Geokon 4200ER Manual de usuario

Instruction Manual

Model 4200ER

Extended Range

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.

(Doc Rev. A, 04/07/2017)

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

Extended Range......................................................................................................1

Vibrating Wire Strain Gage...............................................................................1

1. INTRODUCTION ......................................................................................................1

2. GAGE INSTALLATION ...........................................................................................2

2.1. Adjusting the Desired Range.......................................................................2

2.2. Placing the Gage in Concrete.......................................................................3

2.3. Using Pre-cast Briquettes or Grouting.........................................................5

2.4. Cable Protection and Termination...............................................................5

2.5. Lightning Protection ....................................................................................5

3. TAKING READINGS................................................................................................7

3.1. Operation of the GK-403 Readout Box.......................................................7

3.2. Operation of the GK-404 Readout Box.......................................................7

3.3. Operation of the GK-405 Readout Box.......................................................8

3.4. MICRO-6000 Datalogger ............................................................................9

3.5. Measuring Temperatures ............................................................................9

4.DATA REDUCTION FOR THE GK-403, GK-404 or GK-405 in POSITION D......10

4.1. Conversion of Readings to Strain Changes .................................................10

4.2. Temperature Corrections .............................................................................11

4.3. Shrinkage Effects.........................................................................................12

4.4. Creep Effects................................................................................................12

4.5. Effect of Autogenous Growth......................................................................12

5. TROUBLESHOOTING..............................................................................................13

APPENDIX A - SPECIFICATIONS...............................................................................14

A.1 4200ER Strain Gage......................................................................................14

A.2 Thermistor (see Appendix C also)................................................................14

APPENDIX B - THERMISTOR TEMPERATURE DERIVATION .............................15

APPENDIX C - HIGH-TEMPERATURE THERMISTOR LINEARIZATION............16

LIST of FIGURES, TABLES and EQUATIONS

Figure 1 - Model 4200ER Vibrating Wire Strain Gage.................................................................. 1

Figure 2 Adjusting the range.......................................................................................................... 2

Figure 3 - Attaching VCE-4200X Strain Gages to Rebar............................................................... 3

Figure 3B - Alternative Method for attaching VCE 4200X straingages to rebar........................... 4

Figure 4 - Lightning Protection Scheme......................................................................................... 6

Table 1 - Embedment Strain Gage Readout Positions.................................................................... 7

Figure 5 GK405 Readout Unit....................................................................................................... 8

Table 2 - Embedment Strain Gage Datalogger Parameters............................................................ 9

Equation 1 - Strain Calculation, channel D .................................................................................. 10

Table A-1 Strain Gage Specifications.......................................................................................... 14

Equation C-1 Convert Thermistor Resistance to Temperature.................................................... 15

Table C-1 Thermistor Resistance versus Temperature................................................................ 15

Table B-2: High Temperature Thermistor Resistance versus Temperature. ................................ 16

1. INTRODUCTION

The Geokon Model 4200ER Vibrating Wire Strain Gage is designed primarily for high range

strain measurements in mass concrete, in structures such as foundations, piles, bridges,

dams, containment vessels, tunnel liners, etc. See Figure 1. Strains as high as 10,000

mirostrain can be accomodated

Figure 1 - Model 4200ER Vibrating Wire Strain Gage

Note the small collar under the shrink tube at one end.

The primary means of gage placement is direct embedment in concrete by pre-attaching the

gage to rebar or tensioning cables, pre-casting the gage into a concrete briquette which is

subsequently cast into the structure, or grouting into boreholes in the concrete.

Strains are measured using the vibrating wire principle: a length of steel wire is tensioned

between two end blocks that are firmly in contact with the mass concrete. Deformations in

the concrete will cause the two end blocks to move relative to one another, altering the

tension in the steel wire. This change in tension is measured as a change in the resonant

frequency of vibration of the wire. Electromagnetic coils that are located close to the wire

accomplish excitation and readout of the gage frequency.

Portable readouts or dataloggers available from Geokon, such as the model, GK-403, GK

404 or MICRO-6000 datalogger, used in conjunction with any of these vibrating wire strain

gages, will provide the necessary voltage pulses to pluck the wire and convert the measured

frequencies so as to display the reading directly in digits which can then be converted into

microstrain using the calibration coefficients supplied.

This manual contains installation instructions, readout and data reduction procedures, and

troubleshooting guidelines.

