8,2. MEASUREMENT TOOLS AND TECHNIQUES
491
cameras can capture wave breaking and air entrainment, and time-lapse
photography can document some features of sediment transport.
8.2.5
Pressure Measurement
Static and dynamic pressure measurements may be required in some laboratory experiments. For example, we might be interested in pressure forces
induced by waves breaking on a vertical wall or we might want to study pore
pressures within a rubble-mound breakwater or beneath a sandy foreshore.
Static pressure (or total pressure under steady flow conditions) is easily measured using simple instruments, such as U-tube manometers, that
give the pressure directly as vertical “head” of whatever fluid is used in
the manometer. These instruments are discussed in Hydraulic Laboratory
Techniques (1980).
Measurement of dynamic pressures is accomplished using commercially
available pressure transducers connected to appropriate signal conditioning
units. Dynamic pressure gauges must provide a time history of pressure;
and in the case of wave impacts on a structure, the instrument may need
to sample at rates up to 1 MHz with little distortion in order to capture
the shape of the pressure impulse. Of course, pressures that vary at slower
rates do not require the high frequency response of the sensors intended to
measure impact pressures.
When selecting pressure transducers for a particular laboratory application, it is important to have a “feel” for the possible range of pressures
that the instrument is expected to measure. This will assure selection of
an instrument having sufficient amplitude range and frequency response.
Other selection criteria include size, cost, durability, and reliability.
If a pressure gauge needs to be checked for accuracy, a simple static test
can be conducted by submerging the gauge to a known water depth and
comparing the gauge output with the calculated static pressure. Checking
dynamic response is more difficult and requires some type of “dynamic
pressure calibrator” (Burgi 1987).
8.2.6
Stress and Force Measurement
Waves and currents exert forces on solid bodies such as moored ships, floating breakwaters, oil platforms, piers, concrete armor units, etc. In order
to study these effects in the laboratory, instruments are required that can
accurately measure stresses and forces at the reduced scales of the model.
Laboratory measurement of stresses in structural members is usually
accomplished using surface mounted strain gauges. The use of strain gauges
in coastal engineering laboratory studies is fairly routine, with the major
491
cameras can capture wave breaking and air entrainment, and time-lapse
photography can document some features of sediment transport.
8.2.5
Pressure Measurement
Static and dynamic pressure measurements may be required in some laboratory experiments. For example, we might be interested in pressure forces
induced by waves breaking on a vertical wall or we might want to study pore
pressures within a rubble-mound breakwater or beneath a sandy foreshore.
Static pressure (or total pressure under steady flow conditions) is easily measured using simple instruments, such as U-tube manometers, that
give the pressure directly as vertical “head” of whatever fluid is used in
the manometer. These instruments are discussed in Hydraulic Laboratory
Techniques (1980).
Measurement of dynamic pressures is accomplished using commercially
available pressure transducers connected to appropriate signal conditioning
units. Dynamic pressure gauges must provide a time history of pressure;
and in the case of wave impacts on a structure, the instrument may need
to sample at rates up to 1 MHz with little distortion in order to capture
the shape of the pressure impulse. Of course, pressures that vary at slower
rates do not require the high frequency response of the sensors intended to
measure impact pressures.
When selecting pressure transducers for a particular laboratory application, it is important to have a “feel” for the possible range of pressures
that the instrument is expected to measure. This will assure selection of
an instrument having sufficient amplitude range and frequency response.
Other selection criteria include size, cost, durability, and reliability.
If a pressure gauge needs to be checked for accuracy, a simple static test
can be conducted by submerging the gauge to a known water depth and
comparing the gauge output with the calculated static pressure. Checking
dynamic response is more difficult and requires some type of “dynamic
pressure calibrator” (Burgi 1987).
8.2.6
Stress and Force Measurement
Waves and currents exert forces on solid bodies such as moored ships, floating breakwaters, oil platforms, piers, concrete armor units, etc. In order
to study these effects in the laboratory, instruments are required that can
accurately measure stresses and forces at the reduced scales of the model.
Laboratory measurement of stresses in structural members is usually
accomplished using surface mounted strain gauges. The use of strain gauges
in coastal engineering laboratory studies is fairly routine, with the major