Critical aspects of using vibrating wire piezometers in geotechnical monitoring of dams: how to meet basic operating principles of the devices and avoid common errors that compromise monitoring
the conversion equation of the instrument. For example, RST Instruments dataloggers are pre-set with the default equation for RST instruments, as indicated in their manual.
• Adjustment of initial reading: During installation, the piezometer must be positioned in an environment where only atmospheric pressure acts on the sensor to set the initial reference reading.
This reference point is essential for subsequent measurements.
• Noise verification: Real-time signal analysis can help identify noise sources near the installation point. High-voltage equipment or electromagnetic sources can introduce detectable interference in both
time-domain and frequency-domain graphs[20,21] .
4.5. Verification of sensor performance during operation Once the sensor is installed and operational, periodic verifications are necessary to ensure reliable readings:
• Signal-to-noise ratio (SNR) monitoring: The SNR measures the ratio between the signal amplitude from the sensor and the highest detected noise amplitude. Values above 10 indicate a good-
quality signal, while values close to 1 suggest significant noise interference, which can compromise the readings[20,21]
.
• Signal decay analysis: The time required for the vibration signal to return to its resting state is an indicator of the sensor’s mechanical integrity. Rapid decays may indicate structural issues with the
vibrating wire, while prolonged vibrations are common in sensors installed at greater depths or in clayey soils[20,21]
.
• Evaluation of multi-frequency noise: In environments with high levels of interference, such as dam galleries or areas near high-voltage power lines, noise may appear across different frequency bands.
VSPECT technology can distinguish the sensor’s resonant frequency and filter characteristic noise, such as 50 Hz or 60 Hz signals from the power grid[20,21]
.
• Electrical resistance testing: The resistance of the sensor’s coils and thermistor should be regularly measured and compared to the reference values in the calibration certificate. Readings around
180 ± 10 for the coil and approximately 3,000 for the thermistor indicate that the sensor is functioning within the recommended standards[4]
. Conducting these field tests helps detect and correct potential anomalies, such as signal loss or
increased noise due to external interference. Additionally, regular analysis ensures measurement accuracy and extends the sensor’s service life, enhancing the overall reliability of the geotechnical monitoring system.
4.6. Preventive maintenance and corrective actions In addition to the previously mentioned verifications, preventive maintenance is crucial to prevent failures:
• Replacement of cables and connectors: Worn or oxidized components should be replaced to avoid the amplification of external noise. The use of shielded twisted-pair cables and protected connectors is
recommended to minimize interference[23] . Vol XXXIII Issue 3 | Dam Engineering | 119
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