MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER
| DATA REDUCTION |
17
6.
DATA REDUCTION
6.1
PRESSURE CALCULATION
The digits displayed by the
GEOKON
Models GK-404 and GK-405 readout boxes
on channel B are based on the equation:
EQUATION 1:
Digits Calculation
Note that in the above equation, the period is in seconds;
GEOKON
readout
boxes display microseconds. For example, a piezometer reading of 8000 digits
corresponds to a period of 354
µs
and a frequency of 2828 Hz.
Digits are directly proportional to the applied pressure, as can be seen by the
following equation:
Pressure = (Current Reading – Initial Zero 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% F.S., 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 C.
The instrument’s calibration report (a typical example of which is shown in
Appendix D), shows the data from which the linear gauge 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.
2:
TABLE 2:
Engineering Units Multiplication Factors
Note:
Due to changes in specific gravity with temperature, the factors for
mercury and water in the above table are approximate.
6.2
TEMPERATURE CORRECTION
The materials used in the construction of
GEOKON
vibrating wire piezometers
have been carefully selected to minimize thermal effects; however, most units
From
To
psi
"H
2
O
'H
2
O
mm H
2
0
m H
2
0
"HG
mm HG
atm
mbar
bar
kPa
MPa
psi
1
.036127 .43275
.0014223 1.4223
.49116
.019337 14.696
.014503 14.5039 .14503
145.03
"H
2
O
27.730
1
12
.039372
39.372
13.596
.53525
406.78
.40147
401.47
4.0147
4016.1
'H
2
O
2.3108
.08333
1
.003281
3.281
1.133
.044604 33.8983 .033456 33.4558 .3346
334.6
mm H
2
0
704.32
25.399
304.788 1
1000
345.32
13.595
10332
10.197
10197
101.97
101970
m H
2
0
.70432
.025399 .304788 .001
1
.34532
.013595 10.332
.010197 10.197
.10197
101.97
"HG
2.036
.073552 .882624 .0028959 2.8959
1
.03937
29.920
.029529 29.529
.2953
295.3
mm HG
51.706
1.8683
22.4196 .073558
73.558
25.4
1
760
.75008
750.08
7.5008
7500.8
atm
.06805
.002458 .029499 .0000968 .0968
.03342
.001315 1
.000986 .98692
.009869 9.869
mbar
68.947
2.4908
29.8896 .098068
98.068
33.863
1.3332
1013.2
1
1000
10
10000
bar
.068947 .002490 .029889 .0000981 .098068 .033863 .001333 1.0132
.001
1
.01
10
kPa
6.8947
.24908
2.98896 .0098068 9.8068
3.3863
.13332
101.320 .1
100
1
1000
MPa
.006895 .000249 .002988 .0000098 .009807 .003386 .000133 .101320 .0001
.1
.001
1
