Stain diameter of drops in μm	Spread factor*	Drop diameter actual, in μm
100	1.7	59
200	1.8	109
300	1.9	155
400	2.0	200
500	2.1	243
600	2.1	285

* Refering to water at 20ºC and about 40% relative humidity (RH), and droplets reaching the water-sensitive paper sedimentation velocity.

The spread factors have been assessed by the magnesium oxide (1) and the silicon-oil method (2).

For droplet sizing a glass insert has been created, with a view field of 1 cm sq, a cross graticule with notches at 200 μm (0.2 mm) spacing. It fits exactly into the metal frame of the linen tester lens depicted opposite. A more expensive tubular lens with notches at 100 μm spacing is also available. Its view field is 0.432 cm sq. The relative humidity (%) and the pH on value of the water are of no major influence on spread factors on water-sensitive papers.

For accurate droplet sizing it is recommended that the spread factor be assessed for each spray liquid as it may vary with different concentrations of the same product.

(1) The measurement of airborne droplets by the magnesium oxide method, K.R. May, Journal of Scientific Instruments 24, 128-130.

(2) Bestimmung und Bewertung von Tröpfchengrössenspektren bei Pflanzenschutzdüsen, J. Jaske, Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, 22.2.1970, 17-24.