428 



MEASURING OF AIR. 



The measuring of velocity depends on the measuring of two 

 different pressures on either side of an orifice or on the up- 

 stream side and in the throat of a venturi tube. From the 

 dift'erence of these two pressures, the drop across the instru- 

 ment — the velocity can be calculated and therefrom the weight 

 or volume of air passing. 



— F\G.jft-. — 



Regarding the calibrating of orifices or nozzles, special 

 reference has to be made to the calibration plant of the Central 

 Alining and Investment Corporation, erected at the Ferreira 

 Mines, a short description of which appeared in the Journal of 

 the Smith African Institution of r.n</inccrs (iott'> 10. 



All temperatures should be measured by inserting the mer- 

 cury bulb direct into the stream. Thermometer pockets should 

 be avoided whenever possible. An arrangement as shown in 

 Fig. 5 has proved satisfactory for use with highly compressed 



