IEEE Instrumentation & Measurement - September 2023 - 34

shown in Fig. 2. Under conditions based on the refractive index
of water and the constant refractive index of suspended
particles in water, these principles are shown in (2) and (3):
I K V INRR= ∗ ∗ ∗∗
2
 +



1
cos θ
2
2
I K AN IRM= ∗∗ ∗ 0




(2)
(3)
where: IR is the light intensity of the scattered light; KR is the
Rayleigh scattering coefficient, which is a constant; KM is the
Mie scattering coefficient, which is a constant; V is the volume
number of individual particles; N is the volume number of a
single particle; I0 is the incident light intensity; and θ is the angle
between the scattered light and the incident light.
In the case where the emitter and receiver are fixed, the θ is
also a fixed value. Therefore, whether Rayleigh scattering or Mie
scattering occurs, the intensity of scattered light produced by the
direction is proportional to the concentration N of suspended
particles in the water column. Scattered light turbidity measurement
method is more suitable for the measurement of low sand
content water quality. Since the sand content of river channels is
mostly within 20 kg/m³, and the sand content in water bodies
is relatively low compared to that in the Yellow River basin, it is
more ideal to use scattered light measurement. Considering the
compactness and portability of the instrumentation, the backward
scattering method is used, and the transmitter and receiver
are placed on the same side to reduce the space size.
Another important parameter in the optical measurement
of scattering method is the incident light intensity, which
corresponds to different measurement ranges and accuracies
depending on the wavelength band of the emitting light
source. The range of spectral bands commonly used is between
350-2500 nm, and with the increase of sand content in the water
body, the reflectance of each wavelength band will increase
to different degrees. Therefore, according to the needs of the
field environment, the chosen spectrum band is particularly
imp-ortant for the measurement accuracy and range of the
sand content of the water body. The main light sources commonly
used today are composite and monochromatic light
sources (Table 1).
The compound light sources mainly include incandescent
lamp and tungsten lamps in which the filament heats up to operate.
The efficiency of this method is low and will generate
heat in the surrounding environment, thus affecting the measurement
accuracy. Monochromatic light sources are mainly
laser diodes and light-emitting diodes, which work through the
(a)
(b)
Fig. 3. (a) Sediment collector. (b) Sediment sensor.
Table 1 - Light source comparison table
Light source
Incandescent/tungsten lamps
Light emitting diode
Laser diode
34
Stability
general
good
good
Service
life
general
length
length
Calorific
value
loud
low
low
Influenced by
chromaticity
large
small
small
IEEE Instrumentation & Measurement Magazine
Sensitivity to
particles
small
large
large
Drive
circuit
easy
easy
involved
September 2023
PN junction in the carrier compound to produce light radiation.
They have high efficiency and significantly reduced power
consumption and heat dissipation, and this single-wavelength
light source has a very narrow spectral bandwidth, very sensitive
to current. Light sources of different wavelengths are
obtained by adding chemicals such as gallium, arsenic, phosphorus,
nitrogen, etc. Given that laser has the characteristics of
strong directionality, good monochromaticity, high brightness
and good coherence, which can effectively improve the accuracy
and precision of sand content measurement, laser diode is
most often used as the emitting light source [9].
For the same sand sample with different concentrations,
the emissivity of light sources in different frequency bands is
not the same. Specifically, 600-900 nm short wavelength light
sources have a large increase in reflectivity at low sand content,
where there are two reflection peaks, but at high sand content,
the increase in reflectivity is small. On the other hand, 1800-
1940 nm long wavelength light sources have a low increase
in reflectivity at low sand content, and as the sand content
increases, the reflectance of 1800 nm wavelengths is larger
than that of 1940 nm wavelengths under the same conditions,
because the wavelengths around 1940 nm have a strong absorption
effect. In summary, considering the sand content of
river channels in the range of 0.1-20 kg/m³, the 850 nm NIR
short-wave laser diode was finally selected as the emitting
light source.
Measuring Equipment
Based on the above measurement principle, a set of on-line
sediment measurement equipment was developed, the core of
which was composed of two parts: the sediment collector and
the sediment sensor, shown in Fig. 3. The sediment sensor can

IEEE Instrumentation & Measurement - September 2023

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