Wellerson Bastos, Marcos Pimenta Filho, Luiza Almeida, Marina Ferreira & Mauro Santos Junior


where:


• CF: linear calibration factor (e.g. kPa/digit); • L: subsequent frequency readings taken in the field (in digits or Hz2 • L0: zero reading taken during installation (in digits or Hz2


/1000);


• Ct: thermal correction coefficient (e.g. °C/digit); • T1: current sensor temperature (°C); and • T0: initial reference temperature (°C).


The Roctest PW piezometer[15] uses a linear equation similar to most of the instruments discussed.


However, to convert frequency readings in Hz to a unit similar to “digit,” referred to as Linear Unit (L), the calculation is performed using Equation 3.6:


(3.6)


/1000);


where:


• L: Reading in linear units; • K: Piezometer constant (1.0156); and • F: Frequency (Hz).


Although these equations share similar structures, the arrangement of terms and the signs used vary. For


example, if a datalogger configured with the RST Instruments equation is used with a GEOKON or Encardio- Rite piezometer, the result will be incorrect even if the frequency readings are the same. This issue can be avoided by properly configuring the dataloggers and accurately entering the calibration parameters. Some readouts, such as the Encardio-Rite EDI-54V[18]


, RST Instruments VW2106[6]


and Geosense


VWR1[19], come with default settings that may not be compatible with all piezometers unless adjusted. Furthermore, devices that use VSPECT technology, such as CampbellSCI’s VW Analyzer[20] and Geokon’s GK- 406[21], apply Fourier transforms to identify resonant frequencies and eliminate noise. However, even with this


advanced technology, it is essential to correctly input the thermal coefficients and calibration factors. The use of the correct equations for converting frequency and temperature into pressure is crucial to avoid


errors in pore pressure readings. Calibration certificates provide the necessary conversion coefficients and must be followed rigorously. Additionally, the monitoring team must be aware of operating conditions, such as temperature and atmospheric pressure, to ensure that readings reflect real field conditions. Implementing a continuous verification program ensures the reliability of the geotechnical monitoring system.


3.2. Correction factors for environmental variations The readings of vibrating wire piezometers are susceptible to environmental factors such as temperature and barometric pressure, which can compromise the accuracy of pore pressure measurements.


114 | Dam Engineering | Vol XXXIII Issue 3


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