Campbell CR23X, Operator'S Manual

Page: 101 / 374

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-13
7.15 WATER LEVEL - GEOKON'S
VIBRATING WIRE PRESSURE SENSOR
The vibrating wire sensor utilizes a change in
the frequency of a vibrating wire to sense
pressure. Figure 7.15-1 illustrates how an
increase in pressure on the diaphragm
decreases the tension on the wire attached to
the diaphragm. A decrease in the wire tension
decreases the resonant frequency in the same
way that loosening a guitar string decreases its
frequency.
Vibrating Wire Measurement Instruction 28
excites the "plucking" and "pickup" coils shown
in Figure 7.15-1 with a "swept" frequency. A
"swept" frequency is a group of different
frequencies that are sent one right after another
starting with the lowest frequency and ending
with the highest. The lowest and highest
frequencies are entered by the user in units of
hundreds of Hz. This swept frequency causes
the wire to vibrate at each of the individual
frequencies. Ideally, all of the frequencies
except the one matching the resonant
frequency of the wire will die out in a very short
time. The wire will vibrate with the resonant
frequency for a relatively long period of time,
cutting the lines of flux in the "plucking" and
"pickup" coils and inducing the same frequency
on the lines to the CR23X. Instruction 28 then
accurately measures how much time it takes to
receive a user specified number of cycles.
The vibrating wire requires temperature
compensation. A nonlinear thermistor built into
the probe is measured using Instruction 4, a
single-ended half bridge measurement with
excitation, and calculated with Instruction 55, a
fifth order polynomial instruction.
Campbell Scientific's AVW1 or AVW4 Vibrating
Wire Sensor Interface provides transformer
isolation between the datalogger and vibrating
wire sensors.
The transformer strips off any DC noise on the
signal, improving the ability to detect cycles.
FIGURE 7.15-1. A Vibrating Wire Sensor