Geokon 4420 User manual

Instruction Manual
Model 4420
VW Crackmeter
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 © 1986-2017 by Geokon, Inc.
Doc Rev U 11/27/2017

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. INSTALLATION ....................................................................................................................................................2
2.1 PRELIMINARY TESTS............................................................................................................................................2
2.2 CRACKMETER INSTALLATION..............................................................................................................................2
2.2.1 Anchors.......................................................................................................................................................2
2.2.2 Installation using Weldable Fixtures..........................................................................................................4
2.2.3 Installation using Groutable Anchors.........................................................................................................5
2.2.4 Installation using Expansion Anchors ........................................................................................................6
2.2.5 Special note regarding installation of Models 4420-1-3MM (.125”), 4420-1-12.5MM (.5”), and 4420-1-
25MM (1”)...........................................................................................................................................................7
2.3 PROTECTION FROM MECHANICAL DAMAGE..........................................................................................................7
2.4 CABLE INSTALLATION AND SPLICING ..................................................................................................................8
2.5 ELECTRICAL NOISE..............................................................................................................................................8
2.6 LIGHTNING PROTECTION .....................................................................................................................................9
3. TAKING READINGS...........................................................................................................................................10
3.1 GK-404 READOUT BOX.....................................................................................................................................10
3.2 GK-405 READOUT BOX.....................................................................................................................................11
3.2.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached.............................................................11
3.2.2 Sensors with Bare Leads...........................................................................................................................11
3.2.3 Operating the GK-405 ..............................................................................................................................11
3.3 GK-403 READOUT BOX (OBSOLETE MODEL)....................................................................................................12
3.3.1 Connecting Sensors with 10-pin Bulkhead Connectors Attached.............................................................12
3.3.2 Connecting Sensors with Bare Leads........................................................................................................12
3.3.3 Operating the GK-403 ..............................................................................................................................12
3.4 MEASURING TEMPERATURES.............................................................................................................................12
4. DATA REDUCTION ............................................................................................................................................13
4.1 DEFORMATION CALCULATION...........................................................................................................................13
4.2 TEMPERATURE CORRECTION.............................................................................................................................15
4.3ENVIRONMENTAL FACTORS...............................................................................................................................16
5. TROUBLESHOOTING........................................................................................................................................17
APPENDIX A. SPECIFICATIONS.........................................................................................................................19
A.1 MODEL 4420 CRACKMETER..............................................................................................................................19
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION.....................................................................20
APPENDIX C. 3D ARRAYS....................................................................................................................................21
APPENDIX D. MODEL 4420HT – HIGH TEMPERATURE VERSION...........................................................25

FIGURES
FIGURE 1-MODEL 4420-1-50/100/150/200/300 VIBRATING WIRE CRACKMETER ....................................................... 1
FIGURE 2-MODEL 4420-1-12/25 DETAIL ..................................................................................................................... 1
FIGURE 3-ANCHOR TYPES WITH DIMENSIONS.............................................................................................................. 2
FIGURE 4-INSTALLATION USING WELDABLE FIXTURES ............................................................................................... 4
FIGURE 5-INSTALLATION USING GROUTABLE ANCHORS.............................................................................................. 5
FIGURE 6-INSTALLATION USING EXPANSION ANCHORS............................................................................................... 6
FIGURE 7-TYPICAL COVER PLATE INSTALLATION ....................................................................................................... 7
FIGURE 8-LIGHTNING PROTECTION SCHEME ............................................................................................................... 9
FIGURE 9-LEMO CONNECTOR TO GK-404 ..................................................................................................................10
FIGURE 10 -LIVE READINGS –RAW READINGS............................................................................................................11
FIGURE 11 -TYPICAL CRACKMETER CALIBRATION SHEET...........................................................................................14
FIGURE 12 -TYPICAL 3D ARRAY..................................................................................................................................21
FIGURE 13 -3D ARRAY,CANTILEVER VERSION...........................................................................................................23
TABLES
TABLE 1-CRACKMETER ANCHOR SPACING DISTANCES............................................................................................... 3
TABLE 2-CRACKMETER READING RANGES.................................................................................................................. 3
TABLE 3–DIMENSIONS OF EXTENDED RANGE COVERS................................................................................................ 8
TABLE 4-ENGINEERING UNITS CONVERSION MULTIPLIERS........................................................................................13
TABLE 5THERMAL COEFFICIENT CALCULATION CONSTANTS .....................................................................................15
TABLE 6-SAMPLE RESISTANCE ...................................................................................................................................18
TABLE 7-RESISTANCE WORK SHEET...........................................................................................................................18
TABLE 8-CRACKMETER SPECIFICATIONS ....................................................................................................................19
TABLE 9-THERMISTOR RESISTANCE VERSUS TEMPERATURE......................................................................................20
TABLE 10 -THERMISTOR RESISTANCE VERSUS TEMPERATURE FOR MODEL 4420HT ..................................................26
EQUATIONS
EQUATION 1- DIGITS CALCULATION ............................................................................................................................13
EQUATION 2-DEFORMATION CALCULATION...............................................................................................................13
EQUATION 3-THERMALLY CORRECTED DEFORMATION CALCULATION......................................................................15
EQUATION 4-THERMAL COEFFICIENT CALCULATION .................................................................................................15
EQUATION 5-RESISTANCE TO TEMPERATURE .............................................................................................................20
EQUATION 6-HIGH TEMPERATURE RESISTANCE TO TEMPERATURE............................................................................25

