Geokon 4200 Series User manual

©2018, GEOKON. All rights reserved.
Document Revision: BB | Release date: 12/4/18
Model 4200 Series
Vibrating Wire Strain Gauges
Instruction Manual

WARRANTY STATEMENT
GEOKON warrants its products to be free of defects in materials and workmanship,
under normal use and service for a period of 13 months from date of purchase. If the
unit should malfunction, it must be returned to the factory for evaluation, freight
prepaid. Upon examination by GEOKON, if the unit is found to be defective, it will be
repaired or replaced at no charge. However, the WARRANTY IS VOID if the unit shows
evidence of having been tampered with or shows evidence of being damaged as a
result of excessive corrosion or current, heat, moisture or vibration, improper specifi-
cation, misapplication, misuse or other operating conditions outside of GEOKON’s
control. Components that wear or are damaged by misuse are not warranted. This
includes fuses and batteries.
GEOKON manufactures scientific instruments whose misuse is potentially dangerous.
The instruments are intended to be installed and used only by qualified personnel.
There are no warranties except as stated herein. There are no other warranties,
expressed or implied, including but not limited to the implied warranties of merchant-
ability and of fitness for a particular purpose. GEOKON is not responsible for any
damages or losses caused to other equipment, whether direct, indirect, incidental,
special or consequential which the purchaser may experience as a result of the instal-
lation or use of the product. The buyer’s sole remedy for any breach of this agreement
by GEOKON or any breach of any warranty by GEOKON shall not exceed the purchase
price paid by the purchaser to GEOKON for the unit or units, or equipment directly
affected by such breach. Under no circumstances will GEOKON reimburse the
claimant for loss incurred in removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or
software, however, GEOKON neither assumes responsibility for any omissions or
errors that may appear nor assumes liability for any damages or losses that result
from the use of the products in accordance with the information contained in the
manual or software.
No part of this instruction manual may be reproduced, by any means, without the written consent of GEOKON. The
information contained herein is believed to be accurate and reliable. However, GEOKON assumes no responsibility
for errors, omissions or misinterpretation. The information herein is subject to change without notification.
The GEOKON® wordmark and logo are registered trademarks with the United States Patent and Trademark Office.

I
TABLE OF CONTENTS
1. INTRODUCTION............................................................................................................................................1
2. MODELS ................................................................................................................................................................2
2.1 MODELS 4200 AND 4200L ........................................................................................................2
2.2 MODELS 4200-6 AND 4200-7 .................................................................................................3
2.3 MODEL 4202............................................................................................................................................3
2.4 MODEL 4210............................................................................................................................................4
3. PRIOR TO INSTALLATION...............................................................................................................5
3.1 ADJUSTING GAUGE TO THE DESIRED RANGE .......................................................5
3.2 GAUGE AND CABLE ASSEMBLY ...........................................................................................5
3.3 PRELIMINARY CHECK ....................................................................................................................5
4. INSTALLING THE GAUGES IN CONCRETE..................................................................7
4.1 DIRECT ATTACHMENT TO REBAR.......................................................................................7
4.2 SUSPENSION METHOD .................................................................................................................7
4.3 ALTERNATIVE SUSPENSION METHOD...........................................................................8
4.4 USING PRE-CAST BRIQUETTES, SHOTCRETE, OR GROUTING.................9
4.5 CABLE SPLICING AND TERMINATION............................................................................9
4.6 LIGHTNING PROTECTION............................................................................................................9
5. TAKING READINGS ............................................................................................................................. 11
5.1 MICRO-6000 DATALOGGER .................................................................................................. 11
5.2 EMBEDMENT STRAIN GAUGE READOUT POSITIONS.................................. 11
5.3 GK-404 READOUT BOX.............................................................................................................. 11
5.3.1 OPERATING THE GK-404 ........................................................................................................... 12
5.4 GK-405 READOUT BOX.............................................................................................................. 12
5.4.1 CONNECTING SENSORS WITH 10-PIN BULKHEAD CONNECTORS
ATTACHED ....................................................................................................................................... 13
5.4.2 CONNECTING SENSORS WITH BARE LEADS................................................................... 13
5.4.3 OPERATING THE GK-405 ........................................................................................................... 13
5.5 GK-403 READOUT BOX (OBSOLETE MODEL) ........................................................ 14
5.5.1 CONNECTING SENSORS WITH 10-PIN BULKHEAD CONNECTORS
ATTACHED ....................................................................................................................................... 14
5.5.2 CONNECTING SENSORS WITH BARE LEADS................................................................... 14
5.5.3 OPERATING THE GK-403 ........................................................................................................... 14
5.6 MEASURING TEMPERATURES ........................................................................................... 14
6. DATA REDUCTION ................................................................................................................................15

