8.2. MEASUREMENT TOOLS AND TECHNIQUES
477
capacitors with the anticipated range of sea surface elevations. During
routine operation the wire should be cleaned occasionally to remove any
accumulated film which may change the capacitance of the wire or cause
the water to adhere to the insulation.
Wave Gauge Calibration
Resistance or capacitance wave gauges feature a linear (or nearly linear)
relationship between the sensor output and the elevation of the water level
on the gauge. This relationship can be determined by “calibrating” the
gauge either statically or dynamically. In static calibration the wave gauge
is vertically raised and lowered in known incremental distances relative to
the still water level, and the gauge output at each location is recorded. The
calibration relationship is obtained as a mathematical curve-fit between the
recorded gauge outputs and the corresponding elevations in length units.
Ideally, the relationship is linear and a least-squares linear regression can
be applied to obtain the necessary conversion equation. Sometimes it is
prudent to use a higher-order curve-fit to increase the precision of the wave
gauge calibration.
The wave gauge pictured in Figure 8.1 is attached to a stepper motor
that allows automatic, remote calibration of the gauge. The gauge is placed
in the water and then raised and lowered preset distances under computer
control, and the gauge response at each location is recorded automatically.
Of course, the calibration procedure could also be performed manually.
During static calibration it is very important to not disturb the water
because even the slightest water level fluctuation will impact the quality
of the gauge calibration. It is fairly standard practice to maintain a log of
individual wave gauge calibrations. Such a record can alert the instrument
specialist in the event gauge output quality starts to decline (Mansard and
Funke 1988). The static calibration obtained for resistance and capacitance
wave gauges is considered sufficient for most laboratory wave conditions.
Wave gauges can be dynamically calibrated by mounting them on a
platform, such as a rotating disk, that moves the gauge up and down in the
water column according to a known motion. The mathematical relationship
is determined in the same manner as for static calibration. The advantage of
dynamic calibration comes with the assurance that the gauge will accurately
sense and record the fluctuating sea surface elevations within a level of
accuracy that can be estimated from the calibration. In other words, the
gauge demonstrates good dynamic response to rapid changes in water eve
Dynamic calibration is more commonly associated with field instruments
that measure the acceleration of a floating body, then integrate twice o
displacement.
477
capacitors with the anticipated range of sea surface elevations. During
routine operation the wire should be cleaned occasionally to remove any
accumulated film which may change the capacitance of the wire or cause
the water to adhere to the insulation.
Wave Gauge Calibration
Resistance or capacitance wave gauges feature a linear (or nearly linear)
relationship between the sensor output and the elevation of the water level
on the gauge. This relationship can be determined by “calibrating” the
gauge either statically or dynamically. In static calibration the wave gauge
is vertically raised and lowered in known incremental distances relative to
the still water level, and the gauge output at each location is recorded. The
calibration relationship is obtained as a mathematical curve-fit between the
recorded gauge outputs and the corresponding elevations in length units.
Ideally, the relationship is linear and a least-squares linear regression can
be applied to obtain the necessary conversion equation. Sometimes it is
prudent to use a higher-order curve-fit to increase the precision of the wave
gauge calibration.
The wave gauge pictured in Figure 8.1 is attached to a stepper motor
that allows automatic, remote calibration of the gauge. The gauge is placed
in the water and then raised and lowered preset distances under computer
control, and the gauge response at each location is recorded automatically.
Of course, the calibration procedure could also be performed manually.
During static calibration it is very important to not disturb the water
because even the slightest water level fluctuation will impact the quality
of the gauge calibration. It is fairly standard practice to maintain a log of
individual wave gauge calibrations. Such a record can alert the instrument
specialist in the event gauge output quality starts to decline (Mansard and
Funke 1988). The static calibration obtained for resistance and capacitance
wave gauges is considered sufficient for most laboratory wave conditions.
Wave gauges can be dynamically calibrated by mounting them on a
platform, such as a rotating disk, that moves the gauge up and down in the
water column according to a known motion. The mathematical relationship
is determined in the same manner as for static calibration. The advantage of
dynamic calibration comes with the assurance that the gauge will accurately
sense and record the fluctuating sea surface elevations within a level of
accuracy that can be estimated from the calibration. In other words, the
gauge demonstrates good dynamic response to rapid changes in water eve
Dynamic calibration is more commonly associated with field instruments
that measure the acceleration of a floating body, then integrate twice o
displacement.