RST Instruments VW2100-DP User manual

ELM0005P
All efforts have been made to ensure the accuracy and
completeness of the information contained in this document. RST
Instruments Ltd reserves the right to change the information at any
time and assumes no liability for its accuracy.
Copyright © 2019. RST Instruments Ltd. All rights reserved.
Document Number: ELM0005P
Release Date: July 10, 2019
VW2100 Vibrating Wire
Piezometer
Instruction Manual

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page ii
REVISION TABLE
Rev.
Revision History
Date
Prepared
By
Approved
By
P
Corrected typo in Table 7-1 (0.05% changed to 0.5%);
Corrected linear equation in Section 4.4.1 to include F
factor; corrected minor typos throughout; added Revision
Table; reworded Section 4.3 for clarity.
10-Jul-2019
QR
EG

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
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TABLE OF CONTENTS
1 INTRODUCTION............................................................................................................5
1.1 Model VW2100 .................................................................................................6
1.2 Model VW2100-DP ...........................................................................................6
2 VIBRATING WIRE PRINCIPLE ........................................................................................6
3 CALIBRATION..............................................................................................................7
3.1 Field Calibration Check.....................................................................................8
4 READING PROCEDURES...............................................................................................9
4.1 VW Instrument Readings ..................................................................................9
4.2 Initial Inspection and Check Readings...............................................................9
4.3 Initial Readings .................................................................................................9
4.4 Pressure Equation (Using the VW2106 Readout) ...........................................11
4.4.1 Linear Equation.....................................................................................11
4.4.2 Second Order Polynomial Equation ......................................................12
5 INSTALLATION...........................................................................................................13
5.1 Filter Saturation...............................................................................................14
5.2 Low Air Entry Sintered Stainless-Steel Filters .................................................14
5.3 High Air Entry Ceramic Filters.........................................................................15
5.3.1 One Bar High Air Entry Filters...............................................................15
5.3.2 Two Bar (or Higher) High Air Entry Filters.............................................16
5.4 Installation in Full............................................................................................17
5.4.1 Compacted Clay ...................................................................................17
5.4.2 Granular Materials ................................................................................17
5.5 Installation in Boreholes..................................................................................18
5.5.1 Sand/Bentonite Method.........................................................................18
5.5.2 Fully Grouted Method ...........................................................................20
5.6 Piezometers Drive In Soft Ground...................................................................22
5.6.1 Installation.............................................................................................22
5.7 Cable Identification .........................................................................................23
5.8 Cable Routing.................................................................................................23
5.8.1 Transition from Vertical Borehole to Horizontal Trench .........................23
5.8.2 Horizontal Cable Runs..........................................................................23
5.9 Lightning Protection ........................................................................................24
6 TROUBLESHOOTING ..................................................................................................25
6.1 VW Piezometer Fails to Give a Reading .........................................................25
6.2 VW Piezometer Reading Unstable..................................................................25
6.3 Thermistor Reading is Too Low ......................................................................26
6.4 Thermistor Reading is Too High......................................................................26
7 SPECIFICATIONS........................................................................................................27
8 SERVICE AND REPAIR................................................................................................28
APPENDIX A:VW2100 CALIBRATION SHEET ...................................................................29

VW2100 Vibrating Wire Piezometer
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APPENDIX B:CASAGRANDE STYLE FILTER ASSEMBLY ....................................................30
APPENDIX C:VW2100-DP (DRIVE POINT)PIEZOMETER...................................................31
APPENDIX D:USING THE SECOND ORDER POLYNOMIAL TO IMPROVE THE ACCURACY OF THE
CALCULATED PRESSURE...........................................................................................32
APPENDIX E:THERMISTOR TEMPERATURE DERIVATION ...................................................33
APPENDIX FREFERENCES...............................................................................................34
LIST OF FIGURES
Figure 1-1 Vibrating Wire Piezometer (0.7 MPa)..................................................................5
Figure E-1 Thermistor Resistance versus Temperature.......................................................33
LIST OF EQUATIONS
Equation 1 Linear equation ......................................................................................................11
Equation 2 Second order polynomial equation.........................................................................12
Equation E-1 Convert Thermistor Resistance to Temperature.................................................33
LIST OF TABLES
Table 7-1 General Specifications for All Models.................................................................27
Table 7-2 Specifications for Individual Models ...................................................................28

