Foundation, Concrete and Earthquake Engineering

Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-21

3.2 Correction for the readings:

Temperature Correction

Earth pressure cell is quite sensitive to temperature fluctuations but often temperature changes in the ground are minor and can be ignored. Corrections for temperature effects on the transducer alone can be made using the Thermal Factor (K) supplied on the calibration sheet (Figure 3.1) along with an equation (Equation 3.3) for its proper use.

Temperature Correction = (Current Temperature - Initial Temperature) × Thermal Factor
Pcorrection = (T1-T0) × K

Equation 3.3 - Temperature Correction for the Transducer Only.
The Temperature Correction value is then added to the pressure calculated using Equation 2. For example, an initial temperature of 25° C, a temperature at the time of measurement of 12° C and a Thermal Factor of -0.02814 psi/° C. The thermally corrected pressure is;

119.9 psi = 119.5 + ((12 - 25) × -0.02814)
Hence, the thermally corrected pressure calculation is as follows;

Pcorrected = ((R0 - R1) × C) + ((T1-T0) × K)

Equation 3.4 - Thermally Correct Pressure Calculation

This correction for temperature applies only to the pressure transducer itself and not to the entire cell surrounded by soil or soil and concrete each with its own (different) temperature coefficient of expansion. Commercially it is not practical to measure this effect without incurring huge expenses. Suffice to say that the effect is usually small at depths where the temperature is fairly constant. But where temperatures do vary the effect can be quite large. In practice the best way to compensate for temperatures is to derive a factor from simultaneous measurements of pressure and temperature at times when the temperature is changing and when it can be safely assumed that the applied load is not changing.

Barometric Correction

The pressure transducer used in Earth Pressure Cells is evacuated and hermetically sealed and will respond to barometric pressure fluctuation. However, since the magnitudes are on the order of ±0.5 psi, correction is generally not required. If a correction for these fluctuations is required then it is necessary to record the barometric pressure at the time of each reading.

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