1 88 HOMER E. CHENOWETH. 



in reality, is the pressure tending to burst the pipe as would be 

 measured by the ordinary pressure gage. The dynamic head or 

 pressure, also termed the total or impact pressure, is the sum of the 

 pressures required to overcome the resistance to flow and create 

 the velocity of discharge. For the many occasions where the 

 measurement of volume, or of volume and pressure, is required, 

 more accurate methods of measurement are often desired than 

 have been commonly used, and the experimenter needs something 

 better than the ordinary anemometer to determine the velocity, 

 or a rubber tube connected to a water gage to measure the pres- 

 sure. Especially is this so in fan work where the volume varies 

 from zero with the outlet entirely closed, to a maximum, with an 

 unobstructed discharge. The measuring apparatus must be 

 accurate through this wide range. 



As has been stated, the usual method employed in stating and 

 measuring small pressures is noted by observing the height of a 

 column of water which the pressure will maintain in equilibrium 

 or balance in a U tube, or manometer. The difference in level of 

 the liquid in the manometer is a measure of the static pressure 

 existing at this point in the pipe or duct. A tube of this descrip- 

 tion is turned a "Piezometer." In order to obtain correct piezom- 

 eter readings the most accurate method is to employ a hollow 

 ring connected to the interior of the pipe by six or eight small 

 holes, 0.02 inch in diameter. 



Some form of differential or inclined tube gage filled with 

 gasoline and graduated to read in hundredths of an inch of water 

 is usually employed in place of the ordinary U tube, which is not 

 well adapted for reading small pressure differences. 



We can now measure the static head or pressure, but as this 

 would not be of much value with air under motion, it is necessary 

 to introduce some device in order that the velocity pressure may 

 be obtained. Such an instrument is known as the "Pitot" 

 tube. This Pitot tube when used with suitable pressure meas- 

 uring devices, as already described, is particularly well adapted 

 for measuring both the pressure and volume of air flowing through 

 a pipe or duct. 



The Pitot tube has the advantage of being small, portable, and 

 can be easily and conveniently placed in small air lines. It is on 



