Geokon 4050 User manual

Instruction Manual
Model 4000
(and 4050)
Vibrating Wire Strain Gage
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon, Inc.
The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for
errors, omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1981, 1996, 2004, 2005, 2008, 2009, 2010, 2012, 2013, 2014, 2016 by Geokon, Inc.
(Doc Rev Z, 8/16)

Warranty Statement
Geokon, Inc. 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, Inc. 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, Inc. or any breach of any
warranty by Geokon, Inc. shall not exceed the purchase price paid by the purchaser to Geokon, Inc. 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, Inc. 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. GAGE INSTALLATION -GENERAL...................................................................................................... 2
2.1 PRELIMINARY TESTS ............................................................................................................................. 2
2.2 ARC WELDING THE MOUNTING BLOCKS TO STEEL SURFACES –USING THE SPACER BAR............... 2
2.3 ARC WELDING SEQUENCE.................................................................................................................... 3
2.4 SETTING THE STRAIN GAGE ................................................................................................................. 3
2.5 INITIAL READINGS ................................................................................................................................. 4
3. GAGE PROTECTION .............................................................................................................................. 4
3.1 PROTECTION FROM CORROSION .......................................................................................................... 4
3.2 PROTECTION FROM DIRECT SUNLIGHT AND RAPID CHANGES IN AMBIENT TEMPERATURE................... 4
3.3 PROTECTION FROM MECHANICAL DAMAGE........................................................................................... 4
3.4 CABLE AND CONNECTOR PROTECTION ................................................................................................. 5
3.5. LIGHTNING PROTECTION ..................................................................................................................... 6
4. GAGE LOCATION.................................................................................................................................... 7
4.1. END EFFECTS ...................................................................................................................................... 7
4.2. WELDING EFFECTS .............................................................................................................................. 7
4.3 BENDING MOMENTS.............................................................................................................................. 7
5. SPECIAL APPLICATIONS ................................................................................................................... 11
5.1 INSTALLATION ON DRIVEN STEEL PILES............................................................................................ 11
5.2 INSTALLATION ON STEEL OR CONCRETE SURFACES USING EPOXY CEMENTS................................ 12
5.2.1 Concrete Surfaces: .................................................................................................................. 12
5.2.2 Steel Surfaces: ......................................................................................................................... 12
5.3 INSTALLATION ON CONCRETE SURFACES USING ANCHOR STUDS ..................................................... 13
6. TAKING READINGS.............................................................................................................................. 13
6.1. OPERATION OF THE GK-403 READOUT BOX .................................................................................... 14
6.2 OPERATION OF THE GK-404 READOUT BOX ..................................................................................... 14
6.3 OPERATION OF THE GK-405 READOUT BOX ..................................................................................... 15
6.4. MEASURING TEMPERATURES ............................................................................................................ 15
7. DATA INTERPRETATION ................................................................................................................... 16
7.1 CONVERSION OF THE READINGS TO STRAIN CHANGES.................................................................... 16
7.2 CONVERTING STRAINS TO STRESSES ............................................................................................... 16
8. TROUBLESHOOTING........................................................................................................................... 17
APPENDIX A - SPECIFICATIONS........................................................................................................... 18
A.2 THERMISTOR (SEE APPENDIX CALSO).............................................................................................. 18
APPENDIX B - THEORY OF OPERATION ............................................................................................ 19
APPENDIX C - THERMISTOR TEMPERATURE DERIVATION......................................................... 21
APPENDIX D - MODEL 4050 SPECIAL INSTRUCTIONS (12” GAGE LENGTH) .......................... 22
APPENDIX E - MEASUREMENT OF, AND CORRECTION FOR, TEMPERATURE EFFECTS... 23
APPENDIX F - TEMPERATURE CORRECTION WHEN USED ON CONCRETE........................... 24
APPENDIX G - CALCULATION OF AXIAL LOADS AND BENDING STRAINS FROM THREE
STRAIN GAGES, AT 60 DEGREES, ON A CIRCULAR PIPE............................................................ 25
APPENDIX H - TWO STRAIN GAGES MOUNTED ONE ABOVE THE OTHER ............................ 26

