Installation Manual
Models 4911/4911A
Vibrating Wire Rebar Strain Meters
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
omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1989-2019 by Geokon
(Doc Rev R, 1/03/2019)
®
®
.
®
assumes no responsibility for errors,
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 specification, misapplication, misuse or other operating conditions outside of
Geokon's control. Components which wear or which 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 merchantability 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 installation 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.
TABLE of CONTENTS
1. INTRODUCTION ................................................................................................................................................... 1
2. INSTALLATION .................................................................................................................................................... 2
2.1 PRELIMINARY TEST ............................................................................................................................................. 2
2.2 INSTALLING MODEL 4911, “SISTER BAR” ........................................................................................................... 2
2.3 INSTALLING MODEL 4911A ................................................................................................................................. 4
2.4 CABLE INSTALLATION ......................................................................................................................................... 5
2.5 CABLE SPLICING AND TERMINATION ................................................................................................................... 5
2.6 LIGHTNING PROTECTION ..................................................................................................................................... 5
3. TAKING READINGS ............................................................................................................................................. 7
3.1 GK-404 READOUT BOX ....................................................................................................................................... 7
3.1.1 Operating the GK-404 ................................................................................................................................ 7
3.2 GK-405 READOUT BOX ....................................................................................................................................... 8
3.2.1 Connecting Sensors 10-pin Bulkhead Connectors Attached ....................................................................... 8
3.2.2 Connecting Sensors with Bare Leads .......................................................................................................... 8
3.2.3 Operating the GK-405 ................................................................................................................................ 8
3.3 GK-403 READOUT BOX (OBSOLE TE MODEL) ...................................................................................................... 9
3.3.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached ............................................................... 9
3.3.2 Connecting Sensors with Bare Leads .......................................................................................................... 9
3.3.3 Operating the GK-403 ................................................................................................................................ 9
3.4 MEASURING TEMPERATURES ............................................................................................................................. 10
4. DATA REDUCTION ............................................................................................................................................ 11
4.1 STRAIN CALCULATION ...................................................................................................................................... 11
4.2 TEMPERATURE CORRECTION ............................................................................................................................. 11
4.3 ENVIRONMENTAL FACTORS ............................................................................................................................... 13
4.4 SHRINKAGE EFFECTS ......................................................................................................................................... 13
4.5 CONVERTING STRAINS TO LOADS ...................................................................................................................... 13
5. TROUBLESHOOTING ........................................................................................................................................ 14
APPENDIX A. SPECIFICATIONS ......................................................................................................................... 16
A.1 REBAR STRAIN METERS .................................................................................................................................... 16
A.2 THERMISTOR ..................................................................................................................................................... 16
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION ..................................................................... 17
APPENDIX C. DERIVING THE CALIBRATION FACTOR (C) FROM THE TEST DATA ......................... 18
APPENDIX D. SAMPLE CALIBRATION REPORT ........................................................................................... 21
FIGURES
IGURE 1 - MODEL 4911 REBAR STRAIN METER ........................................................................................................... 1
F
FIGURE 2 - MODEL 4911A REBAR STRAIN METER ........................................................................................................ 1
FIGURE 3 - MODEL 4911 “SISTER BAR” INSTALLATION ................................................................................................ 3
FIGURE 4 - MODEL 4911 “SISTER BAR” INSTALLATION DETAIL .................................................................................... 3
FIGURE 5 - MODEL 4911A INSTALLATION ..................................................................................................................... 4