1
1. INTRODUCTION
Geokon Model 4420 Vibrating Wire Crackmeters are designed to measure movement across
joints such as tension cracks in soils, joints in rock and concrete, and the construction joints in
buildings, bridges, pipelines, dams, etc.
The instrument consists of a vibrating wire sensing element in series with a heat treated, stress
relieved spring which is connected to the wire at one end and a connecting rod at the other. The
unit is fully sealed and operates at pressures of up to 250 psi. As the connecting rod is pulled out
from the gage body, the spring is elongated causing an increase in tension, which is sensed by
the vibrating wire element. The increase in tension (strain) of the wire is directly proportional to
the extension of the shaft. This change in strain allows the Model 4420 to measure the opening of
the joint very accurately.
Figure 1 - Model 4420-1-50/100/150/200/300 Vibrating Wire Crackmeter
Models 4420-3, 4420-12.5 and 4420-25 differ slightly from the standard Crackmeter in that they
provide for adjustment of the setting distance with a threaded extension rod and locking nut.
Figure 2 - Model 4420-1-12/25 Detail
CAUTION!Do not rotate the shaft of the Crackmeter more than 180 degrees: This may
cause irreparable damage to the instrument. The alignment pin on the transducer shaft
and slot on the body serve as a guide for alignment. Never extend the crackmeter beyond
its working range.

2
2. INSTALLATION
2.1 Preliminary Tests
Upon receipt of the instrument, the gage should be checked for proper operation (including the
thermistor). The crackmeter normally arrives with its shaft secured at approximately 50% of its
range. This is accomplished by a dowel pin held in place with a piece of tape for crackmeters
with a range of 100 mm (four inches) or smaller (see Figure 2). For crackmeters with a range
greater than 100 mm, a slotted sleeve made of PVC is used. These devices hold the crackmeter in
tension to protect it during shipping. With the shipping spacers still in place, connect the gage to
a readout box and take a reading. (See Section 3 for readout instructions.) The reading should be
stable and in the range of 4000 to 5000 digits. Please note that crackmeters with a 3 mm (.125
inch) range are shipped with the push rod fully retracted and have no shipping spacer to remove.
These gages should read between 2000 to 3000 digits.
Remove the PVC slotted sleeve or dowel pin before proceeding further.
Checks of electrical continuity can be made using an ohmmeter. Resistance between the gage
leads should be approximately 180 ohms, ±10 ohms. Remember to add the cable resistance,
which is approximately 14.7Ωper 1000 ft. (48.5Ωper km) of 22 AWG stranded copper leads at
20 °C. Multiply this factor by two to account for both directions. Resistance between the green
and white conductors will vary based on temperature; see Table 9 in Appendix B for standard
crackmeters, Appendix D, Table 10 for model 4420HT. Resistance between any conductor and
the shield should exceed two megohms.
2.2 Crackmeter Installation
For additional instructions regarding 3D Arrays and Model 4420HT see Appendix C and
Appendix D respectively.
2.2.1 Anchors
Three types of anchors are available from the factory for installing the Model 4420
Vibrating Wire Crackmeter:
Figure 3 - Anchor Types with Dimensions

3
The weldable fixture is designed to install the Crackmeter on steel members. The
machine bolt expansion anchors and groutable anchors are used to install the Crackmeter
on concrete or rock. The anchors are installed at the appropriate spacing distance (Table
1) depending on the anticipated direction of movement (extension or compression).
Sections 2.2.2 through 2.2.4 give detailed instructions for each type of anchor. See
Section 2.2.5 for special instructions for Models 4420-1-3MM (.125”), 4420-1-12.5MM
(.5”), and 4420-1-25MM (1”).
Model & Range Midrange
To Monitor
Extension
To Monitor
Compression
4420-3 mm
(.125")
292.6 mm
(11.52")
291.1 mm
(11.46")
294.1 mm
(11.58")
4420-12.5 mm
(.5")
317 mm
(12.5")
310 mm
(12.2")
325 mm
(12.8")
4420-25 mm
(1")
343 mm
(13.5")
330 mm
(13")
356 mm
(14")
4420-50 mm
(2")
396 mm
(15.6”)
371 mm
(14.6”)
422 mm
(16.6")
4420-100 mm
(4")
554 mm
(21.8")
503 mm
(19.8")
605 mm
(23.8")
4420-150 mm
(6")
645 mm
(25.4")
569 mm
(22.4")
721 mm
(28.4")
4420-200 mm
(8”)
869 mm
(34.2”)
767 mm
(30.2”)
970 mm
(38.2”)
4420-300 mm
(12”)
1186 mm
(46.7")
1034 mm
(40.7")
1339 mm
(52.7")
Note for model 4420HT: Due to the U-joint configuration of 4420HT, the overall
gage assembly length is increased by 35 mm (1.375”). This length should be added to
the anchor spacing distance shown above.
Table 1 - Crackmeter Anchor Spacing Distances
When setting the gage position using a portable readout, use the reading ranges in Table 2
to determine the proper position.
Approximate Midrange
Reading
Approximate Reading to
Monitor Extensions
Approximate Reading to
Monitor Compressions
4500-5000
2500-3000
6500-7000
Table 2 - Crackmeter Reading Ranges
Note that the calibration sheet (Figure 11) supplied with the Crackmeter shows actual
readings at zero, 25%, 50%, 75%, and 100% of the range of extension. These readings
can be used as a guide to set the Crackmeter in any part of its range, either in anticipation
of closure or opening of the crack. Extend the crackmeter until the desired reading is
obtained. Hold the crackmeter in this position while the distance between the cap screws
(set inside the swivel bearings, see Figure 1) is measured. This measurement can serve as
a spacing guide for drilling or welding the anchor points. The alignment pin on the
transducer shaft and slot on the body serve as a guide for alignment. Do not rotate the
shaft of the Crackmeter more than 180 degrees. This may cause irreparable damage
to the instrument.

4
2.2.2 Installation using Weldable Fixtures
Figure 4 - Installation using Weldable Fixtures
Installation instructions:
1) Determine the proper setting distance using the figures in Table 1, or the readings on
the calibration sheet.
2) Prepare the surface of the steel (grinding, sanding, etc.) around the area of each
weldable fixture.
3) Locate the welding fixtures on prepared surfaces.
4) Check the spacing again and tack weld to the member.
5) Remove the PVC slotted sleeve or dowel pin securing the transducer shaft.
6) Thread the cap screw through the swivel bearing and through the half-inch spacer on
each end.
7) Tighten the cap screws into the welding fixtures.
8) Check and record the reading with a portable readout. Use Table 2 or the readings on
the calibration sheet to check the position.

5
2.2.3 Installation using Groutable Anchors
Figure 5 - Installation using Groutable Anchors
Installation instructions:
1) Determine the proper setting distance using the figures from Table 1 or the readings
on the calibration sheet.
2) Using a hammer drill (or other suitable equipment), drill two half-inch holes
approximately three inches deep at the proper locations. Shorter holes may be drilled
if the anchors are cut down accordingly.
3) Push the cap screws through the swivel bearings and spacers on each end of the
crackmeter and then loosely thread them into the groutable anchors.
4) If installing the instrument at the midrange position, leave the PVC slotted sleeve or
dowel pin that secures the transducer shaft in the midrange position installed.
5) Fill the holes with grout or epoxy. For holes drilled overhead use a quick setting grout
or epoxy.
6) Push the anchors in until the tops are flush with the surface.
7) After the grout or epoxy has set, tighten the set screws.
8) Remove the PVC slotted sleeve or dowel pin if it was not removed earlier.
9) Check and record the reading with a portable readout. Use Table 2 or the readings on
the calibration sheet to check the position.

