Campbell AVW1, Инструкция по эксплуатации

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3-1
SECTION 3. VIBRATING WIRE MEASUREMENT
FIGURE 3.1-1. A Vibrating Wire Sensor
3.1 GENERAL
Figure 3.1-1 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 string on a guitar decreases it's
frequency. Therefore, the resonant frequency
of the vibrating wire sensor decreases with
increasing pressure.
The Vibrating Wire Measurement Instruction
excites the "plucking" and "pickup" coils shown
in the above figure 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. The CR10 requires 150 ms to
"sweep" through all of the frequencies. 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 and as it does so it will cut the lines of flux
in the "plucking" and "pickup" coils inducing the
same frequency on the lines to the CR10. After
waiting for the non resonant frequencies to die
out (20 ms) the CR10 accurately measures how
much time it takes to receive a user specified
number of cycles. Knowing the length of time
and the number of cycles, the CR10 then
computes the square of the frequency
expressed in units of kHz
2
(= 1/T
2
where T is the
period in milliseconds).
3.2 MULTIPLIER & OFFSET
The 50 psi vibrating wire pressure sensor used
in this example is the Geokon Model 4500SV-
50. It is vented to the atmosphere so it will not
be affected by changes in barometric pressure.
Each sensor is individually calibrated and has a
unique set of calibration coefficients. A
calibration sheet contains the Model Number,
Serial Number, Gage Factor (psi/digit),
Temperature Coefficient (psi/
o
C), Zero Reading
(digits), zero Period (microseconds), calibration
Temperature (
o
C) and calibration Barometric
Pressure (inches Hg). A Geokon "digit" is
defined as the square of the frequency in kHz
multiplied by 1000 (1 digit = 0.001 kHz
2
= 1000
Hz
2)
. The calibration coefficients for the sensor
with serial number 3998 are given below: