Geokon 4850, Instruction Manual

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12
4. DATA REDUCTION
4.1 Pressure Calculation
To convert digits to pressure the following equation applies;
Pressure = (Current Reading - Initial Reading) × Calibration Factor
Or
P = (R
1
– R
0
) × G
Equation 1 - Convert Digits to Pressure
The Initial Reading is normally obtained during installation (usually the zero reading). The
Calibration Factor (usually in terms of PSI or MPa per digit) comes from the supplied calibration
report. (See Appendix D for a sample report.)
4.2 Temperature Correction
The vibrating wire stress cell is quite sensitive to temperature fluctuations. The Calibration report
shows the temperature correction for the VW transducer only and usually this effect is
insignificant and can be ignored. But, if a correction is desired it can be made using the factors
supplied on the calibration report and Equation 2. However, there can be much larger
temperature effects caused by the mismatch between temperature coefficients of the cell and
surrounding concrete. This effect is not quantifiable in the laboratory, but a theoretical treatment
is given in appendix C.
Temperature Correction = (Current Temperature - Initial Temperature) × Thermal
Factor
Or
P
T
= + (T
1
- T
0
) x K
Equation 2 - Temperature Correction for Transducer Only
In practice, the best way to compensate for temperatures is to derive a thermal correction
factor from simultaneous measurements of pressure and temperature at times when it can
be safely assumed that the applied load is not changing. Perhaps the best time to do this is
while the concrete is curing.
4.3 Barometric Correction
Barometric pressure fluctuations will be sensed by the cells. However, the magnitudes (±0.5 psi)
are usually insignificant.