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page 5
1 INTRODUCTION
The RST Vibrating Wire Piezometer is a stable, robust pressure transducer designed
to allow very accurate remote measurements of piezometric levels and borehole
pressures over extended periods of time and through all conditions. The vibrating
wire pressure transducer output is a frequency signal which is unaffected by line
impedance and/or contact resistance of the conductor. This allows for the accurate
transmission of the frequency signal over very long distances. These types of
vibrating wire sensors can be installed in boreholes or driven into soft ground.
A standard integral thermistor is included within each transducer, which measures
the temperature of the transducer and its surroundings. This temperature information
is used to provide temperature correction to the output pressure readings. A gauge
calibration factor and temperature correction factor are supplied with each
manufactured gauge based on the factory calibrations which are carried out for each
sensor, immediately following manufacture.
A portable vibrating wire readout unit, such as the RST VW2106 Readout Unit, is
used to display the frequency of the vibrating wire which is proportional to the
pressure being applied to the vibrating wire transducer diaphragm. Additionally, the
VW2106 readout unit will display the transducer temperature directly in degrees
Celsius.
Complete data logging systems are available from RST to provide automated data
collection from vibrating wire transducers. Consult RST for more information, if
required.
FIGURE 1-1 VIBRATING WIRE PIEZOMETER (0.7 MPA)
The RST VW piezometer is a Vibrating Wire diaphragm pressure sensor. Pressure
applied to the transducer diaphragm will cause a change in the Vibrating Wire
tension, resulting in a change to the resonant frequency, which is directly
proportional to the pressure change.
The Vibrating Wire sensors are made of two small diameter cylindrical parts joined
by a length of steel tubing. The diaphragm is welded to the front cylinder. A high
strength steel wire (the Vibrating Wire) is clamped to the center of the diaphragm,
then is run through the first cylinder, and then clamped to the base of the second
cylinder which is the end block. The Vibrating Wire is clamped to the diaphragm and
end block by low temperature hydraulic swaging which virtually welds the parts
together without affecting the elastic properties of the wire. All parts of the sensor,
other than the actual Vibrating Wire are machined from a high-grade stainless steel,
selected for its low yield and high corrosion resistance.

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
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The Vibrating Wire is set to a pre-determined tension during the manufacture. The
instrument housing is evacuated and sealed using electron beam welding to ensure
a perfect seal and a long working life. An O-ring placed behind the diaphragm seals
the back of the assembly within the housing. A coil/magnet assembly is built into
every VW transducer which is used in conjunction with the RST readout box, to pluck
the Vibrating Wire and measure the VW’s vibration period.
1.1 MODEL VW2100
The RST VW2100 Vibrating Wire Piezometer is designed to be embedded in earth
fills and concrete or inserted into boreholes and pipes as small as 19 mm (3/4 inch)
in diameter. The VW2100 piezometer consists of a small diameter cylindrical housing
containing a pressure transducer and thermistor. One end is fitted with an insert that
holds a micrometric high air or low air entry filter. The opposite end contains the
cable entry sealed with an epoxy compound. All parts are made of stainless steel.
The entry filter is set in the front end of the housing and sealed with an O-ring. With
the filter in place, the diaphragm is protected from solid particles, and senses only
the fluid pressure to be measured. The filter housing is easily removable for
calibration of the transducer. The filter assembly can also be replaced with a pipe
thread adapter to use the gauge as a pressure transducer (Model VW2100-PT).
1.2 MODEL VW2100-DP
The RST VW2100-DP Vibrating Wire Piezometer is designed to be driven into
unconsolidated fine grain material such as sand, silt, or clay. The external housing is
a thick-walled cylinder fitted with a pointed shoe at one end. The opposite end is
fitted with a male thread adapter at the cable entry, which fits standard “EX” drill
rods. Three port holes above the point are equipped with micrometric filters. The
data cable passes through the threaded end and can be fed up through the drill rods
to the surface. The cable entry is sealed with an epoxy compound. Both high and
low air entry filters are available.
2 VIBRATING WIRE PRINCIPLE
The sensing element of the Vibrating Wire piezometer is a high strength steel wire
attached to the diaphragm. The vibrating wire is excited by two coil/magnets set
around the connecting over tube. In operation, external pressure on the diaphragm
will move the diaphragm a very small amount, which changes the tension on the
vibrating wire. This tension change is directly proportional to the resonant, or natural,
frequency at which the vibrating wire will vibrate.
The VW2106 Readout Unit generates plucking voltages to the coil/magnet in a
spectrum of frequencies, spanning the natural frequency of the vibrating wire. This
plucking allows the vibrating wire to find its current natural frequency related to the
pressure it is currently experiencing. In turn, the oscillation of the vibrating wire
generates AC voltage in the coil. This output signal is amplified by the VW2106
Readout Unit, which also discriminates against harmonic frequencies, to determine
the resonant frequency of the wire.

