8.2. MEASUREMENT TOOLS AND TECHNIQUES
475
and the conductivity of the water. The changes in sea surface elevations
are recorded as changes in conductance .
The main advantage of the twin-wire resistance wave gauge is that the
gauge exhibits good linear response and can achieve a resolution of about
± 0.1 mm (Sharp 1981). It also has low construction costs and long service
life (Hudson, et al. 1979). The main disadvantage of the instrument is
that frequent calibrations are necessary because the conductivity of water
changes with temperature and concentration of dissolved salts2. Other
disadvantages are the small range of linearity, the high cost of auxiliary
equipment, and the distortion in wave shape due to the finite distance
separating the wires.
2 Sharp (1981) warned that salts will dissolve from newly constructed concret
for several days.
. . ..
3 The capacitance wave gauge circuit shown in Markle and Greer
w*5 °n&in
designed and implemented by Gary Howell in 1977 at the University of Ion a (P
communication).
Capacitance Wave Gauge
The sensor portion of the capacitance wave gauge consists of a thin insulated
wire held taut by a supporting rod. The rod is usually 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 sea surface elevation changes, provided the wire insulation
thickness is uniform and free from holes. Electronic circuits are used to
determine variations in sensor capacitance, which in turn are related to sea
surface elevations after frequency-to-voltage conversion and application of
appropriate calibration constants.
Figure 8.1 from Markle and Greer (1992) shows the type of capacitance wave gauge used at the Waterways Experiment Station. The gauge
is mounted on a stepper motor that is used to raise and lower the gauge during static calibration. Markle and Greer (1992) also included a schematic
drawing of this capacitance wave gauge circuitry3 * along with additional
details about the gauge operation.
The two main advantages of the single-wire capacitance wave gauge are
(1) the gauge exhibits good linearity and dynamic response over a reasonable length so that it can be used for fairly large waves, and (2) the gauge
is stable over sufficiently long times (gauge “drift” is not a significant prob
lem). Hudson, et al. (1979) listed other advantages as: minor obstruction
to the wave front, no distortion of the wave shape, and low construction
costs. The main disadvantages of the capacitance wave gauge include the
cost of the auxiliary equipment and the requirement of matching circuit
475
and the conductivity of the water. The changes in sea surface elevations
are recorded as changes in conductance .
The main advantage of the twin-wire resistance wave gauge is that the
gauge exhibits good linear response and can achieve a resolution of about
± 0.1 mm (Sharp 1981). It also has low construction costs and long service
life (Hudson, et al. 1979). The main disadvantage of the instrument is
that frequent calibrations are necessary because the conductivity of water
changes with temperature and concentration of dissolved salts2. Other
disadvantages are the small range of linearity, the high cost of auxiliary
equipment, and the distortion in wave shape due to the finite distance
separating the wires.
2 Sharp (1981) warned that salts will dissolve from newly constructed concret
for several days.
. . ..
3 The capacitance wave gauge circuit shown in Markle and Greer
w*5 °n&in
designed and implemented by Gary Howell in 1977 at the University of Ion a (P
communication).
Capacitance Wave Gauge
The sensor portion of the capacitance wave gauge consists of a thin insulated
wire held taut by a supporting rod. The rod is usually 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 sea surface elevation changes, provided the wire insulation
thickness is uniform and free from holes. Electronic circuits are used to
determine variations in sensor capacitance, which in turn are related to sea
surface elevations after frequency-to-voltage conversion and application of
appropriate calibration constants.
Figure 8.1 from Markle and Greer (1992) shows the type of capacitance wave gauge used at the Waterways Experiment Station. The gauge
is mounted on a stepper motor that is used to raise and lower the gauge during static calibration. Markle and Greer (1992) also included a schematic
drawing of this capacitance wave gauge circuitry3 * along with additional
details about the gauge operation.
The two main advantages of the single-wire capacitance wave gauge are
(1) the gauge exhibits good linearity and dynamic response over a reasonable length so that it can be used for fairly large waves, and (2) the gauge
is stable over sufficiently long times (gauge “drift” is not a significant prob
lem). Hudson, et al. (1979) listed other advantages as: minor obstruction
to the wave front, no distortion of the wave shape, and low construction
costs. The main disadvantages of the capacitance wave gauge include the
cost of the auxiliary equipment and the requirement of matching circuit