HYDEAULICS AND ITS APPLICATIONS 



or across stream thus placing the plane of its orifice either at 



right angles to or parallel to the 

 direction of flow. 



When pointed up-stream, the statical 

 pressure immediately inside the 

 entrance to the tube must balance 

 the statical head of water outside 

 Q together with the pressure equivalent 

 to the velocity head, so that if h' feet 

 be the height of water in the tube above 

 the free service level when turned 

 up-stream, we have h'=kv 2 -r-2g, where 

 k is a constant very approximately 

 equal to unity. 



Similarly, if turned down-stream, 

 and if h" is now the depression in the 

 tube, we should have h"=k r v 2 --Zg. 



Theoretically, with the orifice point- 

 ing across the stream, the level inside 

 the tube should indicate h, the statical 

 head alone. Owing, however, to eddy 

 formation and to a consequent reduc- 

 tion of pressure at the orifice, the 

 level inside the tube is slightly less 

 than h. 



When turned down-stream, eddy 

 formation also affects the reading h", 

 giving this a lower value than the 

 theoretical, while even when pointing 

 up-stream the reading h' is only 

 accurate when the tube is of very fine 

 bore, and is given a conical form so 

 as to divert the oncoming stream 

 with a minimum of disturbance. For 

 this reason, the tube should be cali- 

 brated by observations in water 

 moving with known velocities. Other- 

 wise, even with a well-constructed 

 apparatus, an error of about 5 per 

 cent', is possible, the calculated being greater than the actual velocity. 



FIG. 100. Pitot Tube. 



