Geosense VWNPC-3000 User manual

V1.4 March 15
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NATM STRESS CELLS
VWNPC-3000

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CONTENTS
Page
1.0 INTRODUCTION
1.1 General Description 3
1.2 How it works 4
1.3 Application 5
2.0 CONFORMITY 6
3.0 MARKINGS 7
4.0 DELIVERY 8
4.1 Packaging 8
4.2 Handling 8
4.3 Inspection 8
4.4 Functionality test & zero readings 9
4.5 Storage 10
5.0 INSTALLATION 11
5.1 Cell installation 11
5.1.2. Tangential cell VWNPC-3000 11
5.1.3 Radial cell VWNPC-3010 13
5.1.4 Initial readings 14
5.2 Re-pressurising the cell 14
5.3 Cable 16
5.3.1 Cable installation 16
5.3.2 Cable marking 16
5.4 Tools 16
6.0 DATA HANDLING 17
6.1 Monitoring the readings 17
6.1.1 Portable readouts 17
6.1.2 Data loggers 17
6.2 Data reduction 18
6.3 Calibration certificate 21
6.4 Pressure calculation 22
6.5 Stress value 22
6.6 Temperature compensation 23
7.0 MAINTENANCE 25
8.0 TROUBLESHOOTING 25
9.0 SPECIFICATION 26
10.0 SPARE PARTS 26
11.0 RETURN OF GOODS 27
12.0 LIMITED WARRANTY 28

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It is VITAL that personnel responsible for the
installation and use of the NATM Cells READ and
UNDERSTAND the manual, prior to working with the
equipment.
1.1 General Description
The primary uses for NATM Cells are :-
The measurement of stress in concrete
Particular features of Geosense® Vibrating wire are:-
Reliable long term performance.
Rugged; suitable for demanding environments.
High accuracy.
Insensitive to long cable lengths.
The Pressure sensor is based upon ‘industry standard’ Vibrating Wire technology.
When electronically excited, the sensor produces an output signal in the form of an
alternating current. The frequency of the alternating current can then be readily
converted to a fluid pressure by applying individual calibration factors.
Frequency signals are particularly suitable for the demanding environment of civil
engineering applications, since the signals are capable of long transmission distances
without degradation. They are also somewhat tolerant of damp wiring conditions and
resistant to interference from external electrical noise.
Geosense® NPC-3000 NATM Cells are supplied in various configurations to suit
varying installation environments and techniques. Each cells is fitted with a length of
connecting cable, an internal temperature sensor and a surge arrestor.
1.0 INTRODUCTION
This manual is intended for all users of Geosense® NPC-3000 NATM Cells and provides
information on their installation, operation and maintenance.

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1.2 How it works
1 - Sensor cable
2 - VW sensor (no elasticity)
3 - Stress cell body - comprises of two stainless steel rectangular plates welded
together around their periphery, leaving a thin space between the plates which is then
filled with an relatively incompressible de-aired fluid. Pressure applied normal to the plate
is balanced by a corresponding build up of internal fluid pressure which is measured by
the sensor.
4 - Mounting lug - provided at each corners of the cell body to fix the cells in place while
the shotcrete is applied.
5 - Pressure (pinch) tube - this is filled with a relatively incompressible de-aired fluid
and is connected at one end to the fluid filled space between the plates and the other
end is capped. The purpose of this tube is to inflate the cell once the concrete is cured.
6 - End cap
1
25
3
4
6

