430 



MEASURING OF AIR. 



Since the equation of continuity is now : 



,, - ^V-v-^ W.v 

 Hj — //, — . 



ir - tr = W' M -. 



y a' a' 



and 



Hence 



w- 



i^-exai 



But 



e,)^ 



■^g 







•> 





AT— 1 



(0 ' -A 



p: 



Hence 



P\ . X'l 



(1) 



K~ I 



w --= 



1 - (--) (t)il 







/> 



(D 



In this equation a^ is the measured area of the stream 

 approaching the orifice and c/._, is the contracted area of the stream 

 leaving the orifice. 



H A is put for the u])stream area of the pipe instead of 

 a., and a for the measured area of the orifice and C the coefficient 

 of contraction, then a.^ =■ ca. and we ohtain in taking the square 

 roots of both sides : 



W 



v>-(fy(|:)^ 



w 



2g^ K 



1 



(f)^ f(^-:)'^'^'} 



This form of the equation introduces the correction term 

 for the velocity of approach and it is necessary to examine anv 

 ])articular case to see whether the correction term is important 

 or not. 



In order to reduce this equation to a form adapted to numer- 

 ical calculation we have p, --= p> + h where h is the pressure 

 drop across the orifice and 



^ = 1 + /' 



P'2 Pl 



