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43
D.3 Barometric Correction (required only on non-vented transducers)
Since the standard piezometer is hermetically sealed and unvented, it responds to changes in
atmospheric pressure. That being the case, 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.
Barometric correction equation is as follows;
Barometric Correction =
(Current Barometer - Initial Zero Barometer) × Conversion Factor
Or
P
= (S
-S
) x F
B
1
0
Equation 12 - Barometric Correction
Since barometric pressure is usually recorded in inches of mercury, a Conversion Factor is
necessary to convert to PSI. The Conversion Factor for inches of mercury to PSI is .491. Table 3
lists other common Conversion Factors.
The calculated correction is usually subtracted from the Pressure calculated using Equation 10.
If the engineering units were converted, remember to apply the same conversion to the calculated
barometric correction!
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 when applying a correction that is not required.
An alternative to making barometric correction is to use piezometers that are vented to the
atmosphere.
Equation 13 describes the pressure calculation with temperature and barometric correction
applied.
P corrected = ((R
- R
) x G) + ((T
-T
) x K) - ((S
-S
) x F)
1
0
1
0
1
0
Equation 13 - Corrected Pressure Calculation
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