FIGURE 6 - RECOMMENDED LIGHTNING PROTECTION SCHEME ..................................................................................... 6
FIGURE 7 - LEMO CONNECTOR TO GK-404 ................................................................................................................... 7
FIGURE 8 - LIVE READINGS – RAW READINGS............................................................................................................... 8
FIGURE 9 - REBAR STRAIN METER SCHEMATIC ............................................................................................................18
FIGURE 10 - SAMPLE MODEL 4911 CALIBRATION REPORT ...........................................................................................21
TABLES
ABLE 1 - THERMAL COEFFICIENTS .............................................................................................................................11
T
TABLE 2 - SAMPLE RESISTANCE ...................................................................................................................................15
TABLE 3 - RESISTANCE WORK SHEET ...........................................................................................................................15
TABLE 4 - MODEL 4911A/4911 STRAIN METER SPECIFICATIONS .................................................................................16
TABLE 5 - THERMISTOR RESISTANCE VERSUS TEMPERATURE .....................................................................................17
TABLE 6 - UNBONDED SECTION DIMENSIONS ...............................................................................................................19
TABLE 7 - MICROSTRAIN CONVERSION FACTORS .........................................................................................................20
EQUATIONS
QUATION 1 - DIGITS CALCULATION ............................................................................................................................11
E
EQUATION 2 - APPARENT STRAIN .................................................................................................................................11
EQUATION 3 - LOAD RELATED STRAIN .........................................................................................................................12
EQUATION 4 - ACTUAL STRAIN ....................................................................................................................................12
EQUATION 5 - STRAIN TO LOAD FORMULA ...................................................................................................................13
EQUATION 6 - RESISTANCE TO TEMPERATURE .............................................................................................................17
EQUATION 7 - TOTAL STRAIN CALCULATION ...............................................................................................................18
EQUATION 8 - ZONE TWO CALCULATION .....................................................................................................................19
EQUATION 9 - ZONE THREE CALCULATION ..................................................................................................................19
EQUATION 10 - LOAD IN POUNDS/KILOGRAMS .............................................................................................................19
1
1. INTRODUCTION
Geokon Vibrating Wire Rebar Strain Meters are designed primarily for monitoring the stress
exerted upon reinforcing steel inside concrete structures, such as bridges, concrete piles, and
diaphragm walls. The strain meter comprises a length of high strength steel, bored along its
central axis to accommodate a miniature vibrating wire strain gauge. Both models of strain
meters are robust, reliable, and easy to install and read. They are unaffected by moisture, cable
length, or contact resistance. The long-term stability of these instruments has proven to be
excellent.
Readout of the load or stress is achieved remotely using a portable readout or datalogging system
available from Geokon. Each strain meter is supplied with a calibration report, which shows the
relationship between readout digits and microstrain. The calibration report also shows the initial
no load zero reading.
Model 4911 Vibrating Wire Rebar Strain Meter or “Sister Bar” (Figure 1) comprises a short
length of high strength steel welded between two sections of reinforcing bar. It is designed to be
wire tied in parallel with structural rebar. The small diameter of the 4911 minimizes its affect on
the sectional modulus of the concrete. The cable exits the strain meter through a small block of
protective epoxy.
Figure 1 - Model 4911 Rebar Strain Meter
Model 4911A Vibrating Wire Rebar Strain Meter (Figure 2) comprises a short length of high
strength steel welded between two sections of reinforcing bar. It is designed to be welded
between sections of structural concrete reinforcing bar. The cable exits the strain meter via a
compression fitting.
Figure 2 - Model 4911A Rebar Strain Meter
2
2. INSTALLATION
2.1 Preliminary Test
A preliminary check should be performed before installing gauges in the field. To perform the
preliminary check, complete the following steps:
Warning! Do not lift the strain meter by the cable.
1) Connect the gauge to a readout box. (See readout instructions in Section 3.) Observe the
displayed readout.
2) Compare the zero reading given on the calibration report to the current zero reading. Under
normal circumstances the two readings should not differ by more than 25 digits (10
microstrains), however, shocks incurred during shipping may cause larger shifts. If the
reading is within 100 digits (40 microstrains) of the factory zero, and is stable, it is safe to
proceed with the installation.
3) Pull on the strain meter ends; confirm that digits on the readout rise as the tension increases.
Checks of electrical continuity can be made using an ohmmeter. The resistance between the two
lead wires (usually red and black) should be around 50 ohms. Remember to add the cable
resistance, approximately 14.7Ω for every 1000 ft. (48.5Ω per km) at 20 °C. Multiply these
factors by two to account for both directions.
Resistance between the two thermistor wires (usually white and green) varies with temperature.
Use Table 5 in Appendix B to convert the resistance to temperature and compare the result to the
ambient temperature.
Resistance between any conductor and the shield should exceed two megohms.
If the strain meter fails any of the preliminary tests, see Section 5 for troubleshooting.
