Water Level - Geokon'svibrating Wire Pressure Sensor; A Vibrating Wire Sensor - Campbell Measurement and Control Module CR10 Operator's Manual

PROGRAM
01: P4
01: 5
02: 25
03: 1
04: 1
05: 10
06: 2000
07: 1
08: .001
09: 0
02: P55
01: 5
02: 1
03: 1
04: -53.784
05: 147.97
06: -218.76
07: 219.05
08: -111.34
09: 23.365
7.16 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 7.16-1 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
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
Excite,Delay,Volt(SE)
Reps
2500 mV 60 Hz rejection
Range
IN Chan
Excite all reps w/EXchan 1
Delay (units .01sec)
mV Excitation
Loc [:TEMP C #1]
Mult
Offset
Polynomial
Reps
X Loc TEMP C #1
F(X) Loc [:TEMP C #1]
C0
C1
C2
C3
C4
C5
FIGURE 7.16-1. A Vibrating Wire Sensor
in Figure 7.16-1 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 CR10. 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 is required between the
sensor to the datalogger. The purpose is twofold:
•
12 volts can be used as the potential in the
swept frequency excitation, thus plucking
the wire harder than the maximum 2.5 volt
switched excitation. The result is a larger
magnitude signal for a longer time.
•
A transformer strips off any DC noise on
the signal, improving the ability to detect
cycles.
7-15