LIST of FIGURES, TABLES and EQUATIONS
FIGURE 1-MODEL 4000 VIBRATING WIRE STRAIN GAGE ............................................................................. 1
FIGURE 2-SPACING JIG .................................................................................................................................. 2
FIGURE 3-WELDING SEQUENCE FOR THE MOUNTING BLOCKS .................................................................... 3
FIGURE 4-TYPICAL COVER PLATE INSTALLATION ......................................................................................... 5
FIGURE 5-LIGHTNING PROTECTION SCHEME ................................................................................................ 6
FIGURE 6A -STRAIN GAGES MOUNTED ON CENTRAL WEB MEASURES AXIAL STRAIN AND BENDING
MOMENTS ABOUT BOTH XX AND YY AXES ............................................................................................ 8
EQUATION 1-AXIAL STRESS CALCULATION ................................................................................................... 8
EQUATION 2-STRESS DUE TO BENDING ON AXIS YY...................................................................................... 8
EQUATION 3-STRESS DUE TO BENDING ON AXIS XX...................................................................................... 8
EQUATION 4-MAXIMUM STRESS .................................................................................................................... 8
FIGURE 6B -STRAIN GAGES MOUNTED ON FLANGES (NOT RECOMMENDED ON TUNNEL ARCHES)........... 9
FIGURE 6C -AXIAL STRAIN MEASUREMENT AND BENDING MOMENT ABOUT YY AXIS ONLY......................... 9
FIGURE 6D -AXIAL STRAIN AND BENDING MOMENTS ABOUT XX AXIS ........................................................ 10
FIGURE 6E –AXIAL STRAIN AND BENDING MOMENT AROUND AXIS XX (NOT RECOMMENDED).................. 10
FIGURE 7PROTECTION ON DRIVEN PILES .................................................................................................... 11
FIGURE 8INSTALLATION USING EPOXY ......................................................................................................... 12
FIGURE 9-INSTALLATION ON CONCRETE USING GROUTABLE ANCHORS .................................................... 13
TABLE 1-STRAIN GAGE READOUT STRAIN RANGES ................................................................................... 13
EQUATION 5-APPARENT STRAIN CALCULATION .......................................................................................... 16
EQUATION C-1 CONVERT THERMISTOR RESISTANCE TO TEMPERATURE................................................... 21
TABLE C-1 THERMISTOR RESISTANCE VERSUS TEMPERATURE...................................................................... 21

-1 -
1. INTRODUCTION
The Geokon Model 4000 Vibrating Wire Strain Gage is intended primarily for long-term or
short term strain measurements on structural steel members such as tunnel linings, arches,
struts, piles, sheet piling, etc.
The primary means of attachment is by conventional arc welding, but they may also be used
to monitor strain changes on concrete or rock surfaces using anchors grouted into
boreholes.
Strains are measured using the vibrating wire principle: a length of steel wire is tensioned
between two mounting blocks that are welded to the steel surface being studied.
Deformations (i.e., strain changes) of the surface will cause the two mounting blocks to
move relative to one another, thus altering the tension in the steel wire. The tension in the
wire is measured by plucking the wire and measuring its resonant frequency of vibration.
The wire is plucked, and its resonant frequency measured, by means of an electromagnetic
coil positioned next to the wire. See Figure 1.
Figure 1 - Model 4000 Vibrating Wire Strain Gage
Portable readouts available from Geokon, used in conjunction with the Vibrating Wire Strain
Gage, will provide the necessary excitation to pluck the wire and will convert the measured
frequency so as to display the reading directly in microstrain.
This manual contains installation instructions, readout instructions and recommended
maintenance and troubleshooting procedures. The theory of the gage is also given, along
with some suggestions for data interpretation.
Protective Tube
Instrument Cable
Pluck & Read CoilsThermistor
Wire
Coil & Thermistor Housing
Mounting Block
Wire Grip
Wire Grip
(4 conductor, 22 AWG)
Gage Length
(6.500", 165 mm)
Mounting Block
O-ring Sealed End Block
Set Screw Set Screws (2 places)
(cone point)
(cone point)

2
2. GAGE INSTALLATION -GENERAL
2.1 Preliminary Tests
A preliminary check is advisable, and this is made by placing the coil assembly onto the
gage and connecting it to the GK-401, GK-402 or GK-403 Readout Box. Switch the position
selector to "C" and turn the unit on. While gently pulling on the gage end blocks, observe
the reading; it should be seen to increase with increased tension. Do not apply excessive
tension, (>10Kgm (20lbs)), as the wire could break. The nominal reading range is 1000 to
4000 microstrain. Mid-range is approximately 2500 microstrain.
Check the resistance between the two lead wires (red and black). It should be around 180
ohms. If the gage contains a thermistor, check its resistance between the white and green
lead wires. (The resistance should be around 3000 ohms at 25 degrees C). Check the
reading against that which should be obtained at the existing ambient temperature. See
Appendix C for the resistance versus temperature correlation.
Return any faulty gages to the factory. Gages should not be opened in the field.
2.2 Arc Welding the Mounting Blocks to Steel Surfaces – Using the Spacer Bar
The Model 4000 Vibrating Wire Strain Gage is attached to mounting blocks that must first be
arc welded to the steel surface to be studied. A spacer bar and spacing jig are used to
correctly space apart the two blocks. Figure 2 shows the procedure: the two mounting
blocks are fitted over the ends of the spacer bar, and the jig is used to position them
correctly, while the set screws in the mounting blocks are tightened down onto the spacer
bar. Avoid excessive tightening as this only damages the spacer bar unduly.
The mounting blocks are supplied in pairs; one has a single cone point set-screw, the other
has two cone point set-screws.
Aluminum Block
Mounting Block
Spacer Bar (in slot) Mounting Block
Position
Position
S
etting Distance
Figure 2 - Spacing Jig