II
6.1 READOUT BOX POSITION A .................................................................................................. 15
6.2 READOUT BOX POSITION B................................................................................................... 15
6.3 READOUT BOX POSITIONS D & E.................................................................................... 15
6.4 STRAIN RESOLUTION.................................................................................................................. 16
6.5 TEMPERATURE CORRECTIONS .......................................................................................... 16
6.5.1 MODEL 4200-6 / 4200-7 CORRECTIONS.............................................................................. 17
6.6 SHRINKAGE EFFECTS.................................................................................................................. 17
6.7 CREEP EFFECTS ................................................................................................................................17
6.8 EFFECT OF AUTOGENOUS GROWTH............................................................................. 18
6.9 CONVERTING STRAIN TO LOAD........................................................................................ 18
6.10 EFFECTIVE MODULUS.............................................................................................................. 18
7. TROUBLESHOOTING........................................................................................................................... 19
APPENDIX A. SPECIFICATIONS................................................................................................... 21
A.1 STRAIN GAUGE ................................................................................................................................. 21
A.2 THERMISTOR ...................................................................................................................................... 21
APPENDIX B. THEORY OF OPERATION .............................................................................. 22
APPENDIX C. THERMISTOR TEMPERATURE DERIVATION ....................... 24
APPENDIX D. HIGH-TEMPERATURE THERMISTOR
LINEARIZATION ...................................................................................................... 25
APPENDIX E. NO STRESS STRAIN ENCLOSURE.................................................... 26
APPENDIX F. MODEL 4200HT-T HIGH-TEMPERATURE STRAIN
GAUGE ................................................................................................................................28
APPENDIX G. MEASUREMENT AND CORRECTION OF
TEMPERATURE EFFECTS............................................................................. 29

III
FIGURES
FIGURE 1: ON-SITE INSTALLATION PHOTO ....................................................................1
FIGURE 2: MODEL 4200 VIBRATING WIRE STRAIN GAUGE .........................................2
FIGURE 3: MODEL 4200L VIBRATING WIRE STRAIN GAUGE .......................................2
FIGURE 4: MODEL 4200-6 VIBRATING WIRE STRAIN GAUGE ......................................3
FIGURE 5: MODEL 4200-7 VIBRATING WIRE STRAIN GAUGE ......................................3
FIGURE 6: MODEL 4202 VIBRATING WIRE STRAIN GAUGE .........................................3
FIGURE 7: MODEL 4210 VIBRATING WIRE STRAIN GAUGE .........................................4
FIGURE 8: ADJUSTING THE RANGE OF THE STRAIN GAUGE .......................................5
FIGURE 9: ASSEMBLED STRAIN GAUGE AND COIL HOUSING ....................................5
FIGURE 10: ATTACHING GAUGES TO REBAR .................................................................7
FIGURE 11: SUSPENDING STRAIN GAUGES BETWEEN REBAR ...................................8
FIGURE 12: ALTERNATIVE METHOD FOR ATTACHING GAUGES TO REBAR ...............8
FIGURE 13: LIGHTNING PROTECTION SCHEME ..........................................................10
FIGURE 14: GK-404 READOUT .......................................................................................12
FIGURE 15: LEMO CONNECTOR TO GK-404 .................................................................12
FIGURE 16: GK-405 READOUT .......................................................................................13
FIGURE 17: NO STRESS STRAIN ENCLOSURE .............................................................26
FIGURE 18: MODEL 4200HT-T .......................................................................................28