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1.3 Application
The "New Austrian Tunnelling Method", or NATM, calls for the support of a tunnel by
the rapid application of shotcrete to the freshly exposed ground. The theory behind
this method of support, particularly useful in weaker ground, is that if the inherent
strength of the ground can be preserved, it will be almost self-supporting and will
require much less artificial support in the form of concrete or steel. To preserve the
inherent cohesion of the ground it is necessary to prevent it from breaking up in the
first place and, hence, the need for a rapidly applied layer of shotcrete.
The graph below shows the ground reaction curve which is the amount of support
needed versus the amount of inherent support and ground deformation. In order to
prevent any support deformation it requires a support pressure exerted onto the
tunnel wall to be equal to the original in-situ ground stress.
NATM cells are used to monitor and thus calculate the support pressure and in
combination with other monitoring devices such as a tape extensometer allows an
assessment as to how much shotcrete lining is required and can be used to alter the
amount as construction progresses.
This therefore provides the most cost effective lining to be installed.
Stress
Convergence
Critical convergence

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2.0 CONFORMITY
Geosense Ltd
Nova House
Rougham Industrial Estate
Rougham, Bury St Edmunds
Suffolk , IP30 9ND
United Kingdom
Tel: [phone] Fax: [phone]
Email: [email protected].uk, Web: www.geosense.co.uk
Declaration of Conformity
We Geosense Ltd at above address declare under our sole responsibility that the
Geosense products detailed below to which this declaration relates complies with protection requirements
of the following harmonized EU Directives,
The Electromagnetic Compatibility Directive 2014/30/EU
Restriction on the use of certain Hazardous Substances RoHS2 2017/2102/EU
Waste electrical & electronic equipment WEEE 2012/19/EU
Equipment description Vibrating Wire NATM Stress Cells
Make/Brand Geosense
Model Numbers VWNPC-3000, VWNPC-3010
This equipment has been designed and manufactured with reference to the following standards:
All mechanical drawings used in the production of this equipment are based upon BS 8888
Electrical/electronic drawings are based upon BS 3939.
A technical file for this equipment is retained at the above address
This Declaration of Conformity was prepared according to EN ISO/IEC 17050-1:2004.
Martin Clegg December 2020
Director

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3.0 MARKINGS
Geosense® NPC-3000 NATM Cells are labelled with the following information:-
Manufacturers website & telephone number
Product type
Pressure range
Model
Serial number
Cable length
CE mark
Maximum pressure

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4.3 Inspection
It is important to check all the equipment in the shipment as soon as possible after
taking delivery and well before installation is to be carried out. Check that all the
components detailed on the documents are included in the shipment. Check that the
equipment has not been physically damaged.
ALL Geosense® NPC-3000 NATM Cells carry a unique identification serial number
which is located on the cable close to the pressure sensor body and at the free end of
the cable. All are supplied with individual calibration sheets that include their serial
numbers and these will shipped with the cells.
Calibration Sheets contain VITAL information about the NATM
Cell. They MUST be stored in a safe place. Only copies
should be taken to site.
(Continued on page 9)
4.0 DELIVERY
This section should be read by all users of Geosense® NPC-3000 NATM Cells.
4.2 Handling
Whilst they are a robust devices, Geosense® NPC-3000 NATM Cells are precision
measuring instruments. They and their associated equipment should always be
handled with care during transportation, storage and installation.
Once the shipment has been inspected ( see below ), it is recommended that cells
remain in their original packaging for storage or transportation.
Cable should also be handled with care. Do not allow it to be damaged by sharp
edges, rocks for example, and do not exert force on the cable as this my damage the
internal conductors and could render the installation useless.
4.1 Packaging
Geosense® NPC-3000 NATM Cells are packed for transportation to site. Packaging
is suitably robust to allow normal handling by transportation companies.
Inappropriate handling techniques may cause damage to the packaging and the
enclosed equipment. The packaging should be carefully inspected upon delivery and
any damage MUST be reported to both the transportation company Geosense®.

