Figure 4. Hopper level and draft sensors

ERDC TN-DOER-T6

September 2004

possible to minimize trim-induced error (Welp and Rosati 2000). Sensor specifications state an

accuracy of 0.25 percent of the measurement range (2 to 50 ft) with no temperature gradient.

This measurement method's accuracy is affected by environmental factors (i.e., temperature,

incident angle, surface composition, humidity, presence of nearby structures, and sediment

buildup on the transducer by slurry spray).

Figure 4.

Hopper level and draft sensors

Experience on the McFarland and dredge Wheeler (Jorgeson

and Scott 1994) illustrates that proper placement of these

transducers is critical to optimizing operating efficiency and

accuracy for these types of sensors. On both dredges, the

sensors were located above the maximum hopper slurry level

and away from the distribution points to minimize direct

contact with splashing and spray. Hopper configuration aspects

(i.e. being open or closed, sloping sides, etc.) and the presence

of piping, auxiliary equipment, and structural members can

impact placement alternatives of these sensors. Periodic

cleaning of the transducers on the dredge McFarland assisted in

minimizing error from sediment deposited on the transducer by

spray, but when a sufficient layer of foam was generated on the

slurry surface, inaccurate readings were recorded until the foam

dissipated (this type of foam is illustrated in Figure 6) . This

error component has been observed on other dredges that use

the Silent Inspector.

Figure 5.

An ultrasonic

transducer mounted

over the dredge

An additional measurement error component is introduced

McFarland's hopper

when using ultra-sonic sensors to measure sand load volumes

with a layer of water overlaying the settled sand. The ultra-sonic pulse reflects off the

supernatant water, not the sand surface, thereby introducing error proportional to the water

layer's thickness. Another measurement error component is encountered when an uneven