ROCTEST PW Series User manual

INSTRUCTION MANUAL
VIBRATING WIRE PIEZOMETER
Model PW
Roctest Limited, 2012. All rights reserved.
This product should be installed and operated only by qualified personnel. Its misuse is potentially dangerous. The Company makes no warranty as to the information furnished
in this manual and assumes no liability for damages resulting from the installation or use of this product. The information herein is subject to change without notification.
Tel.: [phone] • 1.877.ROCTEST (Canada, USA) • 33.1.64.06.40.80 (France) • 41.91.610.1800 (Switzerland)
www.roctest-group.com
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TABLE OF CONTENTS
1APPLICATIONS ........................................................................................................ 1
2PRODUCT ................................................................................................................. 1
2.1General description............................................................................................. 1
2.2Description details and specific uses.................................................................. 1
2.2.1Model PWS and PWC ................................................................................. 1
2.2.2Model PWF.................................................................................................. 2
2.2.3Model PWP.................................................................................................. 2
2.2.4Model PWL .................................................................................................. 3
2.2.5Multi-level piezometer.................................................................................. 3
2.2.6Filter types ................................................................................................... 3
2.3Operation principle.............................................................................................. 4
2.4Calibration........................................................................................................... 5
3INSTALLATION PROCEDURE................................................................................. 5
3.1Pre-installation acceptance reading.................................................................... 5
3.2Filter installation.................................................................................................. 6
3.2.1Low air entry sintered stainless steel filter................................................... 6
3.2.2High air entry ceramic filter.......................................................................... 6
3.3Initial reading ...................................................................................................... 7
3.4On site calibration check (optional)..................................................................... 8
3.5Sensor installation .............................................................................................. 9
3.5.1Installation in fill ........................................................................................... 9
3.5.2Installation in borehole............................................................................... 10
3.5.3Piezometers driven in soft ground ............................................................. 12
3.5.4Installation of a multi-level piezometer ...................................................... 13
3.6Datum reading .................................................................................................. 14
3.7Cable installation .............................................................................................. 14
3.7.1Cable identification .................................................................................... 14
3.7.2Generalities on cable routing..................................................................... 14
3.7.3Horizontal cable runs................................................................................. 15
3.7.4Vertical cable runs ..................................................................................... 16
3.8Splices .............................................................................................................. 17
3.9Cable wiring ...................................................................................................... 18
3.10Lightning protection .......................................................................................... 18
4READING PROCEDURE ........................................................................................ 19
4.1Generalities....................................................................................................... 19

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4.2Taking measurements ...................................................................................... 19
4.3Quick verification of measurements.................................................................. 20
5CONVERSION OF READINGS............................................................................... 20
5.1Pressure value.................................................................................................. 20
5.2Temperature value............................................................................................ 22
5.3Temperature and barometric corrections.......................................................... 22
6TROUBLESHOOTING ............................................................................................ 23
6.1Unstable reading............................................................................................... 23
6.2No reading ........................................................................................................ 24
6.3Temperature troubles ....................................................................................... 24
6.4Other troubles ................................................................................................... 25
7MISCELLANEOUS.................................................................................................. 25
7.1Environmental factors ....................................................................................... 25
7.2Conversion factors............................................................................................ 26

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1 APPLICATIONS
The PW series vibrating wire piezometer is designed to measure pore water or other
fluid pressures. It is used to monitor engineering works such as hydraulic structures
(dams, embankments, land fill dikes…), foundations, retaining walls, excavations,
tunnels, etc.
The vibrating wire technology used inside the PW piezometer provides the necessary
ruggedness, reliability, stability and ease of remote monitoring.
2 PRODUCT
2.1GENERAL DESCRIPTION
The PW piezometer is made from a rigid cylindrical body, enclosing the sensing element.
The later consists in a thin flexible diaphragm where a steel wire is attached. All parts of
the sensor other than the wire are machined from a high-grade stainless steel, selected
for its high yield and corrosion resistance.
A standard internal thermistor allows the measurement of the temperature. The PW
piezometer is also fitted with a surge protector and resists electrical and radio frequency
interferences as determined by tests compliant to IEEE and CEI specifications.
The transducer is fitted with several protections against water intrusion: watertight
connector, potting, watertight feed through header, and o-rings.
2.2DESCRIPTION DETAILS AND SPECIFIC USES
Although the principle of all PW series piezometers is the same, there are some
important differences in their designs to fit with their specific applications.
Moreover, at section 2.2.5 of this manual, you will find the vibrating wire multi-level
piezometer, whixh is perfect for measuring pore pressures at various depths beneath a
single surface location. The time proven design of the PW-series sensors allows them to
be tied to the multi-level piezometer core cable. For more information about the
specifications of the multi-level piezometer, please read the datasheet.
2.2.1 MODEL PWS AND PWC
The PWS piezometer model is designed to be embedded in earth fills and concrete, or
inserted into boreholes and pipes as small as 19 mm (¾”).
It consists of a small diameter cylindrical housing containing a pressure transducer and a
thermistor. The cable output is protected by a cable gland and sealed with an epoxy
compound.

