TReNo 22 
immediately in front of the current meter so that it blocked some of the 
air flowing through the current meter, When the impeller had achieved 
a constant angular velocity, the screen was quickly removed and the 
output of the current meter measured as the angular velocity of the 
impeller increased from its original value to its final value. Initially, 
the period between pulses was measured at intervals of approximately 0.2 
sec with an @lectronic counter connected to a paper tape digital recorder. 
The interval was determined by the maximum printing rate of the recorder-5 
lines/sec, The results, however, were subject to a large amount of scatter, 
which was found to be caused by the variation in angular spacing between 
adjacent impeller blades 110%. To eliminate this the output of the current 
meter was modified using a Schmidt trigger=binomial counter circuit so that 
the period per rotation of the impeller could be measured instead of the 
period between pulses. 
Measurements were made as described at six different wind tunnel 
velocities. The velocity was determined from measurements of dynamic 
pressure, wet and dry bulb temperatures, and barometric pressure; the 
dynamic pressure was measured with a pitot static probe connected to a 
differential micro-manometer, 
A calibration of the current meter was also performed in the wind 
tunnel by measuring the output frequency at various known wind tunnel 
velocities and using the method described in reference 23 to convert the 
values measured in air to in=water values. 
From equation (22), we get 
_F- 
/ 
/ 3 
Coe Wy. Shane (30) 
| 
S 
~N 
Xan 
iT 
This can be written as 
Al = 
es 
Pes c an 