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4.4 Functionality test & zero readings
CHECK the Cells ‘Zero Readings’ against
the factory ‘Zero Readings’ on arrival to
ensure they have not changed due to
damage during transportation. To do this,
connect a Vibrating Wire readout to the
bare cable ends (Black and Red
conductors). – See readout manual for
connection guidance.
*NB If the readout display is in
‘Period’ units ( eg 0.03612 ) a
calculation must be performed to
convert to Hz2/1000 ( Linear Digits )
units, since the calibration sheet is
presented in Hz2/1000 units. The
Geosense Readout model VW200
displays the readings in ‘Period’. The
RST readout / logger unit Model
Number VW2106 displays the readings
in Linear digits. See Section 6 of this
manual for more information on units
and conversion routines.
Prior to carrying out a ‘Zero Reading’
CHECK, ensure that the NATM Cells have
been stored in a reasonably stable
temperature for at least 30– 60 minutes.
Record the values displayed on the readout ( and units ) against the NATM Cell serial
numbers. If these ‘out of the box’ CHECK readings show significant differences ( +/-
40 digits ) to the zero pressure values on the calibration sheets, contact Geosense for
assistance. ( It should be noted that the ‘CHECK Readings WILL be affected by the
atmospheric pressure & altitude ).
If components are missing or damaged, contact the delivery company, the supplier
and / or Geosense™.
(Continued from page 8)

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4.4 Storage
Whilst Geosense® NPC-3000 NATM Cells are relatively robust they are a precision
instrument and if they need to be stored care should be taken to protect them against
any impact or weight.
Ideally they should be stored in their original packaging but if not do not stack units
on top of each other or any other item that could damage the sensor or connecting
pipework from the cell body.
It is recommended that they be stored in a dry environment to prevent moisture
migrating along the cables and storage areas should be free from rodents as they
have been known to damage connecting cables.

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It is VITAL that personnel responsible for the installation and
use of the NATM cells READ and UNDERSTAND the manual,
prior to working with the equipment.
**********
As stated before, it is vital to check all the equipment in the shipment soon after
taking delivery and well before installation is to be carried out. Check that all
components that are detailed on the shipping documents are included.
5.0 INSTALLATION
This section of the manual is intended for all users of Geosense® NPC-3000 NATM
Cells and is intended to provide guidance with respect to their installation.
It must be remembered that no two installations will be the same and it is inevitable
that some ‘fine tuning’ of the following procedures will be required to suit specific site
conditions.
5.1.2 Tangential cell (VWNPC-3000)
Typical installation is by using short
pieces of steel rebar grouted inside short
boreholes and protruding into the area where
the lining will be placed.
STEP 1 - place a VWNPC-3000 tangential
stress cell onto two horizontal re-bar mountings.
STEP 2 - carefully bend the pressure tube 90
degrees away from the tunnel wall so that it
protrudes through the shotcrete lining and can
be accessed for re-pressurising.
STEP 3 - tie to the re-bars using soft iron wire
connected to the lugs at the corners of the cell.
5.1 Cell Installation
Cells are positioned on the wall of the tunnel in two ways, one way to monitor tangential
stresses and the other to measure radial stresses.
CELL MUST BE INSTALLED
PERPENDICULAR TO TUNNEL
LINING

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5.1.2 Tangential cell (VWNPC-3000) contd..
STEP 4 - fix the cable firmly to other pieces of
rebar or to the reinforcing mesh (if one is
used) and feed out to the readout location
which typically is a junction box.
The cable is terminated inside this box.
Sufficient cable should be coiled inside the
box to allow it to be pulled out and connected
to a portable readout box.
It is very important that the concrete makes intimate contact with the
stress cell. Therefore the concrete should be sprayed evenly around the
cell ensuring no voids exist.

