Nipher — On Pressxire Measurements in a Fluid Stream. 31 



The practice of using a tube at right angles to the Pitot 

 tube, on the theory that it eliminates velocity effects is well 

 known to be erroneous. It has nothing but time-honored 

 custom to commend it. The rarefaction produced in a tube 

 in this position, is greater than the compression in the Pitot 

 tube. 



The disk collector when used in pipes where the pressures 

 are constant may have a leading in pipe of very small diam- 

 eter, for a distance of half an inch. It may then widen out, 

 in order to give the required stiffness. The wire gauze disks 

 need not then be over an inch in diameter, and the metal 

 disks may be as small as half an inch. These disk collectors 

 may always be tested in an air blast, in order to see whether 

 they are properly designed for specific work. 



When the disk collector is used for collecting wind pres- 

 sure on the sides of buildings, where great and rapid fluctu- 

 ations occur, the leading-in tube must be larger and the disks 

 of metal should be two and a half inches in diameter, the 

 wire gauze disks being two inches larger. The collectors are 

 placed near the building and with the flat side facing the 

 wall or surface. 



A disk collector which is to be placed far above the build- 

 ing, in order to transmit to an air-tight chamber in the 

 building, the real air pressure independent of velocity effects, 

 should have metal disks from 30 to 40 centimeters in diam- 

 eter. The leading in pipe which taps the lower disk should 

 have a bevelled flange to which the lower disk is fastened. 

 The pipe should extend slightly through the disk in order to 

 prevent moisture which may collect between the disks from 

 passing down the tube. The disks of wire screen may have 

 their central parts cut away, forming a free chamber around 

 the mouth of the pipe, between the metal disks. The screen 

 should be of copper and should project an inch beyond the 

 metal disks, in order to prevent compressions and rarefac- 

 tions around the edge from penetrating between the disks. 

 Ordinary window screen may be used. A barometer within an 

 air-ti^ht chamber connected with such a collector above the 

 building will not be affected by wind. 



