concentration measurements. To ensure that the natural sediment glued 
to the artificial dunes did not.break loose and damage the sensor, the 
surface of the dunes was sprayed with a thin film of plastic. The plas- 
tic was thin enough to not alter the surface roughness and strong enough 
to hold the sediment in place. 
To accommodate the anemometer equipment, the yoke support used in 
concentration measurements had to be removed from the support frame 
(Fig. 3). The hot-film probe assembly (Fig. 19) was attached to the 
support frame at a position midway across the flume. The elevation of 
the sensor could be changed by loosening two friction clamps and reposi- 
tioning the probe holder in relation to a scale fixed to the assembly. 
A 6-inch-square opening was cut into the wave suppressent board to allow 
the sensor to be lowered to the flume bottom. 
2. Experimental Procedure. 
The hot-film sensor is extremely sensitive to both water temperature 
and water quality. Therefore, it was necessary to determine a new cali- 
bration curve each day that velocity measurements were made in the flume. 
Water temperature readings made during calibration measurements were com- 
pared to readings made during flume measurements and did not vary. It 
was necessary to use deaerated water in both the calibration and flume 
measurements to prevent air bubbles from adhering to the hot-film sensor, 
thereby either burning out the sensor or altering its heat transfer char- 
acteristics. Periodically, during both calibration and flume measurements 
a record was made of the base voltage; i.e., the voltage for the sensor in 
Still water. These voltage readings were then averaged to obtain a mean 
base voltage which is needed in calculating the calibration curve. 
Both days that velocity measurements were made with the anemometer, 
a 5-percent overheat of the sensor was used. This ensured uniform sensi- 
tivity of the sensor and base voltage of the measurements. The conditions 
for measurements on both days were so similar that the’two calibration 
curves could not be distinguished. This allowed all the calibration data 
to be used for one curve and only one equation used to convert voltage 
into velocity. 
a. Calibration of the Hot-Film Sensor. The hot-film sensor was 
positioned at the center and as close to the downstream face of the cali- 
bration nozzle as possible (approximately one-eighth inch). A high flow 
rate through the flume was obtained by fully opening the needle valve. 
This flow rate was allowed to continue until a steady flow through the 
calibration tank was established (about 5 minutes). The flow through the 
nozzle was monitored by the continuous voltage output of the hot-film 
bridge displayed on the anemometer equipment. When the voltage readings 
became constant with time the flow rate was steady. A 24-second magnetic 
tape record was then made at the rate of 117 samples per second. Simul- 
taneous with the voltage record, a flow rate measurement was made. The 
flow rate was determined by timing the period required to collect 10 to 
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