TDS QUALITY ASSURANCE TESTS

displacement, hopper volume, and location coordinates provided by a Global Positioning System.

These data are stored for later analysis.

Hopper Volume and Draft Determination. Volume of material in the hopper is calculated by

averaging the forward and aft ullage measurements and applying this value to an equation fitted to

the ullage table values. A time-series plot of the hopper volume for the McFarland is shown in

Figure 9. The hopper volume variability during loading on this plot is explained by the fact that

the aft hopper level sensor reading is affected by inflow from the hopper distribution system.

Sometimes when dredging in noncohesive material, the bin water will not completely cover the

sediment load due to mounding above the water plane. Although the ullage sensors work as

intended, the level they report may not be indicative of the average surface level of the load,

introducing error into the volume calculation. This type of error is being investigated on the

McFarland, which often works in noncohesive material.

Weight of the hopper material is

determined by subtracting the un-

loaded vessel weight (including

the weight of residual water in the

hopper) from the loaded vessel

weight. These vessel weights, or

vessel displacement tonnages

(weight of the water volume dis-

placed by the hull), are calculated

by applying draft measurements to

the vessel curves of form. These

curves equate vessel draft to dis-

placement tonnage (time-series

Figure 9.

plot shown in Figure 9).

TDS QUALITY ASSURANCE TESTS: Quality assurance tests are tests that are conducted to

verify the accuracy and/or consistency of sensor or algorithm outputs.

Ullage Measurements. To date, ultrasonic and radar ullage measurements have compared well

with manual tape measurements when the surface of the hopper material consists of just slurry or

water. However, when foam has been encountered on the hopper material surface, the acoustic or

radar pulse is reflected off of the bubble interface instead of the dredged material surface. The

amount of error introduced into the ullage measurement depends on the thickness of the foam.

Water Tests. One of the TDS quality assurance tests being conducted on the McFarland and

Columbus consists of filling the hopper with water (water test) to use the TDS instrumentation to

calculate the average specific gravity of the water therein and compare it to an average value

determined from samples that are taken from the hopper and analyzed. Water samples retrieved

from the hopper at various locations and depths are being measured with a temperature-compensated

hand refractometer capable of measuring specific gravity to an accuracy of 0.001. The sensor-

measured hopper material specific gravity is calculated by dividing the hopper volume by its weight,