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5.1.3 Radial cell (VWNPC-3010)
STEP 5 - Install re-bar sections or nails
adjacent to the cell location so that the radial
cell can be attached to them.
STEP 6 - make a pad of quick setting mortar
onto the surface of the tunnel wall.
STEP 7 - place the VWNPC-3010 radial
stress cell onto the pad and press firmly so
that mortar is displaced from underneath it to
eliminate any air bubbles or spaces between
the cell and the ground.
STEP 8 - carefully bend the pressure tube so
that it will protrude out of the shotcrete lining.
STEP 9 - tie to the re-bars using soft iron wire
connected to the lugs at the corners of the
cell.
STEP 10 - fix the cable firmly to other pieces
of rebar or to the reinforcing mesh (if one is
used) and feed out to the readout location
which typically is within a junction box.
The cable is terminated inside this box.
Sufficient cable should be coiled inside the
box to allow it to be pulled out and connected
to a portable readout box.

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5.1.4 Initial readings
5.2 Re-Pressurising the Cell
During concrete curing, temperatures very often rise and will cause the cell to expand in
the still green concrete. On cooling, the cell contracts leaving a space between it and the
surrounding concrete which, if allowed to remain, would prevent the transmission of
pressures from the concrete to the cell. Therefore the cell will require to be
re-pressurised in order to measure this pressure.
STEP 1 - connect the sensor cable to the readout (for full
details see page 9)
STEP 2 - start crimping the re-pressurisation tube from
the end cap end with the special crimping tool.
Check the readings on the readout as the tube is
crimped. If there is a relatively large gap between the cell
and the surrounding concrete each crimp will result in a
small value increase in pressure. If the cell is already in
good contact with the concrete then a large value will be
seen.
ONCE TEMPERATURE HAS STABILISED
TAKE INITIAL PRESSURE & TEMPERATURE
BEFORE RE-PRESSURISING
DO NOT START CRIMPING THE TUBE
CLOSER THAN 25MM FROM THE END CAP
STEP 3 - plot a graph of pressure against the number of
crimps.
If the gap between the cell and the concrete is relatively
large then the curve will be as graph 1 on page 15.
If the gap between the cell and the concrete is small the
curve on the graph will be as graph 2 on page 15.
BEFORE SHOTCRETING TAKE INITIAL
PRESSURE & TEMPERATURE READINGS

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oSTOP CRIMPING
Increase in pressure
(decrease in digits)
Crimp number
5.2 Re-Pressurising the Cell contd...
It is recommended that results are graphed so that site characterisation can be obtained.
oSTOP CRIMPING
Increase in pressure
(decrease in digits)
Crimp number
GRAPH 1 - a gradual increase in pressure occurs showing that there is initially little
contact between the cell and the concrete. As the cell starts to become in contact with
the concrete the pressure starts to build up quickly. Once this occurs stop crimping.
GRAPH 2 - a quick increase in pressure occurs showing that the cell is already in good
contact with the concrete. Once this occurs stop crimping.

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5.2 Re-Pressurising the Cell contd..
However, it is also possible that the cell is already in good contact with the concrete, so
that crimping will immediately cause a pressure rise in the cell. If this is the case then
cease crimping immediately.
Continued crimping after the cell has made good contact could cause the concrete
around the cell to split open which is not desirable and could lead to erroneous readings.
A slight relaxation of the cell after the re-pressurization procedure is normal and should
drop the cell pressure to a value roughly equal to the concrete stress. From this point on,
the cell pressure should then be equal to the absolute concrete stress. Record the new
initial pressure after the cell has stabilized.
While the concrete is curing it will be a good idea to take simultaneous readings of
temperature and pressure for the purpose of developing a temperature correction factor.
See section 6.
5.3 Cable
5.3.1 Cable Installation
The cable should be protected from accidental damage caused by moving equipment.
This is best done by putting the excess cable inside a junction box (Cables may be
spliced to lengthen them, without affecting gauge readings. Waterproof splice kits are
available from Geosense®.
5.3.2 Cable marking.
Cables should be marked with unique identification. Markings should be repeated at
regular intervals along the cable where multiple cables are to be grouped together, so
that in the event of cable damage, there may be a chance that the identification could be
exposed and the cables re-joined. Multiple cable marks are particularly important close
to the end of the cable. The spacing of markings can vary according to specific site
requirements but a guide of 5m to 10m is commonly applied (available on request).
5.4Tools.
5.4.1 Tools list
Obtain any tools necessary to carry out the installation. The following is a brief list of
tools typically used during the installation of Geosense® NPC-3000 NATM Cells:-
• NATM Crimper - to re-pressurise the cell.
• Wire cutters and strippers
• Vibrating Wire Readout unit for checking the NATM cell function
• Cable Marking system / equipment ( eg coloured PVC Tapes )
• Tie wire/ Cable ties - for installing NATM cells and cable.

