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| DATA REDUCTION | GEOKON
still have a slight temperature coefficient. Consult the calibration report supplied
with the instrument to obtain the coefficient for the individual piezometer.
Since piezometers are normally installed in a tranquil and constant temperature
environment, corrections are normally not required. If this is not the case for the
selected installation, corrections can be made using the internal thermistor for
temperature measurement. See Section 5.3 for instructions regarding obtaining
the piezometer temperature.
The temperature correction equation is as follows:
Temperature Correction =
(Current Temperature – Initial Zero Temperature)
×
Thermal Factor
Or
P
T
= (T
1
– T
0
) × K
EQUATION 3:
Temperature Correction
The calculated correction would then be added to the pressure calculated using
Equation 2. If the engineering units were converted, remember to apply the
same conversion to the calculated temperature correction.
For example, if the initial temperature was 22 °C, and the current temperature is
15 °C, and the thermal factor (K on the calibration report) is +0.1319 kPa per °C
rise. The temperature correction is +0.1319(15–22) = –0.92 kPa. Refer to the
calibration report provided with the instrument for the thermal factor.
6.3
BAROMETRIC CORRECTION (REQUIRED ONLY ON UNVENTED
TRANSDUCERS)
Since the standard piezometer is hermetically sealed, it responds to changes in
atmospheric pressure. Corrections may be necessary, particularly for the
sensitive, low-pressure models. For example, a barometric pressure change
from 29 to 31 inches of mercury would result in approximately one psi of error
(or
≈
2.3 feet if monitoring water level in a well). It is advisable to read and record
the barometric pressure every time the piezometer is read. Having an onsite
barometer also allows the monitoring of barometric changes to judge what
extent they may be affecting the reading. A separate pressure transducer
(piezometer), kept out of the water, may also be used for this purpose.
The barometric correction equation is as follows:
Barometric Correction =
(Current Barometer
–
Initial Zero Barometer)
×
Conversion Factor
Or
P
B
= (S
1
– S
0
) × F
EQUATION 4:
Barometric Correction
The calculated barometric correction is subtracted from the pressure calculated
using Equation 2. If the engineering units were converted, remember to apply
the same conversion to the calculated barometric correction.
Barometric pressure is usually recorded in inches of mercury. The conversion
factor for inches of mercury to psi is 0.491, and from inches of mercury to kPa is
3.386. Table 2 in Section 6.1 lists other common conversion factors.
The user should be cautioned that this correction scheme assumes ideal
conditions. In reality, conditions are not always ideal. For example, if the well is
sealed, barometric effects at the piezometer level may be minimal or attenuated
from the actual changes at the surface. Thus, errors may result from applying a
