Appendix D. Data Reduction; Pressure Calculation - Geokon 8020-59 Instruction Manual

APPENDIX D. DATA REDUCTION

D.1 Pressure Calculation
The digits that are calculated from the voltage and current outputs are based on the equation:
For example, a piezometer reading 8000 digits corresponds to a period of 354 µs and a frequency
of 2828 Hz. Note that in the above equation, the period is in seconds.
Digits are directly proportional to the applied pressure.
(Current Reading - Initial Zero Reading) × Linear Calibration Factor
Since the linearity of most sensors is within 0.2% FS the errors associated with non-linearity are
of minor consequence. However, for those situations requiring the highest accuracy it may be
desirable to use a second order polynomial to get a better fit of the data points. The use of a
second order polynomial is explained in Appendix F.
The calibration report included with each transducer shows the data from which the linear gage
factor and the second order polynomial coefficients are derived. Columns on the right show the
size of the error incurred by assuming a linear coefficient and the improvement that can be
expected by going to a second order polynomial. In many cases, the difference is minor. The
calibration reports give the pressure in certain engineering units. These can be converted to other
engineering units using the multiplication factors shown in Table 3 below.
Digits = �
1
Period
Or
Digits=
1000
Equation 9 - Digits Calculation
Pressure =
Or
P = (R – R
1
Equation 10 - Convert Digits to Pressure
�
2
-3
x 10
2
Hz
) ×G
0
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