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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.
The temperature correction equation is as follows:
Temperature Correction = (Current Temperature – Initial Zero Temperature) × Thermal Factor
Or
P
= (R
– R
) G + (T
– T
) K
corrected
1
0
1
0
EQUATION 4: Temperature Correction
The calculated correction would then be added to the pressure calculated using
Equation 3. If the engineering units were converted, remember to apply the
same conversion to the calculated temperature correction.
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 rather 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
vibrating wire earth pressure cells is
GEOKON
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 barometric pressure at the
0
time of each reading (S
) and subtract the change (S
- S
) from the calculated
1
1
0
pressure reading.
18 | DATA REDUCTION | GEOKON
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