IV
TABLES
TABLE 1: HEAT SHRINK COLOR DESIGNATIONS...........................................................2
TABLE 2: EMBEDMENT STRAIN GAUGE DATALOGGER PARAMETERS....................11
TABLE 3: READOUT POSITIONS......................................................................................11
TABLE 4: EMBEDMENT STRAIN GAUGE FACTORS......................................................15
TABLE 5: STRAIN RESOLUTION ......................................................................................16
TABLE 6: EFFECTIVE MODULUS .....................................................................................18
TABLE 7: RESISTANCE WORK SHEET............................................................................20
TABLE 8: SAMPLE RESISTANCE .....................................................................................20
TABLE 9: STRAIN GAUGE SPECIFICATIONS..................................................................21
TABLE 10: EMBEDMENT STRAIN GAUGE THEORETICAL PARAMETERS .................22
TABLE 11: THERMISTOR RESISTANCE VERSUS TEMPERATURE ..............................24
TABLE 12: THERMISTOR RESISTANCE VERSUS TEMPERATURE FOR MODEL
4200HT ..........................................................................................................25

V
EQUATIONS
EQUATION 1: PERIOD TO DIGITS CONVERSION........................................................... 15
EQUATION 2: THEORETICAL STRAIN ............................................................................. 15
EQUATION 3: APPARENT STRAIN................................................................................... 15
EQUATION 4: CORRECTION FOR TEMPERATURE EFFECTS ON THE GAUGE............ 16
EQUATION 5: TRUE, LOAD-RELATED STRAIN CORRECTED FOR TEMPERATURE .... 16
EQUATION 6: ACTUAL STRAIN........................................................................................ 17
EQUATION 7: STRAIN TO LOAD FORMULA ................................................................... 18
EQUATION 8: RESISTANCE TO TEMPERATURE ............................................................ 24
EQUATION 9: HIGH-TEMPERATURE RESISTANCE TO TEMPERATURE...................... 25
EQUATION 10: TEMPERATURE-INDUCED STRESS ...................................................... 29
EQUATION 11: COMBINED TEMPERATURE AND LOAD-RELATED STRESS.............. 29
EQUATION 12: EXTERNAL LOAD STRESS...................................................................... 29
EQUATION 13: ACTUAL STRAIN ..................................................................................... 29

VI

MODEL 4200 SERIES STRAIN GAUGES | INTRODUCTION | 1
1. INTRODUCTION
GEOKON vibrating wire embedment strain gauges are designed for direct
embedment in concrete. This can be accomplished by attaching the gauge to
rebar or tensioning cables and then casting the gauge into a concrete briquette,
which is subsequently cast into the structure, or grouting the gauge into
boreholes in the concrete.
Strains are measured using the vibrating wire principle. A length of steel wire is
tensioned between two end blocks that are firmly in contact with the mass
concrete. Deformations in the concrete will cause the two end blocks to move in
relation to each other, altering the tension in the steel wire. This change in
tension is measured as a change in the resonant frequency of vibration of the
wire.
Two coils, one with a magnet insert, the other with a pole piece insert, are
located close to the vibrating wire. In use, a pulse of varying frequency (swept
frequency) is applied to the coils causing the wire to vibrate primarily at its
resonant frequency.
Portable readouts and dataloggers are available from GEOKON. These models,
when used in conjunction with vibrating wire strain gauges, will provide the
necessary voltage pulses to pluck the wire. During vibration, a sinusoidal signal
is induced in the coils and transmitted to the readout box where it is conditioned
and displayed.
This manual contains installation instructions, readout and data reduction
procedures, and troubleshooting guidelines.
Note: Do not rotate or pull on the gauge end blocks, because this will alter the
readings and may cause permanent damage.
FIGURE 1: On-site Installation Photo

