THE ELIMINATION OF \^ELOCITY EFFECTS IN 



MEASURING PRESSURES IN A FLUID 



STREAM. 



By FRANCIS E. NIPHER. 

 (Read April 20, 1906.) 



In determining either velocities or pressures in a current of air 

 or water within a pipe, it has always been found exceedingly diffi- 

 cult to obtain a value which has any physical meaning. 



A straight tube thrust through the side of the pipe, terminating 

 at some point within, and connected to a guage, will transmit the 

 pressure, but on account of the draught of the fluid across the open 

 end of the tube, the fluid will also be drawn out, by an atomizer 

 action. 



If on the other hand the tube is in the form of a Pitot tube with 

 its mouth directed towards the advancing current, it transmits both 

 .static and dynamic pressure to the gauge. 



In the case of a building or other structure in a stream of air, if 

 it is desired to find the variation of pressure on the surface due 

 to the wind, any form of barometer gives misleading results by 

 reason of the compressions and rarefactions around the instrument 

 itself. Moreover the wind sweeps across or into various openings 

 leading to the mercury surface or flexible diaphragm. This intro- 

 duces disturbances or errors, which cannot be corrected. Engineers 

 are accustomed to combine with the Pitot tube, a similar tube with 

 its opening at right angles to the stream lines in a pipe, but they 

 usually give warning that the atomizer action of the air blowing 

 across the opening is likely to give misleading results, if the velocity 

 is great. 



In order to study the Pitot tube in a manner that would eliminate 

 statical pressure, a long series of experiments was made from the 

 window of a car. The tube was so mounted that the mouth of the 

 tube could be directed at any angle, the position being determined 



