bouse1984.pdf - 171 DROPLET-SIZE CONTROL I N SPRAYER SYSTEMS L.F BOUSE and J B CARLTON P e s t C o n t r o l Equipment and Methods Research U n i t

bouse1984.pdf - 171 DROPLET-SIZE CONTROL I N SPRAYER...

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171 DROPLET-SIZE CONTROL I N SPRAYER SYSTEMS L.F. BOUSE and J . B . CARLTON P e s t Control Equipment and Methods Research Unit, USDA/SEA-AR, College S t a t i o n , Texas, (U.S.A.) INTRODUCTION S a f e and e f f i c i e n t a p p l i c a t i o n of a g r i c u l t u r a l p e s t i c i d e s i n spray form de- pends l a r g e l y on t h e spray-droplet s i z e . For spray e f f i c i e n c y , t h e d r o p l e t s must be placed on p l a n t f o l i a g e o r o t h e r t a r g e t s i n s u f f i c i e n t amounts and be properly d i s t r i b u t e d t o permit optimum e f f e c t i v e n e s s by t h e p e s t i c i d e . For saEety, p e s t i c i d e s t h a t a r e t o x i c t o humans, animals,or d e s i r a b l e p l a n t s must not d r i f t (remain a i r b o r n e and move away from t h e a p p l i c a t i o n s i t e ) o r deposit on adjacent a r e a s . Droplets t h a t remain airborne f o r long periods increase the p r o b a b i l i t y of a p p l i c a t o r exposure. Information is lacking concerning t h e optimum droplet s i z e and spray coverage €or t h e most e l f e c t i v e and e f f i c i e n t c o n t r o l of s p e c i f i c weeds, woody p l a n t s , i n s e c t s , o r p l a n t diseases. Spray coverage i n t h i s paper r e f e r s t o t h e percen- tage of p l a n t leaf area covered with spray. Researchers usually agree t h a t , f o r t h e c o n t r o l of p l a n t s , f o l i a g e feeding i n s e c t s , and p l a n t diseases, d r o p l e t s should be uniformly d i s t r i b u t e d on t h e p l a n t f o l i a g e . Because high a p p l i c a t i o n r a t e s are impractical and uneconomical, adequate spray coverage on plant f o l i a g e is o f t e n possible only when sprays with a high percentage of small d r o p l e t s a r e used. However, s m a l l d r o p l e t s a r e usually s u s c e p t i b l e t o d r i f t , and spray-drift c o n t r o l r e q u i r e s t h a t small d r o p l e t s account f o r only a very small portion of t h e spray volume. Therefore, adequate spray-coverage and d r i f t - c o n t r o l require- ments sometimes cannot be s a t i s f i e d simultaneously. D r i f t c o n t r o l , spray coverage, and a p p l i c a t i o n r a t e s must a l l be considered i n a r r i v i n g a t t h e b e s t compromise. Three b a s i c f a c t o r s a f f e c t t h e droplet s i z e of p e s t i c i d e sprays -- t h e atomizer design, which determines t h e way i n which l i q u i d is discharged and d r o p l e t s a r e formed; t h e l i q u i d ' s physical p r o p e r t i e s , which a f f e c t t h e behavior of a disin- t e g r a t i n g l i q u i d sheet o r j e t ; and t h e n a t u r e of t h e spraying atmosphere ( r e f . 1). The r e l a t i v e speed and d i r e c t i o n of t h e air with respect t o t h a t of t h e disin- t e g r a t i n g l i q u i d is important when sprays a r e formed i n a high-speed airstream o r released from an a i r p l a n e . This paper w i l l review methods used f o r describing

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