2.2 Installing Model 4911, “Sister Bar”
Sister Bars are usually installed using standard iron tie wire. Ties near the ends and at the onethird points are sufficient if the gauge is being wired to a larger section of rebar or to horizontal
bars. Wiring at the one third points alone is sufficient if the gauge is being wired in parallel to the
structural rebar. Route the instrument cable along the rebar system and tie it off every three to
four feet (one-meter) using nylon cable ties. Avoid using the tie wire on the instrument cable as it
could cut the cable. See Figure 3 and Figure 4 on the following page.
Be sure to note the location and serial numbers of all instruments during the installation process.
This information is necessary in order to apply the proper calibration factors and determine strain
characteristics when reducing data.
3
Figure 3 - Model 4911 “Sister Bar” Installation
Figure 4 - Model 4911 “Sister Bar” Installation Detail
4
2.3 Installing Model 4911A
Normally the strain meter is welded in series with the reinforcing steel that is to be instrumented
on site. The strain meter is of sufficient length that it may be welded in place without damaging
the internal strain gauge element. However, care should still be taken to ensure that the central
portion of the strain meter does not become too hot, as the plucking coil and protective epoxy
could melt. In order to prevent this it may be necessary to place wet rags between the weld area
and the coil housing. Take care not to damage or burn the instrument cable during welding. After
welding, route the instrument cable along the rebar system and tie it off every three to four feet
(one-meter) using nylon cable ties. Avoid using iron tie wire to secure the cable as the cable
could be cut. Figure 5 show a typical 4911 installation.
Be sure to note the location and serial numbers of all instruments during the installation process.
This information is necessary for applying the proper calibration factors and determining strain
characteristics when reducing data.
Figure 5 - Model 4911A Installation
5
2.4 Cable Installation
As noted in the in the previous sections, instrument cables should be routed along the structural
rebar and secured with nylon cable ties every two to three feet (one-meter). Outside of the
instrumented structure, the cable should be protected from accidental damage caused by moving
equipment or other construction activity.
2.5 Cable Splicing and Termination
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.
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.
The cable from the strain meters can also be protected by the use of flexible conduit, which can
be supplied by Geokon.
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 in fact be beneficial. The cable used for making splices
should be a high quality twisted pair type, with 100% shielding and an integral 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 that 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 in strength and electrical properties. Contact Geokon for splicing
materials and additional cable splicing instructions.
2.6 Lightning Protection
Unlike numerous other types of instrumentation available from Geokon, rebar strain meters do
not have any integral 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 and
are somewhat isolated from potentially damaging electrical transients. However, there may be
occasions where some sort of lightning protection is desirable, for example, where the instrument
is in contact with rebar that is 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.
6
Suggested Lightning Protection Options:
• If the instrument is connected to a terminal box or multiplexer, components such as plasma
surge arrestors (spark gaps) may be installed in the terminal box/multiplexer to provide a
measure of transient protection. Terminal boxes and multiplexers available from Geokon
provide locations for the installation of these components.
• 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 strain meter.
See Figure 6 below.
• Plasma surge arrestors can be epoxied into the instrument cable, close to the instrument. 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 6 - Recommended Lightning Protection Scheme
7
3. TAKING READINGS
3.1 GK-404 Readout Box
The GK-404 is a palm sized readout box, which displays the vibrating wire value and the
temperature in degrees centigrade.
3.1.1 Operating the GK-404
Before use, attach the flying leads to the GK-404 by aligning the red circle on the silver
“Lemo” connector of the flying leads with the red line on the top of the GK-404 (Figure
7). Insert the Lemo connector into the GK-404 until it locks into place.
Figure 7 - Lemo Connector to GK-404
Connect each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
To turn the GK-404 on, press the “ON/OFF” button on the front panel of the unit. The
initial startup screen will be displayed. After approximately one second, the GK-404 will
start taking readings and display them based on the settings of the POS and MODE
buttons.
The unit display (from left to right) is as follows:
• The current Position: Set by the POS button. Displayed as a letter A through F.
• The current Reading: Set by the MODE button. Displayed as a numeric value
followed by the unit of measure.
• Temperature reading of the attached gauge in degrees Celsius.
Use the POS button to select position B and the MODE button to select Dg (digits).
(Other functions can be selected as described in the GK-404 Manual.)