-3 -
2.3 Arc Welding Sequence
The steel surface is cleaned using a wire brush to remove all scale, rust, dirt and oil. The
blocks are then removed from the spacing jig and pressed firmly against the steel surface
using the spacer bar as a handle. The edges of the mounting blocks are now welded in the
order as shown in Figure 3.
1
3
4
2
SPACER BAR
WELD
Figure 3 - Welding Sequence for the Mounting Blocks
Avoid excessive heat and DO NOT WELD THE FLAT END SURFACES as this will prevent
removal of the spacer bar. Avoid welding splatter which could stick to the spacer bar. To
speed things up, where many gages are being installed, it is advantageous to have
more than one spacer bar.
After welding, cool the mounting blocks with a water-soaked rag, then slacken the set
screws and slide out the spacer bar. Clean away all welding slag using a chipping hammer
and wire brush. (Optional: Paint over the surface to provide some protection against
corrosion).
2.4 Setting the Strain Gage
When the mounting blocks have been welded in place the strain gage can be slid into the
holes in the mounting blocks. One end of the strain gage has a V-groove in it - make sure
that this end goes inside the mounting block that has the single cone-point set screw.
Tighten hard the cone point set screw into the V-groove. Now clip the coil housing around
the gage and connect to the readout box (Channel C). Set the initial reading on the gage to
the correct level depending on whether compressive or tensile strains are anticipated. The
useable range of the strain gage runs from around 1000 to 4000 microstrains. The mid-
range reading is 2500. The reading can be adjusted by simply pulling or pushing on the free
end of the strain gage. Strain gages are shipped with a reading of around 3000 to 3500.
This level is OK for compressive strains. If tensile strains are to be measured set the initial
reading to around 1500.
When the desired reading has been achieved tighten the two cone-point set screws in the
mounting block: tighten hard down onto the end of the strain gage. Tighten the hose clamp
onto the coil housing: tighten hard using a nut driver. Finish off by tapping the mounting
blocks with, say, the handle of a screwdriver, to remove any installation strains and
stabilize the initial reading. Tap until the reading remains stable

4
2.5 Initial Readings
All readings are referred to an initial reading, so it is important that this initial reading be
carefully taken. Remember to tap the gages as described in section 2.4. It is preferable to
install gages on steel members, which are still in an unloaded condition, i.e., prior to their
assembly into the structure. In this way, the initial readings correspond to zero load,
otherwise, if the member is under load the initial readings will correspond to some unknown
load level.
3. GAGE PROTECTION
3.1 Protection from corrosion at the weld points can be inhibited by applying a coat of
rust-preventative paint.
3.2 Protection from direct sunlight and rapid changes in ambient temperature is
provided best by covering the gages with a layer of insulating material. This can be important
because the gage can heat up and cool down much faster than the underlying steel and this
can give fairly large spurious readings. The thermal coefficient of expansion of the steel
vibrating wire is the same as that for the steel of the structure to which the gage is attached,
so that no temperature correction to the measured strain is required when calculating
load induced strains. However, this is only true if the wire and the underlying steel structure
are at the same temperature. If sunlight is allowed to impinge directly onto the gage, then this
could elevate the temperature of the wire above the surrounding steel and cause large
changes in apparent strain. Therefore, always shield strain gages from direct sunlight.
Also, avoid excessive handling of the gage prior to taking zero readings. Always allow
sufficient time for the gage temperature to stabilize before reading. In any case, it is always a
good idea to record the temperature every time the strain reading is made, so that any loads
and strains, caused purely by temperature changes, can be assessed. (See also Appendix E
and G.) In order to facilitate the measurement of temperature, each strain gage has a
thermistor encapsulated along with the plucking coil. The thermistor is connected to the green
and white conductors. The GK403 and GK404 readout boxes display the temperature directly
in degrees C. An ohmmeter can also be used. (The relationship between resistance and
temperature is shown in Appendix C).
3.3 Protection from mechanical damage can be had by using the cover plates made
by Geokon. (See Figure 4) Two 3/8 x 2 inch long hex-head bolts which will hold the cover
plates should be welded in place hex-head down. The special cover plates are made from
sheet steel formed into a channel shape. To get the correct spacing for the bolts a spacer jig
is available or the cover plate can be flipped over on its back and the holes in the cover
plate used to mark the bolt locations. The holes are spaced at a nominal 21 inches (530mm)
apart: one hole is slotted so that the spacing is not critical. Studs should not be positioned
within 6 inches of the strain gage and excessive force should not be used when tightening
the cover retaining nuts as this will distort the underlying steel surface and can give rise to
spurious strain readings. Also avoid welding anywhere near the gage as this also will cause
large local distortions of the metal Either a special stud welder or an arc welder can be
used to weld