Geokon 4580, Instruction Manual

Page: 18 / 25

12
The calculated 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!
The user should be aware that this correction scheme assumes ideal conditions. In reality,
conditions are rarely ideal. For example, if a well is sealed and not connected hydraulically to the
atmosphere, barometric effects on the water pressure can be attenuated. This will result in an
error due to applying an unnecessary or excessive correction. Geokon recommends, in these
cases, to independently record barometric pressure changes and correlate these with observed
pressure changes to arrive at a correction factor.
An alternative to measuring the barometric pressure and making barometric correction is to use
pressure transducers that are vented to the atmosphere. (as described in Section 4.3.1.) With
vented transducers, barometric pressure finds its way to both the inside and the outside of the
pressure capsule and is thus automatically canceled.
Equation 5 describes the pressure calculation with both temperature and barometric corrections
applied.
P
corrected
= (R
1
- R
0
)
×
G + (T
1
-T
0
)
×
K - (S
1
-S
0
)
×
F
2
Equation 5 - Corrected Pressure Calculation
For example, using the typical calibration report shown in Appendix D, showing a 7 kPa
Pressure Transducer (vented) used to measure changes of water level, ΔL mm:
R
0
= 8168 digits
R
1
=9660 digits
T
0
= 15.1° C
T
1
= 23.8° C
S
0
= 29.3" Hg.
S
1
= 30.8" Hg.
G = 0.001875 kPa/digit
K = -0.0007823 kPa/°C
F
1
= 101.95 (to convert kPa to mm H
2
O)
F
2
= 345.44 (convert "Hg. to mm H20)
Puncorrected = (9660 -8168)
×
0.001875 =2.798 kPa
Ptemperature = (23.8 - 15.1) × -0.0007823 =-0.0068 kPa
Pbarometric = 0 [this is a vented transducer]
Pcorrected = Puncor Ptemperature - Pbarometric
Pcorrected = 2.798 - 0.0068 -0 = 2.791 kPa
ΔL = +284.5 mm of water