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4.
DATA REDUCTION
4.1 PRESSURE CALCULATION
The digits displayed by the
GEOKON
readouts on channel B are based on the equation:
2
1
---------------- -
=
10
digits
Period
EQUATION 2: Digits Calculation
Note that in the above equation, the period is in seconds;
boxes display microseconds. For example, a 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
– R
) × G
1
0
EQUATION 3: Convert Digits to Pressure
The Initial Reading (R
) is normally obtained during installation (usually the zero
0
reading). The Calibration Factor (G, usually in terms of psi or kPa per digit)
comes from the supplied Calibration Sheet (a typical calibration sheet is shown
in Figure 18). To convert the output to other engineering units, multiply the
Calibration Factor by the conversion multiplier listed in Table 3.
From
psi
"H
O
'H
O
mm H
2
2
To
psi
1
.036127 .43275
.0014223 1.4223
"H
O
27.730
1
12
.039372
2
'H
O
2.3108
.08333
1
.003281
2
mm H
0 704.32
25.399
304.788 1
2
m H
0
.70432
.025399 .304788 .001
2
"HG
2.036
.073552 .882624 .0028959 2.8959
mm HG 51.706
1.8683
22.4196 .073558
atm
.06805
.002458 .029499 .0000968 .0968
mbar
68.947
2.4908
29.8896 .098068
bar
.068947 .002490 .029889 .0000981 .098068 .033863 .001333 1.0132
kPa
6.8947
.24908
2.98896 .0098068 9.8068
MPa
.006895 .000249 .002988 .0000098 .009807 .003386 .000133 .101320 .0001
TABLE 3: Engineering Units Multiplication Factors
For example, assume an initial reading of R
7390 and a Calibration Factor of -0.1192 kPa/digit. The calculated pressure is:
204 kPa = (7390 -9101) × -0.1192
(Appendix D shows how a second order polynomial can be used to improve
accuracy.)
Note: Due to changes in specific gravity with temperature, the factors for
mercury and water in the above table are approximate.
MODEL 4800 VW EARTH PRESSURE CELLS | DATA REDUCTION | 17
Models GK-404, GK-405, and GK-406
2
3 –
Hz
or digits
=
----------- -
1000
0 m H
0
"HG
mm HG
atm
2
2
.49116
.019337 14.696
.014503 14.5039 .14503
39.372
13.596
.53525
406.78
.40147
3.281
1.133
.044604 33.8983 .033456 33.4558 .3346
1000
345.32
13.595
10332
10.197
1
.34532
.013595 10.332
.010197 10.197
1
.03937
29.920
.029529 29.529
73.558
25.4
1
760
.75008
.03342
.001315 1
.000986 .98692
98.068
33.863
1.3332
1013.2
1
.001
3.3863
.13332
101.320 .1
= 9101, a present reading of, R
0
readout
GEOKON
3:
mbar
bar
kPa
MPa
145.03
401.47
4.0147
4016.1
334.6
10197
101.97
101970
.10197
101.97
.2953
295.3
750.08
7.5008
7500.8
.009869 9.869
1000
10
10000
1
.01
10
100
1
1000
.1
.001
1
=
1
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