MODEL 4500 SERIES VIBRATING WIRE PIEZOMETER
| DATA REDUCTION |
19
correction that is not required. In these cases,
GEOKON
recommends
independently recording the barometric pressure changes and correlating them
with the observed pressure changes 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.
The equation below shows the pressure calculation with temperature and
barometric correction applied.
P
corrected
= (R
1
– R
0
)G + (T
1
– T
0
)K – (S
1
– S
0
)F
EQUATION 5:
Corrected Pressure Calculation
6.4
MODEL 4500SV, VENTED PIEZOMETERS
13:
FIGURE 13:
Vented Piezometers
The Model 4500SV vented piezometer is designed to eliminate the effect of
barometric pressure changes on water level measurements in wells, reservoirs,
and boreholes that are connected directly to the atmosphere. They are not to be
used where pore water pressures are being measured.
The space inside the transducer is not hermetically sealed and evacuated, as it is
in the standard 4500 model piezometer, instead, it is connected via a tube
(integral within the cable) to the atmosphere. A chamber containing desiccant
capsules is attached to the outer end of this tube to prevent moisture from
entering the transducer cavity. Vented piezometers require more maintenance
then unvented types, since there is always the danger that moisture may find its
way inside the transducer and ruin it.
Installation of the piezometer is accomplished by lowering it to the desired level
in the well, reservoir, or borehole. The piezometer can be placed inside a canvas
bag filled with sand, if desired.
The desiccant capsule chamber needs to be positioned in some kind of housing
to keep it dry.
GEOKON
can provide suitable housings on request.
To keep the desiccant fresh during storage and transportation, the end of the
desiccant chamber is closed off by means of a seal screw before being shipped
from the factory.
THIS SEAL SCREW MUST BE REMOVED BEFORE THE
PIEZOMETER IS PUT INTO SERVICE.
