Manchester Memoirs, Vol. Iviii. (1914), No. 1. 



VII. Some notes on the measurement of air velocities, 

 pressures and volumes. 



By William Cramp, M.I.E.E., M.Sc.Tech. 



{Received and read J'amia>y i^th, igi4.) 



I. Measurement of Air Velocity. 



The measurement of air velocity is usually carried 

 out by means of an anemometer or a so-called " facing 

 gauge" and manometer. The former is well known, and 

 may give fairly accurate results if sufficient precautions 

 are taken. Thus to obtain the air velocity at any point 

 in an air duct many readings must be taken and the 

 results averaged. To obtain the average velocity over a 

 given section of a duct, many positions over the section 

 must be selected and several readings taken at each, 

 when a proper average may be obtained. To attempt 

 such a series, however, is often impracticable, for the 

 following reasons : — 



(i) The anemometer is not a small instrument, and 

 its very presence in the duct is often sufficient 

 to falsify the normal condition in the duct. 



(2) The reading is often required at a position within 

 a pipe where the anemometer cannot be seen 

 at all, much less handled. The absurdity of 

 relying, on it in such a case is obvious when it 

 is remembered that it must be used in con- 

 junction with a stop watch, and that the plane 

 of its wheel must be normal to the current 

 measured. 



May 26th, igi4. 



