Campbell Scientific CR6 CR6 Measurement and Control System - Page 348
Vspect Measurements
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Section 8. Operation vibrating-wire sensor that can be connected directly to CR6. More sensors can be measured by using multiplexers (see Analog Multiplexers (p. 596) ). The figure Vibrating-Wire Sensor (p. 348) illustrates basic construction of a sensor. To make a measurement, plucking and pickup coils are excited with a swept frequency (p. 510). The ideal behavior then is that all non-resonant frequencies quickly decay, and the resonant frequency continues. As the resonant frequency cuts the lines of flux in the pickup coil, the same frequency is induced on the cable connecting the sensor to the datalogger or interface. Measuring the resonant frequency by means of period averaging is the classic technique, but Campbell Scientific has developed static and dynamic spectralanalysis techniques (Vspect (p. 512)tm) that produce superior noise rejection, higher resolution, diagnostic data, and, in the case of dynamic Vspect, measurements up to 333.3 Hz. A resistive-thermometer device (RTD), which is included in most vibrating-wire sensor housings, can be measured to compensate for temperature errors in the measurement. Figure 94. Vibrating-Wire Sensor 8.1.5.1 Vspect Measurements Campbell Scientific has improved on vibrating-wire measurements with Vspecttm measurements. These measurements achieve two to three orders of magnitude improved noise immunity over time-domain period-averaging techniques. Additionally, spectrum analysis gives improved frequency resolution of up to 0.001 Hz RMS during quiet conditions. The Vspect technique measures resonant frequency with the following procedure: 1. Excite the wire with a swept frequency 2. Record the response 3. Perform an FFT (p. 495) on the response and analyze the results to determine resonant frequency. Vspect also provides diagnostic information indicating the quality of the resonant-frequency measurement. The condition of the vibrating-wire sensor can be inferred from these diagnostics. Two classes of Vspect measurements are made: • Static • Dynamic Vspect measurements are made with the CR6 with the following combinations: • Directly on U terminals configured for vibrating-wire measurements. Use 348
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