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A filter can be 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 and
saturation. It can also be replaced with a pipe-threaded adapter to use the gage as a
pressure transducer (PWC Model).
Figure 1: Model PWS piezometer
2.2.2 MODEL PWF
The PWF piezometer model is a thick wall version of the PWS piezometer for use in
direct burial applications. Similar to the PWS model, a pipe-threaded adapter or a filter
can be set in the front end of its housing.
2.2.3 MODEL PWP
The PWP piezometer model is designed for driving into unconsolidated fine grain
materials such as sand, silt, or clay.
The external housing is a thick walled cylinder fitted with a conical head at one end, and
a male thread adapter at the cable entry, fitting "EW" standard drill rods. Four portholes
above the head hold stainless steel filters. The cable passes through the threaded end,
and may be fed through push rods leading to the surface. The cable entry is sealed with
a cable gland and an epoxy compound to avoid any water penetration into the
piezometer.
Figure 2: Model PWP piezometer

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2.2.4 MODEL PWL
The PWL piezometer model is a low pressure sensor. Similar to the PWS model, a
pipe-threaded adapter or a filter can be set in the front end of its housing.
2.2.5 MULTI-LEVEL PIEZOMETER
The multi-level piezometer provides significant advantages in terms of time, resources
and cost reduction related to installation, while providing quick and reliable reading at
various depths.
Moreover, the multi-level piezometer allows the customer to define sensor spacing along
the readout cable providing precise depth control based on site specific requirements.
Junctions between each piezometer and the main cable are reinforced usinf epoxy resin
for a waterproof and durable seal. You will find information about typical installation in
section 3.5.4.
2.2.6 FILTER TYPES
Two types of filters are available: high air entry ceramic or low air entry sintered stainless
steel filters.
The table below summaries the main differences between the two filter models.

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Stainless steel filter
low air pressure entry
pore diameter: ~50 µm
Ceramic filter
high air pressure entry
pore diameter: ~1 µm
Filter generally used. Filter usually installed for use in
unsaturated fine grain material.
Does not allow suction measurements.
If water level drops below the piezometer
and that a suction builds up, the filter can
de-saturate. But as soon as the water level
comes up, it will re-saturate easily.
Allow measuring suction to -100 kPa.
If negative pressure is more important, the
filter will de-saturate and readings will
become incorrect.
Air entry pressure: ~10 kPa Air entry pressure: ~450 kPa
Small time lag. More important time lag.
Easy to saturate and install. Need to be saturated under vacuum.
Allows fine grain infiltration. Helps prevent fine grain infiltration.
Table 1: Differences between filter models
Saturation of the filter is necessary to ensure hydraulic continuity between the
piezometer diaphragm and the pore water in unsaturated soils. It reduces the possibility
of filter clogging and also decreases response time.
2.3OPERATION PRINCIPLE
The sensing element of the pressure transducer is a piano wire attached to a diaphragm.
The latter is in contact with the fluid around and a pressure variation change the
diaphragm position, affecting the wire tension. The tension is directly proportional to the
square of the resonant or natural frequency of the wire.
In operation, plucking voltages are applied to a coil and a magnet located near the wire
in a spectrum of frequencies, spanning the natural wire frequency, thus forcing the wire
into vibration. The oscillation of the wire generates a voltage in the coil. This signal is
amplified by the readout unit, which also discriminates against harmonic frequencies, to
determine the resonant frequency of the wire.