6
2.2.4 Installation using Expansion Anchors
Figure 6 - Installation using Expansion Anchors
Installation instructions:
1) Determine the proper setting distance using the figures from Table 1 or the readings on the
calibration sheet.
2) Using a masonry drill (or other suitable equipment), drill two 3/8 inch (10 mm) diameter
holes, 1.25" (32 mm) deep at the proper locations.
3) Insert the expansion anchors into the holes, with the slotted end down.
4) Insert the provided setting tool, small end first, into the anchor. Expand the anchor by hitting
the large end of the setting tool with several sharp hammer blows.
5) Remove the PVC slotted sleeve or dowel pin securing the transducer shaft.
6) Push the cap screws through the swivel bearings and spacers on each end of the crackmeter
and then tighten the cap screws into the anchors.
7) Check and record the reading with a portable readout. Use Table 2 or the readings on the
calibration sheet to check the position.

7
2.2.5 Special note regarding installation of Models 4420-1-3MM (.125”), 4420-1-
12.5MM (.5”), and 4420-1-25MM (1”)
If the reading is not in the proper range after installation, additional adjustment may be
made using the threaded extension at the end of the transducer shaft. In order for
adjustments to be accurately made, the transducer needs to be attached to the anchor at
the cable end, and free at the opposite end.
To make an adjustment, loosen the locking nut then rotate the threaded rod in or out of
the end of the transducer shaft. The transducer shaft should be gripped while rotating
the threaded rod. The transducer shaft must never be rotated beyond 180 degrees or
gage failure may result! After make an adjustment, align the hole in the swivel bearing
over the anchor and check the reading. Continue to make adjustments until the desired
reading is shown on the readout. Once the desired reading is obtained, push the cap screw
through the swivel bearing and spacer and then tighten into the anchor.
2.3 Protection from mechanical damage
Protecting the crackmeter from damage can be accomplished by using cover plates available
from Geokon, which are made from sheet steel formed into a channel shape (Model 4420-7). The
standard cover plate is long enough to cover the two-inch range crackmeter; longer range
crackmeters utilize multiple cover plates tack welded together.
Use the mounting hardware provided to install the cover plates as follows:
1) Anchor the two 3/8 x 2” long threaded rods in place head down using either groutable or
expansion anchors. The bolts should be spaced at a nominal 21 inches (530 mm) apart. A
spacer jig is available from Geokon, or the cover plate can be flipped onto its back and the
holes in the cover plate can be used to mark the bolt locations. One hole in the cover plate is
slotted, so the spacing is not critical.
2) Place the cover plate over the welded bolts.
3) Install washers, then nuts. Avoid excessive force while tightening the cover retaining nuts.
See Figure 7 for a diagram of the completed assembly.
Figure 7 - Typical Cover Plate Installation

8
For crackmeters with a range greater than two inches, tack weld multiple cover plates together.
They should be positioned so that the extended slots are on both sides to accommodate the range
of motion. The mounting nut and washer should only be loosely tightened to enable the cover to
slide on the 3/8-16 threaded rods. An extra nut is provided as a locknut. Critical dimensions of
the extended range covers are shown in Table 3.
Range
Total
Length
Hole
Spacing
Slot
Lengths
100 mm (4")
36"
32.5"
2"
150 mm (6")
36"
31.5"
3"
200 mm (8")
48"
42.5"
4"
300 mm (12")
60"
52.5"
6"
Table 3 – Dimensions of Extended Range Covers
2.4 Cable Installation and Splicing
The cable should be routed to minimize the possibility of damage due to moving equipment,
debris or other causes. The cable can be protected by the use of 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 gages 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 gage 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, 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 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.
2.5 Electrical Noise
Care should be exercised when installing instrument cables to keep them as far away as possible
from sources of electrical interference such as power lines, generators, motors, transformers, arc
welders, etc. Cables should never be buried or run with AC power lines. The instrument cables
will pick up the 50 or 60 Hz (or other frequency) noise from the power cable and this will likely
cause a problem obtaining a stable reading. Contact the factory concerning filtering options
available for use with the Geokon dataloggers and readouts should difficulties arise.

