148 COMMERCIAL ELECTRICAL TESTING 



Under the column showing the dry bulb temperature 

 of 67 deg, and opposite the barometer reading of 30.1, 

 the weight of air is given as 0.07517. The addition for 

 each 0.01 of an inch of barometer is given as 2.6 in the 

 second line from the top of the page. Multiplying this 

 by 5; i.e., by the excess of the corrected barometer 

 reading over that selected in the table, the result is 13, 

 which number must be added to the weight of air 

 previously found. The wet bulb depression is the* 

 difference between 67 deg. and 59 deg.; viz., 8 deg. The 

 number opposite 8 is 23, which must also be added, 

 making the total weight of air 0.07553. All pressure 

 readings should be corrected for standard air (see page 

 153) by multiplying the actual pressure obtained by the 

 ratio of the weight of standard air to the weight of air 

 at the time of test. The readings of horse-power input 

 to the fan should also be multiplied by this ratio. 



Pressure and Horse Power Curves by Double Tube Method 



A pressure curve may be taken by the double tube 

 method as follows: 



The opening at the outer end of the discharge pipe 

 should be closed and pressure and power readings taken. 

 Under this condition the static and impact pressures 

 should be exactly the same, since no air passes through 

 the fan. Readings should then be taken as the opening 

 is increased by equal increments from closed to wide 

 open, the opening being measured each time. The 

 speed of the fan should be held constant throughout the 

 test, and the air readings and electrical input readings 

 taken simultaneously. 



It will be noted that in a test which is made with a 

 pipe on the discharge side of the fan, the reading of 

 the impact tube is always greater than the static reading. 

 If the pipe is on the suction side, the opposite will be 

 true; the difference between the two readings being the 

 velocity head. The Pitot tube should point against the 

 stream of air in both cases. 



If the readings are taken by means of a U tube, the 

 readings of both sides of the tube should be recorded. 

 It should always be stated whether the readings were 



