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5.3 Barometric Correction (required only on unvented transducers)
Since the standard piezometer is hermetically sealed and unvented, 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
≈
1 PSI of error (or
≈
2.3 feet if monitoring water level in a well). Thus, it is advisable to
read and record the barometric pressure every time the piezometer is read. A separate pressure
transducer (piezometer), kept out of the water, may be used for this purpose.
The barometric correction equation is as follows:
Barometric Correction
=
(Current Barometer - Initial Barometer)
×
Conversion Factor
Or
P
B
= (S
1
-S
0
) x 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 lists other
common Conversion Factors.
Equation 5 shows the pressure calculation with temperature and barometric correction applied.
Pcorrected = (R
1
– R
0
)G + (T
1
-T
0
) K - (S
1
-S
0
) F
Equation 5 - Corrected Pressure Calculation
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 correction that is not required. In these cases, Geokon
recommends independently recording the barometric pressure changes and correlating them with
the observed pressure changes in order to arrive at a correction factor.
An alternative to making barometric correction is to use piezometers that are vented to the
atmosphere. See Section 5.4.