2| MODELS | GEOKON
2. MODELS
GEOKON vibrating wire strain gauges come in a variety of models, all easily
identifiable by the color of heat shrink which covers the protective tubes, as
shown in the table below.*
*Applicable only to products manufactured after September 2016.
TABLE 1: Heat Shrink Color Designations
The following sections describe in brief the various embedment strain gauges
available from GEOKON.
2.1 MODELS 4200 AND 4200L
GEOKON Models 4200 and 4200L are designed primarily for long-term strain
measurements inside mass concrete, in structures such as foundations, piles,
bridges, dams, containment vessels, tunnel liners, etc.
The 4200L is a low-modulus version designed to enable early curing strains to
be measured. The length of the 4200 gauge is 152 mm (6”).
FIGURE 2: Model 4200 Vibrating Wire Strain Gauge
FIGURE 3: Model 4200L Vibrating Wire Strain Gauge
Model Heat Shrink Color
4200 (4,000  Blue
4200L Black
4200 6 (5,000  Red
4200 7 (10,000  Green
4200X Yellow (dependent on the reason for the X designation)

MODEL 4200 SERIES STRAIN GAUGES | MODELS | 3
2.2 MODELS 4200-6 AND 4200-7
Model 4200-6 and 4200-7 strain gauges are supplied fully sealed and pre-
tensioned with the plucking coil mounted. Note the small collar under the shrink
tube at one end.
FIGURE 4: Model 4200-6 Vibrating Wire Strain Gauge
FIGURE 5: Model 4200-7 Vibrating Wire Strain Gauge
2.3 MODEL 4202
Model 4202 is designed for direct embedment in grout, mortar, and small
aggregate concrete. It is also useful for model studies. The length of the 4202
gauge is 50 mm (2”).
FIGURE 6: Model 4202 Vibrating Wire Strain Gauge
Collar
Collar

4| MODELS | GEOKON
2.4 MODEL 4210
Model 4210 is designed for embedment in large aggregate concrete (greater
than 3/4 of an inch). The standard gauge length is 254 mm (10”), other gauge
lengths available include: Model 4212: 305 mm (12”), and Model 4214: 356 mm
(14”).
FIGURE 7: Model 4210 Vibrating Wire Strain Gauge

MODEL 4200 SERIES STRAIN GAUGES | PRIOR TO INSTALLATION | 5
3. PRIOR TO INSTALLATION
3.1 ADJUSTING GAUGE TO THE DESIRED RANGE
GEOKON embedment strain gauges are supplied fully sealed and pretensioned.
Model 4200 gauges are normally supplied with the wire tension set near the
middle of their range. If the range needs to be adjusted for some reason, the
wire tension may be changed by the following procedure:
1. Attach the red and black leads to a readout box that has been set to position
D, reading in microstrain.
2. Grip the small collar under the shrink tube and rotate the end flange as
shown in the figure below.
3. Rotate clockwise to decrease the initial reading; rotate counterclockwise to
increase the reading. For example, if the gauge will see all compression, it
should be set to about 4000 microstrain.
FIGURE 8: Adjusting the Range of the Strain Gauge
Model 4200-6 / 4200-7 Note: Although the readings are taken on position D,
the digits shown must be converted to microstrain manually. To do this, multiply
the observed digit change by the gauge factor given on the calibration sheet
provided with the strain gauge.
3.2 GAUGE AND CABLE ASSEMBLY
Insert the flat part of the gauge into the slot in the coil assembly located at the
end of the cable. Slide the hose clamp over the assembly and tighten.
FIGURE 9: Assembled Gauge and Coil Housing
Alternatively, the coil housing can be glued in place using cyanoacrylate glue. If
this method is chosen, it will no longer be possible to remove the gauge from
the coil housing.
3.3 PRELIMINARY CHECK
A preliminary check should be performed before installing the gauge in the field.
To perform the preliminary check, complete the following steps:
1. Using an ohmmeter, check the resistance between the two lead wires
(usually red and black).
Collar

6| PRIOR TO INSTALLATION | GEOKON
For Models 4200, 4200L, 4200-6, 4200-7, and 4210/12/14 it should be
about 180 ohms.
For Model 4202, it should be about 50 ohms.
Remember to add the cable resistance at approximately 14.7/1000' or
48.5/km at 20 °C. Multiply these factors by two to account for both
directions.
2. Using an ohmmeter, check the resistance between the two thermistor wires
(usually white and green). Using Table 11 on page 24, convert the
resistance to temperature. Compare the result to the current ambient
temperature. (For Model 4200HT see Table 12 on page 25.)
3. Connect the gauge to a readout box. (See readout instructions, Section 5.)
Observe the displayed readout. The reading should be about the midrange
position as defined in Table 2 on page 11.
4. Press on the gauge ends and confirm that it makes the reading decrease.
Return any faulty gauges to the factory. Gauges should not be opened in the
field.