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page 7
The VW2106 Readout Unit measures 100 cycles of vibration with a precise quartz
oscillator and displays a value proportional to the frequency squared, which is called
B-Units (Frequency2x 10-3). The relationship between B-Unit readings and the
pressure being exerted on the diaphragm is expressed by the following equation:
P = CF (L0–L)
where:
P= Corrected Pressure Reading
CF = Linear Calibration Factor in kPa\B-Unit digit. The CF is a unique value for
each manufactured VW sensor, as determined by the initial laboratory
calibration
L0= Initial B-Unit Reading at zero applied pressure on the diaphragm. The L0
is a unique value for each manufactured VW sensor and is determined by
initial laboratory calibration
L= B-Unit Reading under the currently applied pressure on the diaphragm
The Vibrating Wire technology offers the unique advantage of frequency output
signal virtually unaffected by line impedance or contact resistance. Up to 1.5 km of
cable length can be used without signal deterioration.
3 CALIBRATION
All RST vibrating wire piezometers are individually calibrated in the laboratory before
shipment. Each vibrating wire piezometer is calibrated over its full working pressure
range. A Linear Calibration Factor (CF) is established by using the calibration data
points to do a linear regression. In addition, the calibration data is also fitted to a
polynomial regression which provides slightly more accurate data output over the full
reading range. Both formulas are provided on the instrument Calibration Record
sheet for use as appropriate. It is also noted that RST data loggers are set up to use
either formula to calculate the instrument output in engineering units.
As part of the calibration procedure, all vibrating wire piezometers are tested to
150% of the standard working range to prove their function at overpressure. The
sensor calibration is carried out over a temperature range of -20º C to +80º C which
proves their function at a wide temperature range and provides the input data for the
Temperature Correction Factor for each sensor.
A Calibration Record sheet is provided with each vibrating wire sensor for use in
calculating the applied loads on the vibrating wire sensors. The following general
information is contained in the Calibration Record sheet:
•Model, Serial, and Manufacturing Numbers;
•Pressure Range;
•Temperature and Barometric Pressure at time of Calibration;

VW2100 Vibrating Wire Piezometer
Instruction Manual
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RST Instruments Ltd.
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•Work Order Number;
•Cable Information (Length, Meter Markings, Color Code, and Type);
•Thermistor Type;
•Linear Calibration Factor (CF);
•Temperature Correction Factor (Tk);
•Polynomial Gauge Factors (A, B, and C);
•Calibration Data Table;
•Linear and Polynomial Formulas;
•Zero Reading, Temperature, and Barometric Pressure at time of Shipment;
•Calibration Certification.
Refer to Appendix A for an example of a Calibration Record sheet.
3.1 FIELD CALIBRATION CHECK
The following procedure can be used in the field to verify the validity of a vibrating
wire piezometer calibration as supplied on the Calibration Record sheet.
Note that VW2100 piezometers will require the filter stone to be saturated prior to
calibration. Place the VW2100 piezometer in water to saturate the filter stone.
Ensure the entire space between the instrument diaphragm and the filter stone is
filled with water. Refer to Appendix B for more detailed information on the saturation
of VW2100 piezometers with Casagrande style filters, and VW2100-DP piezometers.
1Lower the piezometer to depth in a vertical, water filled borehole using the cable
markings to accurately control and set the depth. A minimum emersion depth of
10 m is recommended to ensure adequate accuracy of the field calibration check.
2Allow 20 to 30 minutes for the piezometer to come to complete thermal
equilibrium in the borehole. Record the B-unit and temperature readings at that
depth using an RST VW2106 readout unit.
3Raise the piezometer a known amount while keeping it fully submerged. If the
temperature reading is changing, allow the instrument to come to the new
thermal equilibrium. This may take another 20 to 30 minutes.
4Record the new B-Unit and temperature readings at the higher elevation.
Calculate the instrument Calibration Factor (CF) (kPa per B-Unit) from this
information, given the change in pressure head and B-Unit readings.
5Compare this field calibration to the calibration factor value provided on the
Calibration Record sheet. The two values should agree within ± 0.5%. Repeat
this calibration check as required to confirm the sensor is in proper working
condition.

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Instruction Manual
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RST Instruments Ltd.
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It is not recommended to install the piezometer if the calibration record sheet CF
value cannot be confirmed by the field calibration test. The instrument will need to be
inspected and undergo a full shop function test and re-calibrated before being
returned to service. Contact RST for further information.
If the diameter of the water filled borehole is too small, the volume of water that the
vibrating wire piezometer cable displaces, when raised or lowered into position,
could potentially raise, or lower the borehole water level. This effect may seriously
impact the accuracy of the above detailed Field Calibration Check. Note that this
potential effect will be further dependent on the available permeability of the borehole
to absorb small amounts of volume change.
To avoid this potential problem, it is recommended that the water filled borehole used
for Field Calibration Checks be large enough in diameter so that the potential error
caused by cable volume displacement will be insignificant in the calculation of the
pressure change. It is recommended to use a borehole diameter that is a minimum of
10 times greater than the wire diameter. A borehole with a moderate degree of
permeability would be preferred to a “tight” borehole.
4 READING PROCEDURES
4.1 VW INSTRUMENT READINGS
It is strongly recommended that the instruction manual for the RST VW2106 Readout
Unit be read thoroughly before proceeding. Failure to become familiar with the
function and operation of the VW2106 Readout could potentially result in damage to
the VW2106 Readout unit and/or the vibrating wire sensors that are connected to it.
4.2 INITIAL INSPECTION AND CHECK READINGS
A full inspection of all received vibrating wire instrumentation equipment is required
immediately upon receipt at site to ensure that the vibrating wire instruments have
not been damaged in any way during shipment and are fully functional/ready for use.
Test readings should be taken of each vibrating wire instrument and compared to the
vibrating wire instrument reading information provided on the Calibration Record
sheet. Any discrepancies should fully investigated and satisfactorily resolved before
the vibrating wire instrument is released for field installation and service.
The individual performing the inspection and initial test readings must be familiar with
the vibrating wire instrument operation and contents of this instruction manual.
4.3 INITIAL READINGS
Vibrating wire piezometers differ from other types of pressure sensors as the core of
the vibrating wire sensor is manufactured with an initial tension. The piezometers
have a positive B-Unit reading without any external pressure being applied. Vibrating
wire piezometers are acutely sensitive to pressure changes at zero point as there is
no zero-point hysteresis to overcome. The determination of vibrating wire instrument

