15 pounds of water at the overflow of the calibration tank. The water 
was weighed on a scale to the nearest 0.05 pound. The flow through the 
flume was then reduced by partially closing the needle valve and a second 
set of measurements made. The procedure was repeated until five voltage- 
velocity points were measured for a calibration curve. 
The results of the two calibration curves are shown in Figure 20. 
The velocity range measured was from 0.0374 to 0.580 foot per second. 
The relationship between velocity and voltage was: 
Ine(E' - E,) = (0.422) ing(Uz) + 7.147 , (14) 
where E' is proportional to the output voltage of the hot-film bridge, 
and E9 is proportional to the mean output voltage for the sensor in 
still water. U,, the actual velocity of the fluid, was calculated by 
dividing the mean velocity determined from flow rate measurements by the 
nozzle coefficient, C,. The log-log relationship shown in Figure 20 is 
consistent with the theoretical results from the manufacturer and the 
results by Das (1968). Because magnitudes of the voltage readings for a 
given flow velocity are dependent on the water temperature, water quality, 
overheat percentage, and amplification of the sensor output signal, no 
attempt was made to compare quantitatively the measured calibration curve 
‘with other published curves. 
b. Determination of the Nozzle Coefficient. The sensor was posi- 
tioned close to the downstream face and at the lower edge of the nozzle. 
After establishing a high, constant flow rate through the nozzle, the 
voltage from the hot-film bridge was recorded. Without interrupting the 
flow, the sensor was then raised a small measured amount and a second 
voltage record made. This procedure was repeated until the sensor was 
at the upper edge of the nozzle. The velocity of flow was estimated for 
each location from a calibration curve similar to Figure 20 but not cor- 
rected by a nozzle coefficient. The velocity profile across the diameter 
of the nozzle for the high flow rate is shown in Figure 21(a). Because 
of the high flow rate, the water surface elevation in the deaeration tank 
was lowering a Significant amount, thereby decreasing the flow rate 
through the calibration tank. This is the reason for the lower measured 
velocity at the top of the nozzle. The mean velocity was determined by 
integrating the velocity profile across the jet. The nozzle coefficient 
was then calculated from: 
ay sg! F 2 
C= Ore) Chee CR. Ohes (15) 
where 
Cy = nozzle coefficient 
Vaan = mean velocity across the jet 
(dia.) jo4 = diameter of the jet 
(dia.). = diameter of the nozzle (1 inch) 
Ue ) = velocity determined from voltage measurements 
50 
