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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
The following calculations are based on using a
Geokon model 4500 Vibrating Wire sensor. An
individual multiplier and offset must be
calculated for each sensor used in a system.
MULTIPLIER
The fundamental equation relating frequency to
pressure is
P = -F
G + B where
x
P = pressure, PSI
G = the Gage Factor obtained from the
sensors calibration sheet in PSI/digit.
The units of a digit are Hz
B = offset
2
2
-3
F
= f
Hz
(10
), where f is frequency.
x
Instruction 28 measures period, T, of the
vibrating wire in milliseconds (ms) and returns a
measured value, X, of
2
X = 1/(T
(ms)
A multiplier of -1000 in Instruction 28 converts
the measurement to digits, as shown below.
3
-F
= -X(-10
) = -f
x
To calculate the multiplier, convert Geokon's
gage factor, G, to the desired units (i.e., feet of
water per digit) and multiply by -1000 digits/kHz
TEMPERATURE CORRECTION
The temperature correction is applied as
follows.
7-14
FIGURE 7.13-1. A Vibrating Wire Sensor
2
-3
(10
).
2
2
-6
2
) = f
(10
)Hz
2
-3
2
(10
)Hz
P
= P + C
T
P
= Pressure corrected for temperature, C
T
C
= Temperature coefficient, PSI/ C
(from Geokon calibration sheet)
t
& t
= Initial and current temperatures, C.
0
1
The temperature coefficient, C, must be
converted to units compatible with the gage
factor, G.
WELL MONITORING EXAMPLE
In this example the vibrating wire sensor is used
to monitor water table height (Figure 7.13-2).
The desired data is the distance from the lip of
the well to the water surface. The sensor is
vented to atmosphere to eliminate
measurement errors due to changes in
barometric pressure. The water level is
expected to stay within 40 to 80 feet of the lip so
the 50 psi pressure sensor is placed
approximately 100 feet below the lip of the well.
The calibration data from Geokon is provided in
Table 7.13-1.
TABLE 7.13-1 Calibration Data for
Gage Factor
(psi/digit)
2
.
0.0151
(t
- t
), where
1
0
Sensor 3998
Temp. Coeff.
(psi/ C)
-0.0698
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