The photoelectric cell produces a signal which is proportional to the 
amount of light received which, in turn, is proportional to the amount 
of light blocked out by suspended sediment. The signal from the photo- 
electric cell is transmitted, amplified, and recorded on an analog paper 
chart recorder and an analog-to-digital data acquisition system set to 
sample at the rate of 58 samples per second. The light source and 
receiver are shown in Figure 2. 
The light source and receiver are mounted on a rigid, aluminum yoke 
support (Fig. 3) held by friction brackets; by loosening four thumbscrews 
the elevation of the support can be varied from below the flume bottom to 
above the flume top while maintaining precise alinement of the optical 
equipment. This flexibility allows calibration of the optical equipment 
and flume concentration measurements to be made without removing: the 
equipment from the supports. The yoke support and brackets are pinned 
to the laboratory ceiling on the same axis as the flume so that the 
optical equipment can swing with the flume or be held stationary in 
space. 
2. Experimental Procedure. 
a. Settling Velocity of the Sediment. The sediment used in the. filxsit 
series of experiments was the material which passed the 0.495-millimeter 
sieve and was retained on the 0.417-millimeter sieve. The fall velocity 
of this sediment was measured in a 2-inch-diameter glass cylinder filled 
with deaerated water at a temperature of 72° Fahrenheit. The time re- 
quired for 220 particles chosen at random to fall 8.59 inches was measured 
with a stopwatch. The average settling velocity (V,), the range of 
velocities, and the standard deviation were then calculated to be 0.035, 
0.0213 to 0.0532, and 0.0059 foot per second, respectively. 
b. Calibration of Optical Concentration Meter. Calibration of the 
optical equipment was done in a 0.5- by 0.5- by 1.0-foot clear, plastic 
calibration tank placed on the top of the swing flume. The 1.0-foot 
dimension of the tank was positioned parallel to the axis of the light 
source and receiver; i.e., the same width as the swing flume. The tank 
was then filled with a measured quantity of deaerated water. A small, 
measured amount of cleaned sediment was added to the tank and the 
sediment-water mixture was stirred mechanically to give a uniform sus- 
pended concentration. The collimated light beam and receiver were posi- 
tioned and a 5-second record of the voltage from the receiver was recorded 
on the analog chart recorder and magnetic tape. Uniformity of concentra- 
tion in the tank was checked by measuring the fraction of light blocked 
by the sediment at various locations in the cross section of the tank. 
The mechanical stirrer was stopped and the sediment allowed to settle. 
Records were then made of the voltage from the light beam passing through 
clear water and the voltage of the ambient light. The fraction of light 
passed was then calculated as the ratio of the voltage with sediment in 
suspension to the difference between the voltages of the beam through 
clear water and the ambient light. Measured amounts of sediment were 
