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In a spraying system using air as the carrier o£ droplets 

 several factors are operative simultaneously which affects the 

 distribution pattern and quantity of chemical deposited. The 

 velocity and volume of the airstream, droplet size, evaporation 

 rate, ground speed and target distance all have individual effects 

 on the results obtained. 



Droplet size and air velocity must be critically balanced to 

 obtain effective and efficient spray application. Spray droplets 

 of a predetermined size must travel at specific speeds in order 

 to remain in an airstream. Many tests throughout the years have 

 determined these critical speeds and have shown that as the velo- 

 city decreases the larger droplets drop out first followed by pro- 

 portionally smaller droplets as the velocity continues to decrease. 

 The rate of velocity decrease of different airstreams has been 

 found to be similar regardless of the volume. Thus, two airstreams 

 with similar velocities at the point of outlet but having drastic- 

 ally different volumes will also have similar velocities at 25 feet 

 from the outlet and both may be only 15 to 20 percent of their 

 original speed at this distance. 



In view of this rapid velocity decrease with distance from the 

 sprayer outlet, droplet size becomes very important in the impinge- 

 ment of droplets on leaves and fruit which directly affects the 

 level of pest control obtained. The impingement of liquid droplets 

 on a solid surface such as a leaf or fruit depends largely upon two 

 factors; 1) the mass or size of the droplet, and 2) the velocity 

 at which it is traveling. Large droplets will impinge at low velo- 

 cities but also are the first to fall out of the airstream as the 

 velocity decreases. The smaller the droplets the higher the velo- 

 city required for their impingement but these droplets are carried 

 farther in slower airstreams. In slower airstreams droplets are 

 subjected to evaporation for a longer period than in high velocity 

 ones, thus decreasing their size further and diminishing their 

 chances of impingement. With conventional airblast sprayers the 

 air velocity at the outlet may be approximately 120 mph while at 

 25 feet away it often drops to 15-20 mph. In these airstreams if 

 proper droplet size is not carefully selected and distributed in 

 calibrating the machine the larger droplets fall out on the lower 

 parts of the trees while the smaller ones are carried farther but 

 not deposited in the upper portion of the tree. The end result is 

 often a poor spray distribution with heavy deposits on the lower 

 leaves and fruit which may be phytotoxic and inadequate pest con- 

 trol in the top of trees. 



The effect of relative humidity (R.H.) on spray deposition 

 has been recognized and of concern to growers since the introduction 

 of airblast sprayers. The effect of a high evaporation rate due to 

 low R.H. on small droplets produced by low volume sprays is far 

 more significant than on larger droplets because droplet size and 

 momentum greatly affects rate of impingement. It has been shown 

 that the rate of evaporation of the total spray volume will be pro- 

 portional to the total sum of the diameters of the droplets compri- 

 sing the spray. Therefore, the smaller the spray droplets the greater 



