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Hopper level sensors
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TDS Sensitivity Analysis
ERDC TN-DOER-T6
September 2004
weight of water surrounding the hull to calculate the entire vessel weight. The earliest American
hopper weight measurement system is documented in the "Yardage Meter Instruction Manual"
(USACE 1959). The "Yardage Meter" used a "bubbler" draft system that measured the hydro-
static pressure at the dredge's keel and converted this measurement into depth. The bubbler
system is designed to maintain a constant flow of air to various "bubbling points" mounted by
the keel. The pressure required to force the air out through the lines and bubbling points is equal
to the hydrostatic pressure at the respective bubbling points. The hydrostatic pressure is then
converted to depth, or draft, using the density of the water the vessel is immersed in (draft equals
pressure divided by density of water). These bubbler systems are still used today, but the special
16-in.-diam pressure gauge with special dial used in the yardage meter has mostly been replaced
by pressure transducers installed in the bubbler lines to measure the back pressures. Other draft
measurement systems consist of pressure transducers mounted directly through the hull by the
keel. These draft measurements are usually taken with at least two pressure sensors as shown in
Figure 4, one mounted forward and one mounted aft on the underside of the vessel (Rokosch
1989). These sensors also measure the pressures (proportional to depth) experienced at the
underside hull locations. Pressure sensors have also been used to measure bin load weights in the
same manner as hopper dredges, or are manually measured by the draft markings on the hull.
Rokosch (1989) reports that advantages of the weight measurement system (a component of the
Rijkswaterstaat's Tons Dry Solids System) is that the system is independent of type of sediment
and requires no manual actions, but the system measures weight from a relatively small
difference in draft. The weight measurement component was reported to be used in maximum
wave heights of 2 to 2.5 m.
Rullens (1993) reports that errors in this measurement method are introduced by:
Incorrect positioning of draft sensors on the hull.
Displacement not measured due to the vessel's hull bending under the hopper load.
Error due to use of the vessel's displacement curve (to convert draft to vessel weight) without
trim correction.
Pressure measurement affected by pressure variations caused by vessel movement through
water.
Pressure measurements affected by wave action.
Error induced by water density differences in converting hydrostatic pressure to draft.
Non-linearity of pressure sensors.
Tons Dry Solids (TDS). TDS measures the hopper-load's volume and weight in order to
determine the quantity of "dry solids" that it contains. By applying the values for the dry solid's
specific density and the in situ water's density in a formula with the hopper-load's weight and
volume (which indirectly measures the hopper-load's average density), the total quantity of the
dry solids can be calculated. Welp and Rosati (2000) describe this measurement method and the
Corps' initial experiences with it. Because the density is calculated by measuring both the
weight and volume of the hopper or bin, the previously described volume and weight
measurement technologies, and respective advantages/disadvantages and capabilities/limitations,
10

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