424 MEASURING OF AIR. 



on its way through the compressor through leakages, condens- 

 ation of vapour, etc., so that the weight discharged by the 

 compressor differs still more from the weight calculated from 

 the temperature of the suction chamber. The consumer of 

 compressed air is only interested in the weight of air discharged 

 by his compressor. Although the indicator diagram shows the 

 volume of air discharged by the machine, it does not give any 

 means of determining the actual weight discharged, for the 

 same reason as above, i.e. that it is impossible to determine 

 the temperature during the discharge stroke. This tem])erature 

 is highest when the discharge valves open and will decrease 

 towards the end of the discharge stroke. 



The difficulties of measuring air were increased as soon as 

 the rotary or turbo compressor appeared as a competitor of 

 the reciprocating compressor. The turbo compressor draws in 

 and discharges the air in one continuous flow and the only 

 means of measuring the output of a turbo-com]Dressor were 

 the methods known as nozzle or orifice measurements. These 

 methods have improved considerably since the turbo-compressor 

 has come forward and ways have been found to adapt these 

 methods also to reciprocating compressors. 



The measuring of air through nozzles or orifices depends 

 on the exact measurement of the pressure, velocity, and temper- 

 ature of air, when flowing through the nozzle or orifice. ( )f 

 these measurements the measuring of pressure and velocity, 

 although simple at first sight, however, re(|uire great care, if 

 reliable results are to l)e oljtained at all. The difficulty of 

 observing and understanding hydro-dynamic phenomena, the 

 inaccuracy of the co-efficients of friction and of contraction 

 resulting therefrom. ha\e led to most varying justifications of 

 methods for measuring \elocity and pressure. Furthermore, 

 there exists a certain discrepancy regarding the definition of 

 pressure inside a flowing li((uid, which accentuates the above 

 mentioned difficulties. 



In physics we decide since Daniel Bernouilli regarding the 

 pressure inside a liquid between, firstly, the hydrostatic pressure, 

 which is defined as the pressure existing between two particles 

 of the liquid when it is not moving, secondly, the hydro-dynamic 

 ])ressure, which is defined as the pressure between two particles 

 of the liquid during motion. 



Bernouilli's equation for motion of lic|uids is : — 



dh + '^+^ = 

 g 7 



— assuming absence of friction and whirls — , where 

 /; = height above datum level 

 c = velocity 

 /) rzn pressure 