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The relationship between the period Nand the strain in the vibrating wire is expressed
by the following equation:
2
9
10
N
K
where = strain in micro-strain
N= vibration period in microseconds
K= gage constant, specific for each type of gage
The vibrating wire technology offers the unique advantage of a frequency output signal
virtually unaffected by line impedance, or contact resistance. Cable length of several
kilometres can be used without signal deterioration.
A portable unit as the MB-3TL (or MB-6T(L)) can be used to read vibrating wire sensors. It
proceeds to all necessary operations: wire excitation, signal conditioning and reading
display. Contact Roctest – Telemac for further information.
2.4CALIBRATION
A calibration data sheet is supplied with each sensor. It enables conversion of gross
readings into pressure values and temperature correction.
All the sensors are individually calibrated over their working range before shipment. The
calibration factors and the thermal coefficient are established by running the calibration
data points through a linear or polynomial regression formula.
During the calibration process, the sensors undergo a mechanical cycling and are tested
at 40 degrees Celsius.
3 INSTALLATION PROCEDURE
3.1PRE-INSTALLATION ACCEPTANCE READING
Reading of all instruments should be taken as the piezometers are received to ensure
they have not been damaged during shipment or handling on site.
Take a reading in linear units with the piezometer in air, without any filter and in the
same position in which it is to be installed. Then compare it with the factory reading
shown on the calibration sheet. The difference should not be more than twenty linear
units. It can come from hits or vibrations during the shipment and/or from the fact that
temperature and barometric pressure on site are certainly different than those at the
factory, during calibration process. However, this has no effect on the linearity of the
sensor.
Take also a reading of the temperature gage to be sure the thermistor is working properly.

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For details about how to take readings or how to convert frequency into linear units, please
refer to chapter 4 page 19 (Reading Procedure).
3.2FILTER INSTALLATION
3.2.1 LOW AIR ENTRY SINTERED STAINLESS STEEL FILTER
The filter is shipped already mounted on the piezometer body. It should be removed by
holding the piezometer in one hand, pulling, and twisting the filter housing with the other
hand.
Note:The filter on a PWP piezometer is removed by unscrewing the conical head.
Immerse the filter and the piezometer pointing upwards in clean water for approximately
fifteen minutes. De-aired water can be used (water may be de-aired by boiling it for a few
minutes or by applying a vacuum). Tap piezometer housing lightly to remove any air
bubbles present.
Reassemble the filter working underwater. For lower pressure range transducers, while
pushing on the filter assembly, check the readings on the readout unit to avoid pressure
variations that exceed the piezometer range and then can damage the sensor.
As soon as the filter is saturated, it must stay submerged until installation.
3.2.2 HIGH AIR ENTRY CERAMIC FILTER
The filter is delivered cased in a metallic housing and pre-saturated in water. Final
installation must be carried out under water to avoid any air intrusion.
Connect the piezometer to a readout unit.
Put the piezometer upside-down in a container filled with clean water. The
piezometer tip should be up and completely submerged.
With the piezometer submerged, firmly grasp the housing with both hands and
push the filter assembly into its tip. Gradually increase the pushing force using
thumbs until it is fully inserted. This step may take a while to be completed.
Some resistance will be met; it is normal and due to the small porosity of the
ceramic filter. While pushing on the filter assembly, check the readings on the
readout unit to avoid excessive pressure variations that could damage the
piezometer. Refer to the calibration sheet for pressure range.
The filter installation is now completed.
Keep the piezometer underwater for at least 24 hours before proceeding with
field installation. This will allow the pressure induced by the filter installation to
decrease. When ready to be installed, the piezometer reading should be close to
the one of acceptance test with the filter removed (see detail in section on initial

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reading).
3.3 INITIAL READING
Before installing the piezometer in its final location on site, an initial reading has to be
taken to correctly convert readings in linear units into pressure measurements when the
piezometer is in operation. This process is also necessary to be able to apply later
temperature and barometric corrections.
Leave the instrument in water for two or three hours for the temperature to stabilize. It is
preferable to leave it overnight provided that arrangements are made to keep the water
at more or less uniform temperature. All sudden changes of temperature and exposure
to the sun must be avoided.
With the water temperature, ambient temperature and piezometer temperature in
equilibrium, raise the piezometer by the electrical cable until it is out of the water except
two centimetres. Take three successive readings to ensure a stable value and record it.
This is the initial reading (called L0). Record ambient temperature (T0) and barometric
pressure (S0).
Compare readings in linear units before and after the filter installation. They should be
close. A small difference on raw readings comes from the fact that temperature and
barometric pressure may have changed in the meantime. A difference of more than ten
linear units may indicate that the filter is not correctly saturated or the pressure induced
during its installation is not dissipated yet.
For details about how to take readings or how to convert frequency into linear units,
please refer to chapter 4 page 19 (Reading Procedure).
A special calculation has to be done if using the polynomial relation to convert raw
readings into pressure measurements. The coefficient C' of the calibration sheet must be
calculated because it depends on temperature and barometric pressure on site, which
are different from those in factory. Use the following relation:
0
2
0BLALC 

where: C' = new calibration factor in kilopascal
A,B = calibration factors (see calibration sheet)
L0= initial reading in linear unit