Campbell Scientific CR10 CR10 Measurement and Control - Page 99

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES PROGRAM 01: P4 01: 5 02: 25 Excite, Delay,Volt(SE) Reps 2500 mV 60 Hz rejection Range 03: 1 lN Chan 04: 1 Excite all reps dEXchan 1 05: 10 Delay (units .01sec) 06: 2000 mV Excitation 07: 08: 09: .001 1 Loc [:TEMP C #1] Mult 0 Offset o2: 01: 02: 03: o4: 05: 06: o7: 08: 09: P55 Polynomial 5 Reps 1 X Loc TEMP C #1 1 -s3.784 F(X) C0 Loc [:TEMP C #1] 147.97 C1 -218.76 C2 219.0s C3 -111.34 C4 23.365 C5 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 lnstruction 28 excites the "plucking" and "pickup" coils shown in Figure 7.16-1 with a "swept" frequency" A "swept" frequency is a group of ditferent 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. ldeally, all of the frequencies except the one matching the resonant frequency of the wire willdie out in a very short time. The wire willvibrate 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 f ift h order polynomial instruction. Campbell Scientific's AVWl or AVW4 Vibrating Wire Sensor Interface is required between the sensor to the datalogger. The purpose is twofold: o 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 signalfor a longer time. o A transformer strips otf any DC noise on the signal, improving the ability to detect cycles. VIBRATING WIRE STAINLESS STEEL HOUSING 4_CONDUCTOR CABLI DIAPHRAGM INTIRNAL BULKHEAD SEAL PLUCKING AND PICKUP COILS FILTER FIGURE 7.16-1. A Vibrating Wire Sensor 7-15