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page 10
initial readings at the “zero point” is extremely important for the accuracy of the
subsequent readings.
Before installing the vibrating wire piezometer, it is necessary to take the initial zero
reading with no applied load. The initial zero reading can be taken in one of the
following ways:
•With the filter stone removed, or
•With the stone completely saturated and installed.
More information can be found in Section 5.1 and Appendix B.
If the filter stone is saturated, initial zero readings should be taken with the
piezometer exposed to the open air.
CAUTION: DO NOT SUBMERGE THE INSTRUMENT IN WATER TO TAKE THE INITIAL
READINGS.ONLY ATMOSPHERIC PRESSURE SHOULD BE APPLIED TO THE PIEZOMETER
AT THIS TIME.FAILURE TO DO SO MAY IMPACT THE ACCURACY OF THE SUBSEQUENT
READINGS.
The temperature reading from the internal thermistor must also be recorded. The
barometric pressure for piezometers with a total range lower than 2 MPa must also
be recorded. These values are needed to apply the correct correction factors for
changes in temperature and/or barometric pressure, which will impact the reading
accuracy of the vibrating wire piezometers through their intended range.
Initial zero readings are generally obtained immediately prior to installation with no
external pressure and a constant ambient temperature and barometric pressure.
NOTE: BE SURE TO RECORD THE VIBRATING WIRE PIEZOMETER TEMPERATURE AND
BAROMETRIC PRESSURE AT THE SAME TIME THE B-UNIT ZERO READINGS ARE
TAKEN.
The following checks are required to obtain accurate initial zero readings:
•Has the temperature of the vibrating wire piezometer body reached full thermal
equilibrium?
◼Variations in temperature across the mass of the piezometer body may result
in a temperature reading which is not consistent with the entire vibrating wire
instrument. This inconsistency will result in an error to the calculated pressure
being read by the vibrating wire sensor. Allow 20 to 30 minutes for the
temperature of the vibrating wire piezometer to equilibrate. Sources of
temperature fluctuation, such as water flow, may have to be eliminated.
•Is the filter stone saturated?
◼Surface tension effects within the pore spaces of the filter could affect the
zero readings if the filter stone is only partially saturated. This can be a
problem particularly at low pressures (less than 350 kPa). Remove the filter

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Instruction Manual
ELM0005P
RST Instruments Ltd.
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stone to allow direct atmospheric connection with the transducer diaphragm if
there is any question regarding the adequate saturation of the filter stone.
4.4 PRESSURE EQUATION (USING THE VW2106 READOUT)
The VW2106 Readout Unit displays vibrating wire piezometer readings in frequency
units called B-Units, which equal Frequency2x 10-3, where frequency is in Hertz.
The B-Unit values represent absolute pressure and must be corrected for changes in
temperature and barometric pressure. B-Unit changes from the initial zero reading
are converted to the actual pressure changes using equations in Sections 4.4.1 and
4.4.2, which include corrections for temperature and barometric pressure changes.
4.4.1 Linear Equation
𝑷 = 𝑪𝑭(𝑳𝟎− 𝑳)− 𝑻𝑲(𝑻𝟎− 𝑻)+ 𝑭(𝑺𝟎− 𝑺)
EQUATION 1LINEAR EQUATION
Where:
P= Corrected Pressure in kPa
CF = Calibration Factor in kPa/B-Unit (From the VW Piezometer Calibration
Record sheet for each individual sensor)
L0, L= Initial and Current B-Unit reading (Frequency2x 10-3)
TK= Temperature Correction Factor in kPa/ºC (From the VW Piezometer
Calibration Record sheet in each individual sensor)
T0, T = Initial and current temperature readings in (ºC)
F= Barometric Pressure Constant = 0.1 kPa/mbar
S0, S = Initial and Current Barometric pressure readings in mbar
Example for a 350 kPa Range Piezometer
CF = 0.11594 kPa/B-Unit
Li = 8776 B-Unit
L= 7200 B-Unit
TK= - 0.03413 kPa/ºC
Ti= 22.9 ºC
T= 5.0 ºC
F= 0.1 kPa/mbar

