Geokon 4500AL, Инструкция по эксплуатации, Страница: 31 / 39

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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). Thus 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 in order 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 Barometer)

Conversion Factor
Or:
PB = (S1-S0) 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.
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 which is not required. In these cases we recommend
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 corrections is to use piezometers that are vented to the
atmosphere. (See Section 6.4.) However, vented piezometers only make sense if the piezo is in
an open well or standpipe and the user is only interested in the water level. If the piezo is buried
it is not certain that the full effect of the barometric change will be felt immediately at the
instrument and is more likely to be attenuated and delayed, in which case a vented piezo would
automatically apply a correction that is too large and too soon.
Equation 5 shows the pressure calculation with temperature and barometric correction applied.
Pcorrected = (R1 – R0)G + (T1-T0) K - (S1-S0) F
Equation 5 - Corrected Pressure Calculation