PLEASE NOTE THE FOLLOWING:



Do not rotate or pull on the gage end blocks as this will alter the readings and may

cause permanent damage.

2. GAGE INSTALLATION

The 4200ER strain gages are supplied fully sealed and pre-tensioned with the plucking coil

mounted. A preliminary check is advisable and this is made by connecting to the readout

box and observing the displayed readout (see readout instructions, Section 3). The

observed reading should be around the mid-range position (see Table 1). Pressure on the

gage ends should make this reading decrease.

Check the resistance between the two lead wires (usually red and black). For the model

4200ER it should be around 180 ohms. Remember to add the cable resistance at

approximately 14.7/1000' or 48.5/km (at 20°C, multiply by 2 for both directions). If the

gage contains a thermistor, check its resistance (usually the white and green lead wires)

with an ohmmeter. Check the reading against that which should be obtained at the existing

ambient temperature. See Appendix B for the resistance to temperature conversion and

resistance v. temperature table.

Return any faulty gages to the factory. Gages should not be opened in the field.

2.1. Adjusting the Desired Range

Gages are supplied with the wire tension set in approximate rmid-range If the range needs to

be at some other position this is accomplished by the following procedure: Attach the red and

black leeds to a readout box with the channel position set in the D and the mode for

microstrain. Grip the small collar under the shrink tube and rotate the end flange adjacent to

it as shown in Figure 2

Figure 2 Adjusting the range

Rotate clockwise to decrease the initial reading and counter-clockwise to increase the

reading. Please remember that although the readings are taken on Channel D, in the digits

output mode, the digits shown are not directly in microstrain but must be converted to

microstrain by multiplying the observed digits change by the gage factor given on the

calibration sheet.

2.2. Placing the Gage in Concrete

The Model 4200ER strain gages are normally set into the concrete structure in one of two

ways: either by casting the unit(s) into the concrete mix directly or by pre-casting the unit(s)

into briquettes that are subsequently cast into the structure.

When casting the gage directly into the structure care must be taken to avoid applying any

large forces to the end blocks during installation. The model 4200ERcan be wired into

position by wiring directly to the tube (see Figure 3). The wires should not be tied too tightly

since rebar and/or tension cables tend to move during concrete placement and vibration.

Care should be taken not to damage the cable with the vibrator. The gage can also be

placed directly into the mix if it can be assured that the orientation will be correct after the

gage placement.

Attached to rebar with blocking Suspended between rebars

Coil Assembly

Instrument Cable

Nylon Ty-rap

Wire Tie

Rebar or Tensioned Cable Nylon Ty-rap

Wire Tie

Wire Tie Instrument Cable

Wire Tie

Coil Assembly

(VCE-4200) (VCE-4200)

Figure 3 - Attaching VCE-4200X Strain Gages to Rebar

Note the following instructions to suspend the model VCE-4200X strain gage between rebar:

1. Wrap a layer of self-vulcanizing rubber tape around the gage in the two places

shown in Figure 3 (around the tie points). The rubber layer serves as a shock

absorber, dampening any vibrations of the suspension system. Sometimes,

without the rubber layers, as the tie wires are tightened the resonant frequency

of the tie wires interferes with the resonant frequency of the gage. This results

in unstable readings or no readings at all. This effect disappears once the

concrete has been placed.

2. Select a length of soft iron tie wire, the kind normally used for tying rebar cages

together. Twist it 2 times around the body of the strain gage, over the rubber strips,

about 3 cm from the gage ends.

3. Twist two loops in the wire, one on either side of the gage, at a distance of about

3cm

from the gage body. Repeat this process at the other end of the gage.

4. Position the gage between the rebar and twist the wire ends twice around the rebar,

then around itself.

5. Tighten the wire and orient the gage by twisting on the loops.

6. Slip on the plucking coil and affix using a hose clamp. Tie the instrument cable off to

one of the rebar using nylon Ty-Raps™.

Wood or

Styrofoam

Block Rubber Tape

Rubber Tape

Alternative Method:

Tie two short pieces of steel rebar to the existing rebar using nylon Tie-wraps, as shown in

Figure 3B. Then tie the strain gage to the short pieces of rebar again using nylon tie wraps.

This method avoids the resonance problems associated with the previous method.

Figure 3B - Alternative Method for attaching VCE 4200X straingages to rebar.

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