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.
Page iii
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
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page iv
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.
Page 6
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
ELM0005P
RST Instruments Ltd.
Page 8
•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.

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P
RST Instruments Ltd.
Page 9
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