426 MEASURING OF AIR. 



separated from the stream and forced to ])roduce work by 

 keeping tlie water level inside the pitot tube at a' certain 

 height above level of stream. The pressure pst represents 

 potential energy and pt therefore equals the sum of the poten- 

 tial and kinetic energy of the unit of volume i.e. the total ener^ 

 of the liquid. 



It will further be noticed that the dynamic pressure as defined 

 technically does not change into static pressure, when the motion 

 ceases, as should be logically expected. 



The definitions given as used technically are, therefore, mis- 

 leading and when using them, care should be taken always 

 to remember their actual meaning. 



If the pressure of a liquid, flowing through a pij-e. has 

 to be observed for the purpose of measuring, it is first of all 

 necessary to define exactly the point where the pressure has to 

 be measured and further, it has to be assumed for the puq)oses 

 of a test that the flow is stationary, i.e. is not a function of the 

 time. According to the first definitions given above, the pressure 

 so determined can l)e only the hydrodynanfic pressure, ;.t'. the 

 pressure which a gauge flowing with the licpiid would show 

 when passing the a])ovementioned ])oint. .\s all ])rol)lems of 

 measuring large quantities of air in technics only liave to deal 

 with air in motion, the static pressure as defined above according 

 to Bernouilli does not come into question at all, as this 

 <lefinition refers only to the jjressure of lifiuids when motionless. 



When measuring the pressure, special attention must be given 

 to the manner in which the pressure is transferred fro;u the 

 stream of gas to the instrument recording the pressure. 



C B 



\ 



A '^^ *■ -A 



! 



— FIG 1 — 



It is generally accepted that a connection as shown in Fig. i 

 gives the exact hydrodynamic pressure of the gas flowing through 

 pijie A A as long as no disturbances of tlie stream lines 

 occur; The down stream side B of the opening C, howe\er, is 