9
2.6 Lightning Protection
The Model 4420 Vibrating Wire Crackmeter, unlike numerous other types of instrumentation
available from Geokon, does not have any integral lightning protection components, i.e.
transzorbs or plasma surge arrestors. Usually this is not a problem however, 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 gage and possibly destroy it.
Note the following suggestions:
•If the gage 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 installation of these components.
•Lighting arrestor boards and enclosures are available from Geokon that install near the
instrument. The enclosure has a removable top, allowing access to the protection board. In
the event that the (LAB-3) is damaged, the user may service the components or replace the
board. A connection is made between this enclosure and earth ground to facilitate the passing
of transients away from the gage. See Figure 8. Consult the factory for additional information
on these or alternate lightning protection schemes.
•Plasma surge arrestors can be epoxy potted into the gage cable close to the sensor. A ground
strap would connect the surge arrestor to earth ground, either a grounding stake or other
suitable earth ground.
Terminal Box/Multiplexer
Instrument Cable LAB-3 Enclosure LAB-3 Board
Model 4420 Crackmeter
Structure
Ground Connections
Surface
(usually buried)
Crack
Figure 8 - Lightning Protection Scheme

10
3. TAKING READINGS
3.1 GK-404 Readout Box
The Model GK-404 Vibrating Wire Readout is a portable, low-power, handheld unit that is
capable of running for more than 20 hours continuously on two AA batteries. It is designed for
the readout of all Geokon vibrating wire gages and transducers, and is capable of displaying the
reading in either digits, frequency (Hz), period (µs), or microstrain (µε). The GK-404 also
displays the temperature of the transducer (embedded thermistor) with a resolution of 0.1 °C.
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 9). Insert the
Lemo connector into the GK-404 until it locks into place.
Figure 9 - 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 display:
Geokon Inc.
GK-404 verX.XX
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 gage in degrees Celsius.
Use the POS button to select position Band 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 no reading displays or the reading is
unstable, consult Section 5 for troubleshooting suggestions.
For further information, please refer to the GK-404 manual.

11
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 via a cable to the vibrating wire gage 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 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.
3.2.2 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. Choose display mode “B”. Figure 10
shows a typical vibrating wire output in digits and thermistor output in degrees Celsius. If
no reading displays or the reading is unstable, see Section 5 for troubleshooting
suggestions. For further information, consult the GK-405 Instruction Manual.
Figure 10 - Live Readings – Raw Readings

12
3.3 GK-403 Readout Box (Obsolete Model)
The GK-403 can store gage readings and apply calibration factors to convert readings to
engineering units. The following instructions explain taking gage measurements using Mode
"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 gage 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.
3.4 Measuring Temperatures
All vibrating wire transducers 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: Connect an ohmmeter to the green and white
thermistor leads coming from the displacement transducer. (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 14.7 Ωfor every 1000 ft., or 48.5Ωper km at
20 °C. Multiply these factors by two to account for both directions.) Look up the temperature for
the measured resistance in Appendix B, Table 9 for standard crackmeters; Appendix D, Table 10
for model 4420HT.

13
4. DATA REDUCTION
4.1 Deformation Calculation
The basic units utilized by Geokon for measurement and reduction of data from Vibrating Wire
Crackmeters are "digits". Calculation of digits is based on the following equation:
Digits =󰇡1
Period󰇢2x 10-3 or Digits=Hz2
1000
Equation 1- Digits Calculation
To convert digits to deformation the following equation applies:
Duncorrected = (R1- R0) ×G ×F
Equation 2 - Deformation Calculation
Where;
R1is the current reading.
R0is the initial reading, usually obtained at installation.
G is the gage factor, usually millimeters or inches per digit (see Figure 11).
F is an optional engineering units conversion factor, see Table 4.
From→
To
↓
Inches Feet Millimeters Centimeters Meters
Inches
1
12
0.03937
0.3937
39.37
Feet
0.0833
1
0.003281
0.03281
3.281
Millimeters
25.4
304.8
1
10
1000
Centimeters
2.54
30.48
0.10
1
100
Meters
0.0254
0.3048
0.001
0.01
1
Table 4 - Engineering Units Conversion Multipliers
For example, the initial reading R0, at installation of a crackmeter is 2500 digits. The current
reading, R1, is 6000. The gage factor is 0.006223 mm/digit. The deformation change is:
Duncorrected = (6000 −2500) ×0.006223 = +21.78 mm
Note that increasing readings (digits) indicate increasing extension.
To use the Polynomial Gage factors given on the Calibration Sheet, use the value of R0and Gage
Factors A and B with D set to zero to calculate the new value of C. then substitute the new value
of R1and use A, B and the new value of C to calculate the displacement D.

14
Figure 11 - Typical Crackmeter Calibration Sheet