VW2100 Vibrating Wire Piezometer
Instruction Manual
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RST Instruments Ltd.
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Si= 1003.1 mbar
S= 995 mbar
P= [(0.11594) x (8776 - 7200)] - [(-0.03413) x (22.9 - 5.0)] + [0.1 x (1003.1 - 995)]
= [182.72] - [-0.61] + [0.81]
= 184.14 kPa
NOTE: BAROMETRIC COMPENSATION IS NOT REQUIRED WITH VENTED AND
DIFFERENTIAL PRESSURE TRANSDUCERS.
4.4.2 Second Order Polynomial Equation
𝑃 = 𝐴(𝐿)2+ 𝐵(𝐿)+ 𝐶 − 𝑇𝐾(𝑇0− 𝑇)+ 𝐹(𝑆0− 𝑆)
EQUATION 2SECOND ORDER POLYNOMIAL EQUATION
Where:
P= Corrected Pressure in kPa
A= Polynomial Gauge Factor A in kPa/B-Unit2(Second Order Polynomial
Expression derived from the VW Piezometer Calibration data, for each
individual sensor)
B= Polynomial Gauge Factor B in kPa/B-Unit (Second Order Polynomial
Expression derived from the VW Piezometer Calibration data, for each
individual sensor)
C= Polynomial Gauge Factor C kPa (Second Order Polynomial Expression
derived from the VW Piezometer Calibration data, for each individual
sensor)
NOTE: POLYNOMIAL GAUGE FACTOR CMUST BE CALCULATED USING THE SITE
ZERO READINGS,AS PER THE EQUATION BELOW.
C=-[A(L0)2+B(L0)]
L0, L= Initial and Current B-Unit reading (Frequency2x 10-3)
TK= Temperature Correction Factor in kPa/ºC (From the VW Piezometer
Calibration Record sheet in each individual sensor)
T0, T = Initial and current temperature readings in (ºC)
F= Barometric Pressure Constant = 0.1 kPa/mbar
S0, S = Initial and Current Barometric pressure readings in mbar

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Instruction Manual
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RST Instruments Ltd.
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Example for a 350 kPa Range Piezometer
A= - 4.1484E-07 kPa/(B-Unit2)
B= - 0.10991 kPa/B-Unit
C= 996.58 kPa
L= 7200 B-Unit
TK= -0.03413 kPa/ºC
T0= 22.9 ºC
T= 5.0 ºC
F= 0.1 kPa/mbar
S0= 1003.1 mbar
S= 995 mbar
P= [(-4.1484 E-07) x (7200)2] + [-0.10991 x 7200] + [996.58]
+ [-0.03413 x (5.0 - 22.9)] - [0.1 x (995 - 1003.1)]
= [-21.51] + [-791.35] + [996.58] + [0.61] - [-0.81]
= 185.14 kPa
5 INSTALLATION
Vibrating wire piezometers can be installed in various ways to suit the individual
application. Specific guidelines for piezometer installation have been developed by
various agencies and technical specialists. Appendix F provides a list of references.
The following instructions summarize the generally accepted practice for:
•Filter saturation;
•Cable identification;
•Piezometers installed in clay fill, granular material, or boreholes;
•Cable routing.
It is not recommended that vibrating wire piezometers be installed in wells or
standpipes where an electrical pump and/or a power supply cable is present or
nearby. Electrical interference from these sources can cause unstable readings.
Ground fault currents from this type of equipment can easily damage the sensitive
low voltage vibrating wire piezometers. Additional steps must be performed on site to
ensure complete isolation and adequate grounding of the instrumentation circuits if

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installation under these conditions is unavoidable. The instrument shield wire should
be well grounded, but isolated from sources of external electrical interference.
In situations where vibrating wire piezometers and packers are used at the same
time in standpipes or wells, special care must be taken to avoid damaging or cutting
the cable jacket with the packer equipment or tools. Any cuts in the cable jacket will
allow water entry, which can potentially result in damage or failure of the vibrating
wire sensor.
5.1 FILTER SATURATION
High air entry ceramic or low air entry sintered stainless steel filters are available.
The filters are intended to protect the delicate diaphragm area of the vibrating wire
piezometer while allowing the transmission of external pressures. The filters and
bottom cavity of the piezometer body must be saturated to allow the accurate
transmission of hydraulic pressures to the vibrating wire diaphragm. Filter saturation
provides the following reading advantages:
•There is no fluid movement in a saturated environment - only pressure
transmission. This reduces the possibility of the filter becoming clogged with
debris due to oscillating water movement;
•Decreased response times due to pressure changes, which means increased
sensor sensitivity;
•Ensure hydraulic continuity between the pore water and the piezometer
diaphragm in unsaturated soils, which will provide the highest accuracy of
pressure measurement.
5.2 LOW AIR ENTRY SINTERED STAINLESS-STEEL FILTERS
Total saturation of the filter is necessary for accurate reading results. For the
standard filter supplied, the low air entry filter, saturation will start to occur as the
piezometer is lowered into the water. Water will be forced into the filter, compressing
the air in the space between the filter stone and the pressure sensitive diaphragm.
Given enough time, this air will dissolve into the water until the space below the
diaphragm and within the filter is entirely saturated. This could take multiple days,
which could mean slightly inaccurate reading results the first few days.
The following procedure will speed up the filter saturation process and allow accurate
readings to be taken immediately:
•Turn the vibrating wire piezometer upside down. Remove the end filter assembly,
which is held in place with an internal O-ring.
•Submerge the inverted piezometer in a bucket of flat water (water which has
been sitting for 24 hours). This will fill the space above the piezometer diaphragm
with water.
•While keeping the piezometer submerged, slowly replace the filter housing onto
the inverted piezometer end, allowing the water to be forced out through the filter
sinter.

