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TECHNICAL REFERENCE DR O P S I Z E T E R MI N O L O G Y AN D I MPAC T DROP SIZE TERMINOLOGY IMPACT Terminology is often a major source of discrepancy and confusion in understanding drop size. To accurately compare drop sizes from one nozzle to another, the same diameters have to be used. Drop size is usually expressed in microns (micrometers). Following are the most popular characteristic diameters and their definitions. Impact, is the measure of force imparted on a surface by a spray pattern at a given distance. It can be expressed in several ways. All definitions are derived from the most basic equation of total impact force. This is the force that any flow, at any pressure, is capable of making on a surface. This does not account for orifice shape, nozzle type, fluid properties and other factors. VOLUME MEDIAN DIAMETER (VMD) I = KxQx P also expressed as D v0.5 and Mass Median Diameter (MMD) A means of expressing drop size in terms of the volume of liquid sprayed. The Volume Median Diameter drop size when measured in terms of volume (or mass) is a value where 50% of the total volume of liquid sprayed is made up of drops with diameters larger than the median value and 50% with smaller diameters. Total theoretical impact = constant (based on units) x flow (at pressure P) x square root of pressure (P) I kg(f) K .0526 .024 .24 .745 Q gpm lpm lpm lpm P I = total theoretical spray impact lbs.(f) psi kg/cm 2 bar MPa Newtons Newtons K = constant SAUTER MEAN DIAMETER (SMD) also expressed as D 32 A means of expressing the fineness of a spray in terms of the surface area produced by the spray. The Sauter Mean Diameter, is the diameter of a drop having the same volume-to-surface area ratio as the total volume of all the drops to the total surface area of all the drops. NUMBER MEDIAN DIAMETER (NMD) also expressed as D N0.5 A means of expressing drop size in terms of the number of drops in the spray. This means that 50% of the drops by count or number are smaller than the median diameter and 50% of the drops are larger than the median diameter. Q = flow rate P = liquid pressure The constant (K), is a unit conversion based on the measurement system used. The conversions are listed in the chart above. Example: I = .0526 x 3.5 gpm x √150 psi I = 2.25 lbs(f) is available for distribution throughout the pattern Contact your local sales engineer for assistance in determining impact in your application. More drop size data is available on all types of spray nozzles. For more information contact your local Spraying Systems Co. sales engineer. spray.com | 1.630.665.5000 A9