MODEL 4200 SERIES STRAIN GAUGES | INSTALLING THE GAUGES IN CONCRETE | 7
4. INSTALLING THE GAUGES IN CONCRETE
GEOKON strain gauges are typically set into concrete in one of two ways:
1. Cast the units directly into the concrete mix (see Sections 4.1 through 4.3)
2. Cast them into briquettes that are subsequently cast into the concrete
structure (see Section 4.4).
When casting the gauge directly into the concrete mix, take care to avoid
applying any large forces to the end blocks during installation. This is most
imperative when installing Model 4202 gauges.
Model 4202 Note: Do not wrap an iron tie wire around the body of the gauge;
doing so could cause damage due to its delicate construction. Instead, use the
holes in the end blocks to affix the gauge to the rebar, being sure that the gauge
is not tensioned or compressed in the longitudinal direction.
Model 4200L Note: This gauge is specifically designed to allow strains to be
measured in curing concrete. However, do not bury the gauge more than one
meter deep; doing so could damage the gauge.
4.1 DIRECT ATTACHMENT TO REBAR
1. Place two pieces of wood or polystyrene foam between the gauge and the
rebar as shown in Figure 10 on this page.
2. Use soft iron tie wire, similar to the kind that is normally used for tying rebar
cages together. Run the wire around the body of the strain gauge and
around the rebar. Twist the wire around itself to hold the gauge in place.
3. Tie the instrument cable off to the rebar using nylon cable ties.
Note: Don’t tie the wires too tightly, since rebar and tension cables tend to
move during concrete placement and vibration. Take care not to damage the
cable with the vibrator. The gauge can also be placed directly into the mix if it
can be assured that the orientation will be correct after the gauge placement.
4.2 SUSPENSION METHOD
1. Wrap a layer of self-vulcanizing rubber tape 3 cm from each gauge end, as
shown in Figure 11 on page 8.
Model 4210 Note: This step is not necessary
The layers of rubber serve as a shock absorber, dampening any vibration of
the suspension system. Without the rubber layers, the resonant frequency
of the tie wires might interfere with the resonant frequency of the gauge
when the tie wires are tightened. This can result in unstable readings, or in
no readings at all. However, this problem disappears once the concrete has
been placed.
For a method that avoids this potential problem, see Section 4.3.
2. Use soft iron tie wire, the kind normally used for tying rebar cages together.
Wrap the wire around the rubber strips twice.
3. Twist two loops in the wire, one on either side of the gauge, at a distance of
3 cm from the gauge body. Repeat this process at the other end.
4. Position the gauge between the rebar and twist the wire ends twice around
the rebar, then around itself.
5. Tighten the wire and orient the gauge by twisting the loops between the
gauge and the rebar.
Nylon Cable Tie
Instrument Cable
4200
Coil Assembly
Wood or
Polystyrene
Block
Wire Tie
Wire Tie
Rebar or
Tensioned Cable
FIGURE 10: Attaching Gauges to Rebar

8| INSTALLING THE GAUGES IN CONCRETE | GEOKON
6. Attach the plucking coil using a hose clamp. Tie the instrument cable off to
one of the rebar using nylon cable ties.
FIGURE 11: Suspending Model 4200/4200L/4200HT Strain Gauges Between Rebar
4.3 ALTERNATIVE SUSPENSION METHOD
Tie two short pieces of steel rebar to the existing rebar using nylon cable ties, as
shown in the figure below. Then tie the strain gauge to the short pieces of rebar
again using more nylon cable ties. This method avoids the resonance problems
associated with the previous method.
FIGURE 12: Alternative Method for Attaching Model 4200/4200HT Strain Gauges to
Rebar
Instrument Cable
Coil Assembly
Rebar or
Tensioned Cable
Wire Tie
Wire Tie
Rubber Tape
4200
Wire Tie
Wire Tie Rubber Tape
Nylon Cable Tie
Instrument Cable
Short Lengths
of Rebar
Nylon Cable Ties
Nylon Cable Ties
Nylon Cable Ties