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RST Instruments Ltd.
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◼Note that with a low-pressure range piezometer (less than 350 kPa), it is
recommended that vibrating wire readings be taken with a VW2106 Readout
Unit while the filter housing is being pushed slowly into place, to ensure that
the sensor does not over-range due to this operation.
•The vibrating wire piezometer should be stored in the bucket of water until ready
to install downhole to maintain the filter saturation prior to installation.
•During the installation, the vibrating wire piezometers should be handled as
gently as possible to keep the water in the filter sinter and the bottom chamber
until submerging in the borehole.
CAUTION: VIBRATING WIRE PIEZOMETERS CANNOT BE ALLOWED TO FREEZE WHEN
FULLY SATURATED,OTHERWISE DAMAGE WILL OCCUR TO THE TRANSDUCER
DIAPHRAGM,WHICH WILL INVALIDATE THE TRANSDUCER FUNCTION AND
CALIBRATION.
The O-ring providing the friction fit may become worn and the filter housing may
become loose if the vibrating wire piezometer must undergo multiple removals and
reinstallations of the filter housing. Replace the O-ring immediately if the filter
housing is loose.
Coarser screen housings are available for use on vibrating wire piezometers if salts
or other precipitates are clogging the stainless sinter filter. Screens are less likely to
become clogged by precipitates and other debris found in some water sources.
Note that salts and other dissolved solids can be deposited within a stainless
sintered filter if the filter is allowed to dry out completely. Thoroughly rinse out the
filter with clean distilled water prior to drying to prevent filter clogging.
5.3 HIGH AIR ENTRY CERAMIC FILTERS
The ceramic filter on high air entry piezometers is also removable for de-airing.
Because of the high air entry characteristics of the filter, proper de-airing is
particularly important for this type of filter assembly in order to ensure that accurate
readings can be taken. High air entry filters are available with different air entry
values, which will require different procedures. It is therefore very important to know
which type of high air entry filter is installed.
5.3.1 One Bar High Air Entry Filters
1Remove the filter housing from the piezometer body by carefully twisting and
pulling on the filter housing assembly. Remove the filter housing slowly to avoid
causing a vacuum pressure on the piezometer diaphragm.
2Boil the filter assembly in de-aired water for 30 minutes to force all air out of the
filter and to saturate the filter material. Place the filter into de-aired water when
boiling is completed.

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3Re-assemble the filter housing into the piezometer body under the surface of de-
aired water, while keeping the piezometer oriented with the diaphragm pointing
upward. Take care to ensure that no air is trapped in the transducer cavity.
4Vibrating wire readings must be taken with a VW2106 readout unit while the filter
housing is being pushed slowly into place. Allow any over-range pressures to
fully dissipate before pushing the filter on any further.
5The vibrating wire piezometer with a high air entry filter installed must be stored
in de-aired water until the unit is installed.
CAUTION: VIBRATING WIRE PIEZOMETERS CANNOT BE ALLOWED TO FREEZE WHEN
FULLY SATURATED,OTHERWISE DAMAGE WILL OCCUR TO THE TRANSDUCER
DIAPHRAGM,WHICH WILL INVALIDATE THE TRANSDUCER FUNCTION AND
CALIBRATION.
5.3.2 Two Bar (or Higher) High Air Entry Filters
The proper procedure for de-airing and saturating two bar (or higher) high air entry
filters is complex and difficult to complete properly. It is recommended that it be
performed either at the factory or by carefully following the instructions below:
1Place the assembled piezometer, with the filter housing facing downward, at the
bottom of a vacuum chamber. The vacuum chamber is to have an inlet port at
the bottom to later allow introduction of de-aired water into the chamber.
2Close the valve for the de-aired water inlet and evacuate the chamber. The
piezometer should be monitored with a VW2106 readout unit while the chamber
is being evacuated.
3When the maximum vacuum has been achieved in the vacuum chamber, use the
VW2106 readout unit to read the piezometer until it has also reached the same
maximum vacuum pressure.
4Open the de-aired water inlet valve to allow de-aired water to enter the bottom of
the chamber and reach an elevation of approximately 50 mm above the top of
the piezometer high air entry filter.
5Close the de-aired water inlet valve when the de-aired water has reached the
required height.
6Release the vacuum, allowing the chamber to return to atmospheric pressure.
7Observe the transducer output on the VW2106 readout unit. Up to 24 hours may
be required for the (5 bar high entry) filter to completely saturate and for the
piezometer pressure to return to zero. The saturation of the high entry filter is
considered to be completed at this point.
8After saturation, the transducer must be kept in a sealed container of de-aired
water until ready for installation. If de-aired at the factory, a special plastic cap is