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6.0 DATA HANDLING
The function of an instrument is to provide useful and reliable data.
Accurate recording and handling of the data is essential if it is to be
of any value.
6.1 Monitoring the NATM cell readings
Geosense® NPC-3000 NATM Cells include both pressure and temperature sensors.
NATM cells are installed in a zone where the temperature is likely to fluctuate
significantly and therefore records of both pressure and temperature data should be
recorded. They can then be used to assess any temperature effects on the pressure
readings.
6.1.1 Portable Readouts
Geosense® offer a range of readout and data logging options. Specific operation
manuals are supplied with each readout device.
Below is a brief, step-by-step procedure for use with
the RST VW2106 portable readout.
1. Connect signal cable from the sensor to the
readout following the wiring colour code.
Conductor colours may vary depending upon
the extension cable used. Commonly these
are:
RED =VW +
BLACK =VW -
GREEN =Temp
WHITE =Temp
2. Switch on the unit and, where necessary, select range B
3. The readout displays the Vibrating Wire reading ( in Hz2/1000 - Linear Digits )
and a temperature reading in degrees C.
Whilst it is not critical that the polarity be observed for most Vibrating Wire
instruments, a better signal may be obtained if the correct polarity is adopted. Since
the temperature sensor is a Thermistor, its connection polarity is not important.
6.1.2 Data Loggers
A number of data loggers are available to automatically excite, interrogate and record
the reading from Vibrating Wire instruments. These include devices manufactured by
Geosense® / RST in both single and multi-channel configurations, as well as
equipment manufactured by independent suppliers.
Geosense® configure and supply equipment manufactured by both Campbell
Scientific Ltd and DataTaker Ltd. These are the most commonly adopted third party
manufacturers of data loggers that can be used with Vibrating Wire Instruments.
(Continued on page 18)

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Specific configuration and programming advice can be obtained from Geosense.
6.2 Data Reduction
Overview
The tension of a sensor wire can be measured by detecting the frequency (note) at
which it naturally vibrates. The following is a description of the units commonly used
by the instrumentation industry.
Frequency Units ( Hz ). If the wire is ‘excited’ electronically the frequency at which it
vibrates can be measured. The units used to express frequency are Hertz (Hz) or
Kilohertz (kHz).
The disadvantage of these units is that there is no ‘linear’ conversion from ‘change in
Hertz’ to ‘change in wire tension’.
Linear Digits ( B ). In order to overcome the problem of a linear conversion
described above, the frequency value can be squared, thereby rendering it linear, but
quite large. To reduce its size, it is often divided by 1000 (or multiplied by 10-3). The
expression Hz2/1000 (or Hz2 x 10-3) is the most commonly adopted as a ‘linear’ digital
output.
Period Units ( P ). Some readout devices utilise the ‘counter’ function available in
many common integrated circuits.
Period Units represent the time taken for the wire to vibrate over one full oscillation,
expressed in seconds. Due to the very small size of the number generated most
equipment manufacturers display the unit multiplied by either 1000 ( 103 ) or
10000000 ( 107).
The relationship between Period units and Frequency units is expressed as
P = 1
Frequency
Period units are convenient to measure but to convert the readings to units of
pressure, calibration factors must be applied to the recorded values. For most
Vibrating Wire sensors, these factors are unique and are detailed on the sensor
calibration sheet. A unique calibration sheet is supplied with all Geosense® NPC-
3000 NATM Cells (see page 21).
If the readout display is in Period units ( e.g. 0.03612 or 3612 - depending upon the
readout used ) the first step to producing an engineering value is to convert the
reading to Linear Digits ( Hz2/1000 ) . Two examples of this calculation can be seen
below. The first (1) where the readout includes a decimal point and displays the
Period in Seconds–2 and the second (2) where the readout displays the Period in
Seconds-7
(1) Readout Display =0.03612
Linear Digits (Hz2/1000) = ( 1 / 0.03612 x 10 –2 ) 2 / 1000
(Continued from page 17)
(Continued on page 19)

