8.1. LABORATORY MEASUREMENTS
465
will be needed to learn how to make reliable and accurate measurements
using the instruments.
The task of instrument selection is made easier in situations where there
is a good understanding of the physical characteristics of the process being
studied (Burgi 1987). This understanding helps to establish criteria that
the measuring instrument must meet to assure quality measurements. For
example, impact pressures of a wave breaking against a vertical wall occur
over a very short time period, hence, the instrument recording the impact
pressure must be able to respond accordingly without undue distortion of
the pressure time history.
Laboratory studies involve measurement of static and dynamic quantities. Static quantities, such as length measurements, or slowly changing
quantities, such as steady flow speeds, are relatively easy to measure; and
reliable results can be achieved using simple instruments and measuring
techniques. Measurements of dynamic quantities, such as pressure or force,
and kinematic quantities, such as unsteady flows or turbulence, are usually
obtained using specialized electronic equipment interfaced to a computer.
Selection Factors
Some (but not all) of the factors to be considered when selecting instruments for coastal engineering related laboratory experiments are enumerated below. Many of these items were discussed in Hydraulic Laboratory
Techniques (1980).
1. The type and range of the variables to be measured must
be reasonably well known so that an instrument can be
selected that has good response over the projected measurement range.
2. We must have a good feel for the time rate of change expected in the variables to be measured. This determines
the necessary instrument frequency response.
3. We need to know the approximate quantity of data to be
recorded (time span or number of data points). The selected instrument and recording system must be able to
handle the projected data stream without difficulty, and it
must have ample storage capacity.
4. The minimum required spatial coverage of the variables
of interest will determine the minimum number of instruments needed for the experiment and the necessary data
channels that must be recorded (assuming point measurements).
465
will be needed to learn how to make reliable and accurate measurements
using the instruments.
The task of instrument selection is made easier in situations where there
is a good understanding of the physical characteristics of the process being
studied (Burgi 1987). This understanding helps to establish criteria that
the measuring instrument must meet to assure quality measurements. For
example, impact pressures of a wave breaking against a vertical wall occur
over a very short time period, hence, the instrument recording the impact
pressure must be able to respond accordingly without undue distortion of
the pressure time history.
Laboratory studies involve measurement of static and dynamic quantities. Static quantities, such as length measurements, or slowly changing
quantities, such as steady flow speeds, are relatively easy to measure; and
reliable results can be achieved using simple instruments and measuring
techniques. Measurements of dynamic quantities, such as pressure or force,
and kinematic quantities, such as unsteady flows or turbulence, are usually
obtained using specialized electronic equipment interfaced to a computer.
Selection Factors
Some (but not all) of the factors to be considered when selecting instruments for coastal engineering related laboratory experiments are enumerated below. Many of these items were discussed in Hydraulic Laboratory
Techniques (1980).
1. The type and range of the variables to be measured must
be reasonably well known so that an instrument can be
selected that has good response over the projected measurement range.
2. We must have a good feel for the time rate of change expected in the variables to be measured. This determines
the necessary instrument frequency response.
3. We need to know the approximate quantity of data to be
recorded (time span or number of data points). The selected instrument and recording system must be able to
handle the projected data stream without difficulty, and it
must have ample storage capacity.
4. The minimum required spatial coverage of the variables
of interest will determine the minimum number of instruments needed for the experiment and the necessary data
channels that must be recorded (assuming point measurements).