Wellerson Bastos, Marcos Pimenta Filho, Luiza Almeida, Marina Ferreira & Mauro Santos Junior 3. Correlation Equations


3.1. Frequency and temperature conversion to pressure The conversion of frequency and temperature into pressure in vibrating wire piezometers is an essential process to ensure accurate pore pressure readings. The equations used for this calculation vary between manufacturers and equipment models, but all are based on frequency readings and calibration factors that convert the data into engineering units. DGSI VW[16]


cross terms, considering frequency (Hz2 ) and the interaction between them (Hz × T). This approach provides greater accuracy in scenarios with significant thermal variations: (3.1) where:


• C0 to C5: calibration coefficients provided in the sensor’s calibration certificate; • Hz: frequency reading in hertz squared; and • T: temperature in degrees Celsius.


This approach requires that the coefficients are correctly entered into the dataloggers and readouts, as default configurations using linear equations can lead to measurement errors. On the other hand, piezometers such as those from Geosense[7]


, GEOKON 4500[4] RST VW2100[17] , Encardio-Rite[5] and use more common linear equations because they are practical and provide acceptable


error margins. It is important to note that Equation 3.2 is presented according to the terms used by GEOKON but is also valid for other instruments, with only the variable names differing.


GEOKON, Encardio-Rite and Geosense (Equation 3.2): (3.2) where:


• G: linear calibration factor (e.g. kPa/digit); • R1: subsequent frequency readings taken in the field (in digits or Hz2 • R0: zero reading taken during installation (in digits or Hz2


/1000);


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


piezometers use a polynomial equation (Equation 3.1) that incorporates linear, quadratic and ), temperature (T2


/1000);


112 | Dam Engineering | Vol XXXIII Issue 3


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