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Status of Current Hopper and Bin Measurement Technologies
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MEASUREMENT IN THE PIPELINE (cont.) - doert60003
ERDC TN-DOER-T6
September 2004
dense) and the final deposition manner. Cohesive soils tend to increase in volume when removed
from their in situ position. Hydraulic dredges usually bulk up sediment more than mechanical
dredges due to water entrainment. New work material tends to have higher initial bulking in the
placement area than maintenance material because it is usually in a more consolidated in situ
state. A general rule of thumb is the larger the grain size, the lower the bulking factor (for
relative approximations, sand can bulk up 1.0 to 1.2, silt 1.2 to 1.8, and clay 1.5 to 3.0 (USACE
Dredging Fundamentals PROSPECT Course, 1998)).
Measurement in the pipeline consists of an inline production meter that measures the slurry flow
velocity and density before it enters the hopper. Measurement in the means of conveyance
involves determining the volume and/or weight of the hopper load. Several American dredges
measure one or the other of these parameters to quantify production, but most U.S. hopper
dredges measure both to calculate the load's average density or some derivative thereof, i.e., tons
dry solids (Welp and Rosati 2000). Various methods (described later in this technical note) are
used to measure the hopper load weight and volume.  Determining barge production by
"measurement in the bin" usually involves quantification of bin load volume. Bin load weight is
also used as a production parameter on barges, but on a more limited basis. This technical note
describes these different measurement methods and discusses their respective advantages and
disadvantages, capabilities, and limitations.
MEASUREMENT IN THE PIPELINE: A "measurement in the pipeline" dredge production
meter is a system that determines slurry velocity and density, and uses these two values to
calculate dredge production. The system usually consists of a density meter, flow meter, data
recorder, and an output display that indicates production in units of mass or volume per unit
time. The major types of flowmeters used on hopper dredges include electromagnetic and
Doppler acoustic devices, while slurry density is usually measured by a nuclear density device.
The electromagnetic flow meter works on the principle of electromagnetic induction and is
designed to measure the flow of conductive liquids in a pipe. Two electromagnetic coils
surround a pipe made of anti-magnetic materials and produce a magnetic field at right angles to
the flow direction. As a conductive liquid passes the metering section, the lines of force from the
magnetic field are cut, producing a low-level voltage at the stainless steel pick-up electrodes. The
electrodes measure the potential difference, which is proportional to the flow rate and
independent of the solids concentration (Herbich et al. 1992). The Doppler flow meter uses the
theory of the 'Doppler effect'; i.e. there is an apparent change in the frequency of sound, light, or
radiowaves as a function of motion. These meters consist of a piezoelectric crystal transducer, a
Doppler frequency receiver, and a transmitter. The transmitter sends a continuous ultrasonic
signal at an angle to the direction of flow through the pipe wall and into the liquid stream. The
sound waves are reflected back to the receiver by particles, bubbles, or other discontinuities in
the liquid. The difference between the transmitted and the reflected frequencies, called the
`Doppler shift,' is analyzed and the flow rate of the slurry is displayed (Herbich et al. 1992). The
nuclear density gauge (Figure 1) measures density using the energy-absorption method. A
radioactive source emits gamma-ray energy through the discharge pipe. The rays are absorbed in
proportion to the density of the slurry, and a detector handles the gamma ray energy. The
transmitted energy is finally converted into a linearized output, which indicates density changes.
2

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