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4. DATA REDUCTION
4.1 Pressure Calculation
The basic units utilized by Geokon, and displayed on the readout box Channel B, for
measurement and reduction of data from a Pile Tip Pressure Cell are "digits". Calculation of
digits is based on the following equation:
Digits =
�
1
Period
�
2
x 10
-3
or Digits
=
Hz
2
1000
Equation 1 - Digits Calculation
To convert digits to pressure the following equation applies;
Pressure
=
(Initial Reading - Current Reading)
×
Calibration Factor
Or
P = (R
0
- R
1
)
×
C
Equation 2 - Convert Digits to Pressure
The Initial Reading is obtained as described in Section 2.1. The Calibration Factor (usually in
terms of PSI or MPa per digit) comes from the supplied Calibration Sheet.
4.2 Temperature Correction
The vibrating wire sensor is relatively insensitive to temperature fluctuations and usually the
effect of temperature is insignificant and can be ignored. But, if desired, correction for
temperature effects on the sensor can be made using the factors supplied on the calibration sheet
and Equation 3. Also, there are spurious temperature effects caused by the mismatch between
temperature coefficients of the cell and surrounding concrete. This effect is not quantifiable in
the laboratory and, hence, no correction factor for this effect can be supplied.
Temperature Correction
=
(Current Temperature - Initial Temperature)
×
Thermal Factor
Or
P
T
= (T
1
-T
0
) x K
Equation 3 - Temperature Correction
4.3 Barometric Correction
Barometric pressure fluctuations will be sensed by the cells. However, the magnitudes (±0.5 psi)
are usually insignificant.