Campbell CRVW3, Инструкция по эксплуатации

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CRVW3 3-Channel Vibrating-Wire Datalogger
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5.1 Measurement Theory
The CRVW3 uses Campbell Scientific VSPECT™ technology (U.S. Patent
No. 7,779,690) to measure standard, single-coil-circuit vibrating-wire sensors.
FIGURE 5-2. CRVW3 Cable Connection and Entry Points
A vibrating-wire sensor is designed by its manufacturer so that its output
corresponds to changes of the phenomenon in the environment being
measured. This is done by converting environmental changes into changes of
the tension on a suspended metal wire which is located inside the sensor. The
wire’s natural frequency changes as the tension changes, therefore a change in
frequency may be detected as changes in the measured phenomenon. A
magnetic coil circuit is placed near the suspended wire and senses the motion
of the wire by picking up the changes in the magnetic field caused by wire
vibrations.
To measure a vibrating-wire sensor, three main steps are taken by the
measurement system: 1) Electrical energy is inserted into the coil circuit,
creating a changing magnetic field that excites (plucks, or inserts energy into)
the wire. 2) The excitation terminates so that the wire is left to vibrate at its
characteristic resonant frequency (natural frequency). The coil circuit is used to
electrically capture the motion of the wire’s vibrations during this stage.
3) The measurement system processes the time-series waveform captured from
the coil to assess the frequency of the wire’s vibration. The basic output of a
vibrating-wire sensor is frequency in Hz. Conversion from frequency to
engineering units is possible based on a knowledge of how the sensor operates,
and that is provided by the sensor manufacturer.
FIGURE 5-3. Internal View of a Typical Vibrating-Wire Piezometer
Sensor