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
To ensure that the data accurately reflects field conditions, it is crucial to apply the appropriate corrections consistently. Temperature is one of the main factors affecting piezometer readings. The vibrating wire, made of steel,
expands or contracts with thermal variations, altering its vibration frequency. All analyzed manufacturers, including DGSI, GEOKON, Soil Instruments, Encardio-Rite, Roctest and RST Instruments, incorporate thermistors in their sensors to monitor internal temperature and allow real-time corrections. In addition to thermal correction, barometric pressure also impacts readings, especially in absolute
piezometers, which measure total pressure (the sum of pore pressure and atmospheric pressure). Barometric correction is essential for sensors installed in areas exposed to significant atmospheric variations, such as mountainous regions, dams and deep ventilated boreholes. To correct this effect, many dataloggers and readouts offer options to record barometric pressure using embedded or external sensors. In the case of absolute piezometers, such as those from GEOKON, RST Instruments and Encardio-Rite,
it is recommended that the reference reading be taken using a barometer located near the installation point. The difference between the atmospheric pressure recorded on-site and that recorded during factory calibration can be applied using a linear correction factor. The barometric correction equation can be represented as Equation 3.7:
(3.7) where:
• Pcorr: corrected pressure; • Pmeasured: uncorrected sensor reading; • Bsite: barometric pressure measured at the site; and • Bcalib: barometric pressure recorded during calibration.
Devices such as the Geosense VWR1 and the RST Instruments VW2106 feature configurable modes that
automatically perform barometric correction. However, dataloggers with default configurations, without the inclusion of Bsite and Bcalib, may produce discrepancies in readings, especially during abrupt atmospheric pressure changes due to weather fluctuations. In addition to thermal and barometric corrections, electromagnetic interference can impact readings. Modern equipment, such as the VWAAnalyzer with VSPECT technology, applies Fourier transforms to separate frequencies of interest from noise signals, providing accurate readings even in high-interference environments, such as dam galleries near power transmission lines. To ensure reliable measurements, the use of shielded cables and routing away from noise sources is recommended. The signal-to-noise ratio (SNR)[20]
should be monitored regularly, with ideal values above 10
and preferably above 50. In addition to correction factors and installation practices, periodic verification of electrical resistance and sensor continuity helps identify physical issues, such as damage to connectors and cables. The installation
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