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applied to the piezometer tip to maintain the saturation level. The plastic cap
must be removed immediately before installation.
CAUTION: VIBRATING WIRE PIEZOMETERS CANNOT BE ALLOWED TO FREEZE WHEN
FULLY SATURATED,OTHERWISE DAMAGE WILL OCCUR TO THE TRANSDUCER
DIAPHRAGM,WHICH WILL INVALIDATE THE TRANSDUCER FUNCTION AND
CALIBRATION.
5.4 INSTALLATION IN FULL
5.4.1 Compacted Clay
1Excavate a vertical trench or recess approximately 50 cm deep in the clay
material. Form a horizontal cylindrical hole in the sidewall of the excavated trench
near the bottom. The hole diameter should be slightly smaller than the
piezometer body to ensure a snug fit when the piezometer is inserted in the hole.
2Push the piezometer into the hole in the trench side, and into the host clay
material. Smear the filter ceramic with a thin paste of the saturated clay material
if necessary, to ensure continuity of the saturated air entry filter and pore water.
3Place the cable with the utmost care to avoid any damage due to kinking or
stretching. Look the cable and route it out of the trench. Make sure it rests on a
bed of hand placed and lightly compacted screened clay. Ensure that the cable
does not come into direct contact with itself or other cables in the area. Always
maintain a few centimeters of compacted clay material between any two cables.
4Backfill the trench with screened clay containing no particles larger than 3 mm in
dimension. The backfill should have a water content and density equal to that of
the surrounding material.
5Ensure that the cable is well protected from any potential damage caused by any
angular fill material, compacting equipment, and any settlement that might occur
due to construction work or subsequent fill placement.
5.4.2 Granular Materials
1Excavate a vertical trench or recess about 50 cm deep in the granular material.
2Place the piezometer horizontally in the center of the trench or recess.
3Loop the cable and backfill the bottom 10 cm of the trench around the piezometer
with screened granular material not exceeding 3 mm in dimension.
4Above that level, the trench can be backfilled in 10 cm lift with the same granular
material that was excavated. The granular backfill should contain the same
moisture content and be compacted to the same density as the surrounding fill.
Care must be exercised to not subject the piezometer instrument to damage
during compaction work.

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5In rock fill (particle sizes greater than 10mm), the large interstitial voids will not
allow fine backfill materials around the piezometer to stay in place. The fine filter
materials will migrate into the rock fill, eventually leaving the piezometer body in
direct contact with the angular rock fill material. It will be necessary to place a
graded filter zone around the piezometer to ensure that the filter materials will not
be moved. Fine grained clean sand, grading to pea gravel or larger, will be
required around the piezometer instrument. The particle size of the backfill will
have to increase in size outwards toward the rock fill. The sand placed around
the piezometer instrument and cable should range in size from 0.5 to 3mm in
diameter and should not be angular.
◼Note that it may be necessary or advisable to use geotextile filter fabric layers
and/or envelopes to provide hard boundaries when attempting to place a fine
grained zoned backfill around a piezometer within courser fill materials. This
practice will ensure that fine grained backfill materials used within a graded filter
will not become mobilized and wash away.
5.5 INSTALLATION IN BOREHOLES
5.5.1 Sand/Bentonite Method
The method used to install a piezometer in a borehole depends on the technical
requirements for the instrument, the drilling method that was employed, the particular
downhole conditions, and the materials which the installation must be carried out in.
The general method described below will have general applicability to most
installations. However, the Field Engineer must be aware of the unique conditions
that may be present in the subject borehole, which could make downhole
installations a major challenge. Conditions such as artesian pressures, squeezing
ground, shear zones, and borehole wall instabilities will impact the piezometric
instrumentation method chosen and installation techniques required. Refer to
Appendix F for references of descriptions of other potential instrumentation methods.
General Installation Methodology
The drill casing is drilled 30 cm below the required piezometer installation elevation.
If the piezometer is intended to measure the pore water pressure at a specific
horizon, it may be necessary to drill hole to 90 cm below the required piezometer
elevation to provide room for the placement of a bentonite bottom seal.
After the drilling is completed to the required depth, the drill cuttings and other
downhole debris must be removed from inside the drill casing. The borehole is
washed to bottom, inside the drill casing, until the water emerging runs clear.
If the borehole walls are stable enough to remain open, the drill casing can be
withdrawn a certain distance above the hole bottom to allow the piezometer
installation to proceed in the open length of the borehole. This is the desired method
because the work will able to proceed in much easier fashion.
The piezometer installation will have to proceed with multiple small withdrawals of
the drill casing to minimize the risk of losing the installation if the borehole walls are
considered to be unstable or likely to cave or collapse. This method is described