The GK-404 will continue to take measurements and display readings until the unit is
turned off, either manually, or if enabled, by the Auto-Off timer. If the no reading
displays or the reading is unstable, see Section 5 for troubleshooting suggestions.
For further details, please refer to the GK-404 manual.
8
3.2 GK-405 Readout Box
The GK-405 Vibrating Wire Readout is made up of two components: The Readout Unit,
consisting of a Windows Mobile handheld PC running the GK-405 Vibrating Wire Readout
Application; and the GK-405 Remote Module, which is housed in a weatherproof enclosure and
connects to the vibrating wire gauge to be measured. The two components communicate
wirelessly using Bluetooth®, a reliable digital communications protocol. The Readout Unit can
operate from the cradle of the Remote Module, or, if more convenient, can be removed and
operated up to 20 meters from the Remote Module.
3.2.1 Connecting Sensors 10-pin Bulkhead Connectors Attached
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.2.2 Connecting Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.2.3 Operating the GK-405
Press the button labeled “POWER ON (BLUETOOTH)”. A blue light will begin
blinking, signifying that the Remote Module is waiting to connect to the handheld unit.
Launch the GK-405 VWRA program by tapping on “Start” from the handheld PC’s main
window, then “Programs” then the GK-405 VWRA icon. After a few seconds, the blue
light on the Remote Module should stop flashing and remain lit. The Live Readings
Window will be displayed on the handheld PC. Figure 8 shows a typical vibrating wire
output in digits and thermistor output in degrees Celsius. If the no reading displays or the
reading is unstable, see Section 5 for troubleshooting suggestions.
For further information, consult the GK-405 Instruction Manual.
Figure 8 - Live Readings – Raw Readings
9
3.3 GK-403 Readout Box (Obsolete Model)
The GK-403 can store gauge readings and apply calibration factors to convert readings to
engineering units. The following instructions explain taking gauge measurements using Modes
“B”. Consult the GK-403 Instruction Manual for additional information.
3.3.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.3.2 Connecting Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.3.3 Operating the GK-403
1) Turn the display selector to position “B”.
2) Turn the unit on.
3) The readout will display the vibrating wire output in digits. The last digit may change
one or two digits while reading.
4) The thermistor reading will be displayed above the gauge reading in degrees
centigrade.
5) Press the “Store” button to record the value displayed.
If the no reading displays or the reading is unstable, see Section 5 for troubleshooting
suggestions.
The unit will automatically turn off after approximately two minutes to conserve power.
10
3.4 Measuring Temperatures
All vibrating wire strain meters are equipped with a thermistor, which gives a varying resistance
output as the temperature changes. The white and green leads of the instrument cable are
normally connected to the internal thermistor.
The GK-403, GK-404, and GK-405 readout boxes will read the thermistor and display the
temperature in degrees C.
To read temperatures using an ohmmeter:
1) Connect an ohmmeter to the green and white thermistor leads coming from the strain meter.
(Since the resistance changes with temperature are large, the effect of cable resistance is
usually insignificant. For long cables a correction can be applied, equal to approximately
14.7Ω per 1000 ft. (48.5Ω per km) at 20 °C. Multiply these factors by two to account for
both directions.)
2) Look up the temperature for the measured resistance in Appendix B, Table 5.
11
4. DATA REDUCTION
Steel (K
steel
):
12.2 ppm/°C
6.7 ppm/°F
Concrete (K
):
≈10 ppm/°C
≈5.5 ppm/°F
Difference (K):
2.2 ppm/°C
1.2 ppm/°F
4.1 Strain Calculation
The basic units utilized by Geokon for measurement and reduction of data are “digits”.
Calculation of digits is based on the following equation:
2
Digits =
1
x 10-3 or Digits=
T
Equation 1 - Digits Calculation
Where;
T is the period in seconds.
Hz is the frequency in cycles per second.
To convert digits to strain the following equation applies:
Hz
1000
2
ε
apparent
= (R
- R
1
) × C
0
Equation 2 - Apparent Strain
Where;
R0 is the initial reading in digits, usually obtained at installation or at the commencement of a
test.
R1 is the current reading in digits.
C is the calibration factor from the supplied calibration report. (For an example of a typical strain
meter calibration report see Figure 10 in Appendix D.)