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=7664.8
(2) Readout Display = 3612
Linear Digits (Hz2/1000) = ( 1 / 3612 x 10 –7 ) 2 / 1000
=7664.8
If the readout displays ‘Frequency’ values, ( e.g. 2768.5 Hz ) only a simple calculation
is required to convert the readings to Linear Digits.
Linear Digits (Hz2/1000) = ( 2768.5 ) 2 / 1000
=7664.6
Certain data loggers store their Vibrating Wire data in Linear Digits but further divided
by 1000. In this case the data would have to be multiplied by a further 1000 to
maintain the standard Linear Digits (Hz2/1000) format for standard calculations.
There are many ways to achieve the conversion from recorded data to useful
engineering values. The following are included as a guide only and as a basis for
alternative approaches.
Linear Calculation
This is the most simple calculation to convert ‘raw’ data to engineering units. It can
be easily carried out using a simple calculator. It assumes that the reading is in
Linear Digits ( Hz2/1000 ). Where this is not the case, the reading should be
converted to these units prior to application of the calibration factors. For most
applications this equation is perfectly adequate and is carried out as follows:
Pressure ( KPa) = Linear Factor for KPa (k) x ( Current Reading - Base Reading )
Polynomial Calculation
The polynomial calculation can be more precise as it accommodates any slight
deviation from a perfect linear correlation. However to use the polynomial equation
the “C” Constant for the environment must be calculated using a “Site Zero” Reading
at atmospheric pressure.
Once the Site “C” Constant is established the polynomial formula can be used to
convert Raw Data to Engineering Units.
The above formula essentially gives the relative change in the variable being
measured
Where the Pressure is required in an alternative format, mH2O for example, a simple
conversion using standard conversion factors can be applied to each factor or at the
end of the equation. (1 psi = 0.7031 mH2O for example).
(Continued from page 18)
(Continued on page 20)

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An instrument calibration sheet similar to the example on page 21 includes the
following information:
Model This refers to the Geosense® model number.
Serial Number This is a unique sensor identification number that can be found
on the cable just behind the NATM cell body and, for long
cables, at the end of the cable.
Works ID Unique works batch and range code
Cal Date Date the calibration was performed
Baro Barometric Pressure at the time of calibration
Temp oCTemperature at which the NATM cell was calibrated
DPI No. Serial number of the Digital Pressure Indicator used in
conjunction with the pressure generator
Readout No. Serial Number of the readout used to display the transducer
output
R/O Cal Date The date on which the Readout was calibrated to a traceable
standard
Applied Pressure Pressure applied to the transducer as part of the calibration
cycle in both psi and kPa
Readings [digit] Readings from the transducer as pressure is applied and as
pressure is reduced, in steps. The average is calculated.
Calculated Pressure Calculation of the applied pressure using the calculated Linear
and Polynomial for comparison with the actual Applied
Pressure.
Non Lin. % FSO Non Linearity expressed as a percentage of the transducers
Full Scale.
Calibration Factors ‘Linear’ and ‘Polynomial’ factors are provided for a selection of
engineering units ( other units can be calculated directly from
the kPa values ). Examples of calculated values are detailed
on the next page.
(Continued from page 19)
(Continued on page 22)