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For example, assume an initial reading of R
= 9101, a present reading of, R
= 7390 and a
0
1
Calibration Factor of -0.1192 kPa/digit. The calculated pressure is:
204 kPa = (7390 -9101) × -0.1192
(Appendix E shows how a second order polynomial can be used to improve accuracy.)
4.2 Temperature Correction
The vibrating wire 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 (see Figure 18) along with an equation for its proper use. See Equation 4.
Temperature Correction = (Current Temperature - Initial Temperature) × Thermal Factor
Or
P corrected = (R
- R
)G + (T
-T
)K
0
1
1
0
Equation 4 - Temperature Correction for the Transducer Only.
The Temperature Correction value is then added to the pressure calculated using Equation 3. For
example, assume an initial temperature of 25° C, a temperature at the time of measurement of
12° C and a Thermal Factor of +0.03852 kPa/° C. The thermally corrected pressure is:
203.5 kPa = 204 + (12 - 25) x 0.03852
Note that this correction for temperature applies only to the pressure transducer, 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. The effect is usually small at depths where the temperature is fairly
constant, but where temperatures do vary the effect can be quite large. For more information see
Appendix D which gives a theoretical treatment.
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.
4.3 Barometric Correction
The pressure transducer used in Geokon Vibrating Wire Earth Pressure Cells is evacuated and
hermetically sealed and will respond to barometric pressure fluctuation. If a correction for these
fluctuations is required then it is necessary to record the initial barometric pressure (S
) and the
0
barometric pressure at the time of each reading (S
) and subtract the change (S
- S
) from the
1
1
0
calculated pressure reading.
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