For example: Assuming an initial reading (R0) of 8000 digits, a current reading (R1) of 7700, and
a calibration factor (C) of 0.343 microstrains per digit; the strain calculation would be as follows:
ε
apparent
= (7700 - 8000) × 0.343 = - 102.9 µε (compression)
4.2 Temperature Correction
The assumption with strain meters embedded in concrete is that the strain in the meter is equal
to the strain in the concrete. However, when the temperature changes the concrete expands and
contracts at a rate slightly less than the rate of the steel of the vibrating wire. The coefficients of
this expansion are shown in the table below.
concrete
Table 1 - Thermal Coefficients
12
Because of the difference in these two coefficients, a correction is required to the apparent
strains, equal to the difference of the two coefficients. The equation for this correction is as
follows:
ε
load related
Where;
T0 is the initial temperature recorded at the time of installation.
T1 is the current temperature.
K is the thermal coefficient from Table 1.
The strains thus calculated are due to load changes only.
Using the same theoretical example from Section 4.1, where R0 = 8000 digits on channel B, and
R1 = 7700 digits on channel B, with the addition of temperature, T0 = 20 °C and T1 = 60 °C
(during the concrete curing); the strain corrected for temperature (i.e. due to load changes only)
can be calculated as follows:
ε
load related
Please note that the actual strain undergone by the concrete, (e.g., that which would be measured
by a tape measure,) is given by the formula:
Apparent strain =
(7700 – 8000) x 0.343 = – 103 µstrain (compression)
Load related strain =
(7700 – 8000) x 0.343 + (60 – 20) x (12.2 – 10) = – 15 µstrain (compression)
Actual strain =
(7700 – 8000) x 0.343 + (60-20) x (12.2) = +385 µstrain (tension)
From the above example, it can be seen that while the concrete was actually expanding by 385
microstrains due to the temperature increase, the apparent strain was 103 microstrains in
compression, and the actual change of strain due to increased stress in the concrete was only 15
microstrains compression.
= (7700 – 8000) 0.343 + (60 – 20) (12.2 – 10) = – 14.9 µstrain (compression)
ε
actual
= ((R
Equation 3 - Load Related Strain
= ((R
1
Equation 4 - Actual Strain
- R
1
- R
) × C) + ((T
0
) × C) + ((T
0
- T
1
- T
1
) ×K
0
) × K)
0
)
steel
13
4.3 Environmental Factors
Since the purpose of the strain meter installation is to monitor site conditions, factors which may
affect these conditions should always be observed and recorded. Seemingly minor effects may
have a real influence on the behavior of the structure being monitored, and may give an early
indication of potential problems. Some of these factors include, but are not limited to: blasting,
rainfall, tidal levels, traffic, temperature and barometric changes, weather conditions, changes in
personnel, nearby construction activities, excavation and fill level sequences, seasonal changes,
etc.
4.4 Shrinkage Effects
A well know property of concrete is its propensity to shrink as the water content diminishes and
to swell as it absorbs water. This shrinkage and swelling can give rise to large strain changes that
are not related to load or stress. The magnitude of these strains can be several hundred
microstrains.
It is difficult to compensate for these unwanted strains. An attempt may be made to keep the
concrete under a constant condition of water content, but this is frequently impossible on
concrete structures exposed to varying weather conditions. The shrinkage and/or swelling effect
may be measured by casting a strain gauge inside a concrete block that remains unloaded, yet
still exposed to the same moisture conditions as the active gauges. Strains measured on this
gauge may be used as a correction factor.
4.5 Converting Strains to Loads
The load in any structural element to which the strain meter is attached is given by the formula:
L = E µ A
Equation 5 - Strain to Load Formula
Where;
L is the load.
E is the elastic modulus of the structural element in the appropriate units.
µ is the strain in microstrains.
A is the cross-sectional area in the appropriate units.
When installing strain gauges in concrete piles it is standard practice to install them in pairs on
either side of the neutral axis. This allows any strains imposed by bending to be corrected by
taking the average strain of the two gauges. It is also standard practice to install a pair of strain
gauges close to the top of the pile. The measured strain of these two gauges is used to calculate
the modulus of the concrete.