MODEL 4200 SERIES STRAIN GAUGES | INSTALLING THE GAUGES IN CONCRETE | 9
4.4 USING PRE-CAST BRIQUETTES, SHOTCRETE, OR GROUTING
An alternative to casting the units into the concrete mix directly is to precast the
gauges into briquettes of the same mix as the mass concrete and then place the
briquettes in the structure prior to concrete placement. The briquettes should be
constructed no more than three days prior to installation, but no less than one
day prior, and should be continuously cured with water while awaiting
placement in the mass concrete.
GEOKON strain gauges may also be used in shotcrete, as well as in holes drilled
in rock or concrete that are subsequently grouted. When used in shotcrete,
special care should be taken to protect the lead wires, such as encasing them in
conduit or heavy tubing. The gauges may be placed by packing the immediate
area around the gauge by hand and then proceeding with the shotcrete
operation.
4.5 CABLE SPLICING AND TERMINATION
The cable from the strain gauges can be protected by using flexible conduit,
which can be supplied by GEOKON.
Terminal boxes with sealed cable entries are available from GEOKON for all types
of applications. These allow many gauges to be terminated at one location with
complete protection of the lead wires. The interior panel of the terminal box can
have built-in jacks or a single connection with a rotary position selector switch.
Contact GEOKON for specific application information.
Because the vibrating wire output signal is a frequency rather than a current or
voltage, variations in cable resistance have little effect on gauge readings;
therefore, splicing of cables has no ill effects, and in some cases may be
beneficial. The cable used for making splices should be a high-quality twisted
pair type, with 100% shielding and an integrated shield drain wire. When
splicing, it is very important that the shield drain wires be spliced
together. Always maintain polarity by connecting color to color.
Splice kits recommended by GEOKON incorporate casts, which are placed
around the splice and are then filled with epoxy to waterproof the connections.
When properly made, this type of splice is equal or superior to the cable itself in
strength and electrical properties. Contact GEOKON for splicing materials and
additional cable splicing instructions.
Cables may be terminated by stripping and tinning the individual conductors and
then connecting them to the patch cord of a readout box. Alternatively, a
connector may be used which will plug directly into the readout box or to a
receptacle on a special patch cord.
4.6 LIGHTNING PROTECTION
Unlike numerous other types of instrumentation available from GEOKON,
embedment strain gauges do not have any integrated lightning protection
components, such as transorbs or plasma surge arrestors. Usually this is not a
problem, as these types of gauges are installed within concrete or grout and are
somewhat isolated from potentially damaging electrical transients. However,
there may be occasions where some sort of lightning protection is desirable.
One such example is where the gauge is in contact with rebar that may be
exposed to direct or indirect lightning strikes. In addition, if the instrument cable
is exposed, it may be appropriate to install lightning protection components, as
the transient could travel down the cable to the gauge and possibly destroy it.

10 | INSTALLING THE GAUGES IN CONCRETE | GEOKON
SUGGESTED LIGHTNING PROTECTION OPTIONS
Lighting arrestor boards and enclosures are also available from GEOKON.
These units install where the instrument cable exits the structure being
monitored. The enclosure has a removable top to allow the customer to
service the components or replace the board in the event that the unit is
damaged by a lightning strike. A connection is made between the enclosure
and earth ground to facilitate the passing of transients away from the
gauge. See the figure below.
Plasma surge arrestors can be epoxied into the instrument cable, close to
the sensor. A ground strap then connects the surge arrestor to an earth
ground, such as a grounding stake or the rebar itself.
Consult the factory for additional information on available lightning protection.
FIGURE 13: Lightning Protection Scheme