SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

 

 

PROGRAM

01:

P4

Excite,Delay,Volt(SE)

01:

5

Reps

02:25 2500 mV 60 Hz rejection Range

03:

1

IN Chan

04:1 Excite all reps w/EXchan 1

05:

10

Delay (units .01sec)

06:

2000

mV Excitation

07:1 Loc [:TEMP C #1]

08:

 

.001 Mult

09:

0

Offset

02:

P55

Polynomial

01:

5

Reps

02:1 X Loc TEMP C #1

03:1 F(X) Loc [:TEMP C #1]

04:

-53.784

C0

05:

147.97

C1

06:

-218.76

C2

07:

219.05

C3

08:

-111.34

C4

09:

23.365

C5

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.

7.16WATER 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

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.

FIGURE 7.16-1. A Vibrating Wire Sensor

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