14
5. TROUBLESHOOTING
Maintenance and troubleshooting of rebar strain meters is confined to periodic checks of cable
connections and maintenance of terminals. Once installed, the strain meters are usually
inaccessible and remedial action is limited. Gauges should not be opened in the field.
Should difficulties arise, consult the following list of problems and possible solutions. Return
any faulty gauges to the factory. For additional troubleshooting and support contact Geokon.
Symptom: Thermistor resistance is too high
It is likely that there is an open circuit. Check all connections, terminals, and plugs. If a cut is
located in the cable, splice according to instructions in Section 2.5.
Symptom: Thermistor resistance is too low
It is likely that there is a short. Check all connections, terminals, and plugs. If a short is
located in the cable, splice according to instructions in Section 2.5.
Water may have penetrated the interior of the strain meter. There is no remedial action.
Symptom: Strain Meter Readings are Unstable
Is the readout box position set correctly? If using a datalogger to record readings
automatically, are the swept frequency excitation settings correct?
Is there a source of electrical noise nearby? Likely candidates are generators, motors, arc
welding equipment, high voltage lines, etc. If possible, move the instrument cable away from
power lines and electrical equipment or install electronic filtering.
Make sure the shield drain wire is connected to ground whether using a portable readout or
datalogger. Connect the shield drain wire to the readout using the blue clip. (Green for the
GK-401.)
Does the readout work with another strain meter? If not, it may have a low battery or
possibly be malfunctioning.
Symptom: Strain Meter Fails to Read
Is the cable cut or crushed? Check the resistance of the cable by connecting an ohmmeter to
the gauge leads. Table 2 shows the expected resistance for the various wire combinations;
Table 3 is provided for the user to fill in the actual resistance found. Cable resistance is
approximately 14.7Ω per 1,000 feet (48.5Ω per km); multiply this factor by two to account
for both directions. If the resistance is very high or infinite (megohms), the cable is probably
broken or cut. If the resistance is very low (<20Ω), the gauge conductors may be shorted. If a
cut or a short is located in the cable, splice according to the instructions in Section 2.5.
Does the readout or datalogger work with another gauge? If not, it may have a low battery or
possibly be malfunctioning.
15
Vibrating Wire Sensor Lead Grid - SAMPLE VALUES
Red
Black
White
Green
Shield
≅50Ω
≅50Ω
3000Ω at
25°C
3000Ω at
25°C
Black
White
Green
Shield
Red
Red
Black
White
N/A
N/A infinite infinite infinite
infinite infinite N/A
infinite infinite
infinite infinite infinite
infinite
N/A infinite
infinite infinite infinite infinite N/A
Table 2 - Sample Resistance
Vibrating Wire Sensor Lead Grid - SENSOR NAME/## :
Red Black White Green Shield
Green
Shield
Table 3 - Resistance Work Sheet
16
Model:
4911 “Sister Bar”
4911A
Range1:
3000 µε
Rebar Sizes Available2:
#4
#5, #6, #7, #8, #9, #10, #11, #14
Sensitivity:
0.025% FSR
Accuracy:
0.25% FSR
Linearity:
0.25% FSR
Operating Temperature:
-20 to +80 °C
Operating Frequency:
1400-3200 Hz
Coil Resistance:
50Ω, ±5Ω
Length:
914 mm (36")
1105 mm (43.5")
Materials:
Grade 60 Rebar, (60 ksi yield), and High Strength Steel
Two twisted pair (Four conductor) 22 AWG
Foil shield, PVC jacket, nominal OD=6.3 mm (0.250")
APPENDIX A. SPECIFICATIONS
A.1 Rebar Strain Meters
Electrical Cable:
Table 4 - Model 4911A/4911 Strain Meter Specifications
Notes:
1
The standard factory setting allows for 2000 µε in compression and 1000 µε in tension. Other
initial settings available upon request.
2
Consult the factory for other sizes available.
