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5. DATA REDUCTION
5.1 Pressure Calculation
The digits displayed by the Geokon Models GK-403, GK-404, and GK-405 Readout Boxes on
channel B are based on the equation:
Digits =
�
1
Period
�
2
x 10
-3
or Digits
=
Hz
2
1000
Equation 1 - Digits Calculation
For example, a piezometer reading 8000 digits corresponds to a period of 354
µ
s and a frequency
of 2828 Hz. Note that in the above equation, the period is in seconds. Geokon readout boxes
display microseconds.
Digits are directly proportional to the applied pressure.
Pressure
=
(Current Reading - Initial Reading)
×
Linear Calibration Factor
Or
P = (R
1
– R
0
)
×
G
Equation 2 - Convert Digits to Pressure
Since the linearity of most sensors is within ±0.2% FS, the errors associated with nonlinearity are
of minor consequence. However, for those situations requiring the highest degree of 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 D.
The instrument’s calibration report, a typical example of which is shown in Figure 18, 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 report gives the pressure in certain engineering units. These can be converted to
other engineering units using the multiplication factors shown in Table 2.
