MEASURING OF AIR. 



427 



bound to disturb tbe stream lines to a certain extent, wbich 



can be expressed in percentage of the kinetic energy V. — 



If the piping connecting C with the instrument has the same 

 diameter as C the error resulting from the disturbances will 

 be transferred to the instrument. Under the conditions under 

 which compressed air for mining purposes is generally trans- 



mitted. the term V.y is very small in comparison with the hydrody- 



namic pressure. If, for example, 20 lbs. per second of air of loolljs. 

 absolute pressure, 650'^ F., absolute temperature and a density 



>j^*- 



T 



FIG.2 



of 0.4155 111 licr cubic ft. are flowing through a 12 inch pipe 

 line, we find the velocity to be 61.3 ft. per second and the 

 kinetic energy 24.35 ^^^- P^^' s(j. ft. respectively, or 0.169 lbs. per 

 sq. in. The kinetic energy is therefore 0.168 % of the 

 hydrodynamic ]jressure and as the error can be only a small 

 percentage of the kinetic energy, it is negligible. Furthermore 

 it can be altogether done away with by providing as shown in 

 Fig. 2 round the pipe a circular space of such size that the 

 shocks due to the air striking the down stream edge of the 

 opening are comj^letely annihilated through eddies. 



n 



— FIG.3.— 



With the exception of the pitot tube as shown in Fig. 3, 



which gives the hydrodynamic pressure plus kinetic energy, all 



connections, projecting into the stream lines for instance, such 



as shown in Fig. 4. — give readings which are more or less 



different from the actual pressure on account of disturbances of 

 the flow and should not be used. 