A.2 Thermistor
(See Appendix B also)
Range: -80 to +150° C
Accuracy: ±0.5° C
17
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
201.1K
-50
16.60K
-10
2417
+30
525.4
+70
153.2
+110
187.3K
-49
15.72K
-9
2317
31
507.8
71
149.0
111
174.5K
-48
14.90K
-8
2221
32
490.9
72
145.0
112
162.7K
-47
14.12K
-7
2130
33
474.7
73
141.1
113
151.7K
-46
13.39K
-6
2042
34
459.0
74
137.2
114
141.6K
-45
12.70K
-5
1959
35
444.0
75
133.6
115
132.2K
-44
12.05K
-4
1880
36
429.5
76
130.0
116
123.5K
-43
11.44K
-3
1805
37
415.6
77
126.5
117
115.4K
-42
10.86K
-2
1733
38
402.2
78
123.2
118
107.9K
-41
10.31K
-1
1664
39
389.3
79
119.9
119
101.0K
-40
9796 0 1598
40
376.9
80
116.8
120
94.48K
-39
9310
+1
1535
41
364.9
81
113.8
121
88.46K
-38
8851 2 1475
42
353.4
82
110.8
122
82.87K
-37
8417 3 1418
43
342.2
83
107.9
123
77.66K
-36
8006 4 1363
44
331.5
84
105.2
124
72.81K
-35
7618 5 1310
45
321.2
85
102.5
125
68.30K
-34
7252 6 1260
46
311.3
86
99.9
126
64.09K
-33
6905 7 1212
47
301.7
87
97.3
127
60.17K
-32
6576 8 1167
48
292.4
88
94.9
128
56.51K
-31
6265 9 1123
49
283.5
89
92.5
129
53.10K
-30
5971
10
1081
50
274.9
90
90.2
130
49.91K
-29
5692
11
1040
51
266.6
91
87.9
131
46.94K
-28
5427
12
1002
52
258.6
92
85.7
132
44.16K
-27
5177
13
965.0
53
250.9
93
83.6
133
41.56K
-26
4939
14
929.6
54
243.4
94
81.6
134
39.13K
-25
4714
15
895.8
55
236.2
95
79.6
135
36.86K
-24
4500
16
863.3
56
229.3
96
77.6
136
34.73K
-23
4297
17
832.2
57
222.6
97
75.8
137
32.74K
-22
4105
18
802.3
58
216.1
98
73.9
138
30.87K
-21
3922
19
773.7
59
209.8
99
72.2
139
29.13K
-20
3748
20
746.3
60
203.8
100
70.4
140
27.49K
-19
3583
21
719.9
61
197.9
101
68.8
141
25.95K
-18
3426
22
694.7
62
192.2
102
67.1
142
24.51K
-17
3277
23
670.4
63
186.8
103
65.5
143
23.16K
-16
3135
24
647.1
64
181.5
104
64.0
144
21.89K
-15
3000
25
624.7
65
176.4
105
62.5
145
20.70K
-14
2872
26
603.3
66
171.4
106
61.1
146
19.58K
-13
2750
27
582.6
67
166.7
107
59.6
147
18.52K
-12
2633
28
562.8
68
162.0
108
58.3
148
17.53K
-11
2523
29
543.7
69
157.6
109
56.8
149
Table 5 - Thermistor Resistance Versus Temperature
55.6
150
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
1
T=
A+B(LnR)+C(LnR)
Equation 6 - Resistance to Temperature
Where;
T = Temperature in °C.
LnR = Natural Log of Thermistor Resistance.
A = 1.4051 × 10-3
B = 2.369 × 10-4
C = 1.019 × 10-7
Note: Coefficients calculated over the −50 to +150° C. span.
-273.2
3
18
APPENDIX C. DERIVING THE CALIBRATION FACTOR (C) FROM THE TEST DATA
Geokon Rebar Strain Meters are calibrated by loading them in a testing machine; hence, the
gauge factor (C) must be determined after converting loads to strains. This is done as follows:
The central section of the rebar strain meter, (the unbonded length) which is 7.5" or 19.05 cm
long, contains a vibrating wire sensor located axially at the midsection. See Figure 9.
Figure 9 - Rebar Strain Meter Schematic
To convert the observed change in readout digits (∆R) into a strain (ε
the unbonded section (7.5 in./19.05 cm), requires the following equation:
(
ε
ε
=
t
Equation 7 - Total Strain Calculation
Where;
L1 is 2.000" (5.08 cm).
L2 is 5.000" (12.7 cm).
