearer to 1.0 the better.
Driftables is another good measure. The first chart has 43% driftables whereas the second has only 6%.
Driftable: In this context driftables are the percentage of the spray volume that contains droplets smaller than 100 micron and which almost certainly will not reach their target: Wasted spray.
Low VMD are often heavily influenced by high driftables.
On the other hand number of droplet per millilitre is a very poor measure. In the example the first chart has more than twice the number of drops per millilitre than the second but the second would deliver much better results.