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below and it will be obvious why longer drill rod or casing pulls will be more desirable
if possible.
In general, boreholes in bedrock are more stable than boreholes in soil. Boreholes in
cohesive soils are also more stable than boreholes in less cohesive, granular soils.
Bentonite Plug Method
Bentonite chips are recommended for downhole backfill work because they are a
made from solid bentonite which will not hydrate as quickly when exposed to water
compared to bentonite pellets, which are a manufactured product. Bentonite pellets
will become sticky very quickly when exposed to water and can easily clump
together, bridging inside the casing well above the target zone. Use of either
bentonite products for downhole seals should be limited to holes which are less that
20 meters, due to the difficulty involved with this method.
CAUTION: DO NOT ROTATE THE DRILL CASING.THE DRILL CASING CANNOT BE
ROTATED WHEN BEING PULLED.ROTATING THE DRILL CASING WILL LIKELY RESULT IN
DAMAGE TO THE INSTALLED PIEZOMETERS.
1Place a 60 cm bentonite seal at the bottom of the borehole to seal (if required).
2Raise the drill casing 15 cm and start placing the bentonite chips until the
bentonite level is 30 cm below the required piezometer elevation.
3Pull the drill casing as the bentonite is set in place. Be very careful not to bridge
or plug the drill casing with the bentonite.
◼This is accomplished by ensuring the bentonite level is at all times below the
casing bottom and by slowly dropping the bentonite chips one at a time down
the hole. Feeding the bentonite chips in too rapidly will result in bridging of
the chips in the drill casing or borehole. Bridging will make completing
downhole installations extremely difficult.
◼Tamping is not required because the natural swelling of the chips will provide
an adequate seal to the borehole walls once the bentonite chips are in place.
4Lower a cylindrical weight down the drill casing to the top of the bentonite plug to
ensure the hole is clear of any obstructions prior to setting filter sand in place for
the piezometer zone.
5Rinse the borehole with clean water to remove any obstructions or debris.
6Place 30 cm of fine, clean sand in 15 cm increments by dropping from surface.
The drill casing will also have to be pulled as the sand back-filling proceeds.
7Lower the piezometer into the hole and take the initial readings, as described in
Section 4.3.
8Raise the drill casing 15 cm and backfill the hole around the piezometer with fine,
clean sand. Repeat until the sand is 30 cm above the top of the piezometer.

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9Take a second reading on the piezometer.
10 Lift the casing in 15 cm increments and backfill with bentonite chips until a
minimum four-foot seal has been placed. Keep the piezometer cable taut to
prevent the bentonite chips from adhering to the wall of the drill casing during the
bentonite chip placement. Drop the bentonite chips into the hole one at a time to
avoid bridging.
◼If more than one piezometer is to be installed in the drill hole, the intervening
distance between the top of the first piezometer zone and the bottom of the
next piezometer zone can be backfilled with either cement grout or
cement/bentonite grout delivered by tremie method. The second piezometer
can then be constructed in the same general manner as described above.
11 Top off the borehole collar with grout and a protective steel collar casing once all
the drill casing has been removed from the hole.
5.5.2 Fully Grouted Method
The fully grouted method of piezometer installation involves the installation of the
vibrating wire piezometers directly within a cement-bentonite grout mixture. This
method has now become widely accepted based on the technical theory and on
extensive field testing and application. It provides a simple and accurate method to
obtain precision piezometric monitoring results. Refer to Mikkelson & Green (2003)
and Contreras et al. (2008) in Appendix F for more details on this method.
The general method described below was taken from the two above technical papers
and outlines the basic concepts and methodology of the Fully Grouted Method:
When using the fully grouted method, it is very important that proper filter saturation
is performed. This ensures that there are no air-filled voids in the filter and that
cement-bentonite grout will not be able to plug the filter stone. The best practice is to
install the piezometers upside down with the filter tips facing upwards which will
ensure that the water stays inside the filter stone. The piezometer can be inverted
and tied off to its own cable or it can be inverted and taped onto a PVC pipe which
can be used as either a downhole carrier pipe or as a tremie pipe for grout delivery.
The design of a bentonite-cement mixture is intended to approximate the strength
and deformation characteristics of the surrounding soil or rock (rather than the
surrounding permeability). The strength of the grout can be controlled by adjusting
the Water-Cement ratio which is easy to control in the field. The water and cement
are mixed first prior to adding any bentonite. This ensures that the water-cement
ratio stays fixed and the strength/modulus of the mix is more predictable. Any type of
bentonite drilling mud can be combined with Type I or II Portland Cement to make
the mix. The quantity of bentonite powder will vary depending on the grade of the
bentonite, the mixing agitation, the water pH, and the water temperature. As the
bentonite solids content increases, the mix density increases and the permeability
decreases.
The final mix point has to be carefully monitored to ensure that the completed grout
remains pumpable. Although the grout mix has a target bentonite content, it may be