L3 is 0.500" (1.27 cm).
ε
is the total strain of the unbonded section.
t
ε
is the strain in zone one, determined empirically from the equation for the vibrating wire
1
sensor, i.e., ε
ε
, ε
are the strains in zones two and three, respectively, dependent on the load and cross-
2
3
sectional area, see Equation 8 and Equation 9.
= ∆R × 0.359 × 10-6 where ∆R is the change in readout digits.
1
1×L1
)+(
ε
L
1+L2+L3
2×L2
)+(
ε
3×L3
) for the entire length of
t
)
19
P
Rebar Size
Diameter
a2
a3
#4
1.264 cm2
2
1.60 cm2
#5
1.973 cm2
2
2.303 cm2
#6
2
2.516 cm2
2
2.852 cm2
#7
2
3.542 cm2
2
3.877 cm2
#8
2
4.729 cm2
2
5.065 cm2
#9
2
6.077 cm2
2
6.413 cm2
#10
2
7.580 cm2
2
7.916 cm2
#11
2
9.239 cm2
2
9.580 cm2
#12
1.714 in2
1.766 in2
#14
1.750 in
2.352 in2
2.404 in2
ε
=
a
×E
Equation 8 - Zone Two Calculation
P
ε
=
a
×E
Equation 9 - Zone Three Calculation
Where;
P is the load in pounds or kilograms.
a2 and a3 are the cross-sectional areas in inches2 or cm2. See Table 6.
Ε is the Young's Modulus, 30×106 psi or 2.1×106 kg/cm2 (or MPa × 10.197)
P is also given by the following equation:
P=∆R×F
Equation 10 - Load in Pounds/Kilograms
Where;
P is the applied load in pounds or kilograms.
∆R is the corresponding change in readout digits.
F is the calibration factor expressed as lbs. or kg. per readout digit.
0.500 in.
1.27 cm
0.625 in.
1.59 cm
0.750 in.
1.905 cm
0.875 in.
2.222 cm
1.000 in.
2.54 cm
1.125 in.
2.858 cm
1.250 in.
3.175 cm
1.375 in.
3.493 cm
1.500 in
Table 6 - Unbonded Section Dimensions
0.196 in2
0.3058 in2
0.390 in
0.549 in
0.733 in
0.942 in
1.175 in
1.432 in
0.248 in
0.357 in
0.442 in
0.601 in
0.785 in
0.994 in
1.227 in
1.485 in
20
6
10
5.7
442.030
5.0
390.030
5
2359.0
−
×
×
+
×
+×
×∆=FR
t
ε
(
)
( )
6
1006205.00957.0
−
×+∆= FR
t
ε
Rebar Size
Conversion Formula - English
Conversion Formula - Metric
#4
C = 0.0957 + F x 0.12220
C = 0.0957 + F x 0.2707
#5
C = 0.0957 + F x 0.07888
C = 0.0957 + F x 0.2091
#6
C = 0.0957 + F x 0.06205
C = 0.0957 + F x 0.1373
#7
C = 0.0957 + F x 0.04416
C = 0.0957 + F x 0.0977
#8
C = 0.0957 + F x 0.03310
C = 0.0957 + F x 0.0733
#9
C = 0.0957 + F x 0.02584
C = 0.0957 + F x 0.0572
#10
C = 0.0957 + F x 0.02073
C = 0.0957 + F x 0.0459
#11
C = 0.0957 + F x 0.01700
C = 0.0957 + F x 0.0377
#12
C = 0.0957 + F x 0.01422
C = 0.0957 + F x 0.0315
#14
C = 0.0957 + F x 0.01037
C = 0.0957 + F x 0.0230
By making the various substitutions of Table 6 the relationship between strain in the unbonded
section versus change in readout digits can be obtained.
Using the #6 rebar as an example, with English conventions, yields the following:
Doing these calculations for the other sizes of rebar yields the data shown in Table 7. Use Table
7 to obtain the microstrain/digit gauge factor (C) from the lbs. or kg. per digit gauge factor (F).
Table 7 - Microstrain Conversion Factors
21
APPENDIX D. SAMPLE CALIBRATION REPORT
Figure 10 - Sample Model 4911 Calibration Report
Loading...