Figure 40. Measurement of wave, current, and sediment concentration data
The sensor portion of the capacitance wave gauge consists of a thin insulated
wire held taut by a supporting rod. The rod is constructed of stainless steel with
a minimum cross section to reduce flow disturbance. The wire insulation serves
as a capacitor between the inside conducting wire and the water, which serves as
the "ground". The capacitance varies linearly as the water surface elevation
changes, provided the wire insulation thickness is uniform and not damaged,
(Markle and Greer 1992). Advantages of the single-wire capacitance wave gauge
are: (a) good linearity and dynamic response over a reasonable length that can be
used for fairly large waves; and (b) the gauge is stable over sufficiently long time
periods so that "gauge drift" is not a significant problem (Hughes 1993).
Calibration of linear actuators
Each wave gauge is mounted to a linear actuator that is used to raise and
lower the gauge. The linear actuators can been seen in Figure B-4, Appendix B,
and consist of an electric motor coupled to a rack-and-pinion mechanism which
accurately raises and lowers the gauge. This allows for simultaneous remote
static calibration of all wave gauges. Each linear actuator includes a high
precision potentiometer that provides feedback for the position of the linear
actuator. Calibrating these potentiometers is a semiautomated procedure and is
partially accomplished using the MTS control software. The calibration
procedure is carried out by manually positioning the linear actuator at
approximately 10 elevations evenly spaced along the length of the actuator. At
each elevation, the MTS control software automatically records the output
voltage from the potentiometer. The elevation of the actuator is simultaneously
measured using a high precision measuring device. This elevation is manually
entered into the MTS control software, and this procedure is repeated for each
elevation along the length of the linear actuator. After all positions have been
73
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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