Calibration of ADVs
and receiver as well as the speed of sound in water. Unlike capacitance wave
gauges, ADVs do not require calibration on a regular basis. The manufacturer's
calibrations give reliable results, provided that the acoustic transmitter and
receivers are not damaged. The only parameter that influences the calibration of
the ADVs is the speed of sound in water, which should be compensated for on a
daily basis. Therefore, the water temperature in the facility is monitored daily
and the temperature in degrees Celsius is entered into the ADV software prior to
each experiment.
The calibrations can be verified by towing the ADVs through the water while
driving the bridge along the rails at a constant speed. However, because the
facility holds a large volume of water (1,350 m3, for a water depth of 0.9 m), it is
important to allow the water several hours to become completely quiescent
before conducting these tests.
Custom designed 1.8-m-tall vertical struts and mounting devices were
designed to make it easy to manually raise and lower the ADV's and to minimize
the possibility of damage. Each vertical strut has an adjustable rubber bumper
that can be positioned at the appropriate elevation so that if the ADV is dropped
while being manually raised or lowered, the instrument would stop at the
elevation of the bumper, rather than driving the ADV sensor into the beach.
Considerable care is shown when moving the ADVs to ensure that the orientation
of the sensors is maintained and that the acoustic transmitter and receivers are not
damaged.
Calibration of analog signals
As mentioned previously in Chapter 6, the serial communications port does
not have sufficient capacity to simultaneously transmit digital output signals from
10 ADVs at a frequency of 20 Hz. Therefore, the digital data obtained from the
ADVs is used only for real-time display of single ADV channels. Each ADV
was upgraded with additional hardware to generate analog output voltages, which
are transmitted through ten 130-m-long cables, to the MTS control and data
acquisition system. The analog output voltage from the ADVs ranges from 0 to
5 V and is equivalent to a velocity range of +100 cm/sec. In contrast, the voltage
range of the analog-to-digital converters in the MTS control and data acquisition
system is +10 V. Because the system is linear, +10 V is equivalent to 300 cm/sec
and -10 V is equivalent to -500 cm/sec, as illustrated in Figure 43.
78
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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