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Model 4900
Vibrating Wire Load Cell
Instruction Manual
©2022, GEOKON. All rights reserved.
Document Revision: CC | Release date: 3/9/22
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 that wear or are damaged by misuse are not warranted. This includes fuses and batteries.
GEOKON manufactures scientific instruments whose misuse is potentially dangerous. The instruments are intended to be installed and used only by qualified personnel. There are no warranties except as stated herein. There are no other warranties, expressed or implied, including but not limited to the implied warranties of 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.
No part of this instruction manual may be reproduced, by any means, without the written consent of
GEOKON . The information contained herein is believed to be accurate and reliable. However, GEOKON assumes no responsibility for errors, omissions or misinterpretation. The information herein is subject to change without notification.
The GEOKON® wordmark and logo are registered trademarks with the United States Patent and Trademark
Office.
III
IV
TABLE OF CONTENTS
1.2 LOAD CELL DESIGN AND CONSTRUCTION
................................................................
1.2.1 FRICTION BETWEEN THE BEARING PLATE AND LOAD CELL .......................................
1.2.2 WARPING OF THE BEARING PLATES AND BEARING PLATE DESIGN ......................
2.3 CABLE INSTALLATION AND SPLICING
..........................................................................
....................................................................................................
11
11
3.2 GK-406 VIBRATING WIRE READOUT
............................................................................
12
12
12
3.2.3 ESTABLISHING A BASELINE AND SETTING A GAUGE FACTOR ..............................
13
13
14
4.2 TEMPERATURE CORRECTION FACTOR
......................................................................
15
17
..................................................................................................
18
18
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION
20
B.2 10KΩ THERMISTOR RESISTANCE
..................................................................................
21
APPENDIX C. WIRING AND CONNECTOR PINOUTS
.........................................
22
C.2 GK-403 TO MODULE CONNECTOR
.................................................................................
22
APPENDIX D. LOAD CELL GAUGE FACTOR RECALCULATION
23
V
VI
APPENDIX E. LOAD CELL CALIBRATIONS - EFFECTS OF
......................................................................
E.2 LOAD CELL CALIBRATION PROCEDURES
.................................................................
E.4 EFFECTS OF JACK SIZE ON LOAD CELL READING
..........................................
APPENDIX F. USE OF THE REGRESSION ZERO WHEN USING
.................................................................
APPENDIX G. MODEL 8032-27 AND LOAD CELL WIRING
VII
VIII
FIGURES
LOAD CELLS ON TIEBACKS FOR PERMANENT MONITORING OF LOADS. 1
FIGURE 2: LOAD CELLS ON TIEBACK FOR PROOF TESTING ONLY ............................... 2
LOAD CELLS FOR LOAD MONITORING DURING A PILE LOAD TEST.......... 2
LEMO CONNECTOR TO GK-404 .....................................................................16
FIGURE 8: GK-406 READOUT............................................................................................27
TABLES
TABLE 1:
ENGINEERING UNITS CONVERSION MULTIPLIERS ...................................14
TABLE 2:
MODEL 4900 LOAD CELL SPECIFICATIONS ................................................18
TABLE 3:
3KΩ THERMISTOR RESISTANCE ...................................................................20
TABLE 4:
10KΩ THERMISTOR RESISTANCE .................................................................21
TABLE 5:
STANDARD LOAD CELL WIRING ..................................................................22
TABLE 6:
TABLE 7:
GAUGE FACTOR FOR REMAINING GAUGES ...............................................23
TABLE 8:
EFFECTS OF JACK SIZING ON READINGS ..................................................25
IX
X
EQUATIONS
EQUATION 1:
DIGITS CALCULATION ..............................................................................
EQUATION 2:
LOAD CALCULATION USING LINEAR REGRESSION.............................
EQUATION 3:
LOAD CALCULATION USING POLYNOMIAL ..........................................
EQUATION 4:
LOAD, CORRECTED FOR TEMPERATURE ..............................................
EQUATION 5:
3KΩ THERMISTOR RESISTANCE .............................................................
EQUATION 6:
10KΩ THERMISTOR RESISTANCE ...........................................................
1. INTRODUCTION
1.1 THEORY OF OPERATION
GEOKON load cells are of an annular design primarily for use on tiebacks and rockbolts. They may also be used during pile load tests and for monitoring loads in cross-lot struts and tunnel supports, etc. In practically all cases, the load cells are used in conjunction with a hydraulic jack, which applies the load, and with bearing plates positioned on either side of the load cell.
GEOKON Model 4900 load cells are frequently used for the following:
■
To provide a permanent means of monitoring the load throughout the life of the tieback, rockbolt, strut or support, etc.
■
To provide an electronic output for automatic data gathering.
■
As a check on the load as determined by the hydraulic pressure applied to the jack during proof testing on tiebacks, rockbolts, etc. For this purpose the user should be aware that the agreement cannot be guarantee better than ±20% because of the many variables.
Load cells are positioned so that the tensile load in the tieback or rockbolt produces a compressive load in the load cell. This is done by trapping the load cell between bearing plates positioned between the jack and the structure, either below the anchor plate for permanent installations or above the anchor plate for proof testing. Figure 1 and Figure 2 show the two different installations.
1:
FIGURE 1:
Load Cells on Tiebacks for the Permanent Monitoring
MODEL 4900 VIBRATING WIRE LOAD CELL | INTRODUCTION | 1
Tendon or Rod
Bearing Plates
Loading Shoe
Hydraulic
Jack
Load Cell
Wale
Lock Off Nut
2:
Soldier Pile
Anchor Zone
FIGURE 2:
Load Cells on Tieback for Proof Testing Only
Figure 3 illustrates load cells being used for load monitoring during a pile load test.
3:
2 | INTRODUCTION | GEOKON
FIGURE 3:
Load Cells for load monitoring during a pile load test
1.2 LOAD CELL DESIGN AND CONSTRUCTION
The Model 4900 Load Cell body is constructed in the form of a high strength steel cylinder in which three to six vibrating wire strain gauges are embedded to measure the change of strain in the cylinder as it comes under load. Multiple gauges are needed to account for the effects of off-center or eccentric loading.
The cable is attached to the cell through a waterproof gland. A Kellem's grip strain relief prevents the cable from being pulled out of the cell. Cables have thick PVC jackets and can be terminated in a connector to mate with terminal boxes or readouts. See Appendix C for cable and connector diagrams. Figure 4 below shows a typical load cell.
4:
FIGURE 4:
Model 4900 (Three Gauge)
Additional cable protection can be obtained by either using armored cable or by placing the cable inside flex conduit. Figure 5 shows a typical load cell system.
5:
FIGURE 5:
Typical Load Cell System
Annular load cells, because of their design, are inherently susceptible to varying conditions of end loading, unlike solid load cells, which can be designed with button shaped ends so that the load always falls in a uniform, predictable fashion. Thus, the output and calibration of an annular load cell can be affected by the factors discussed in the subsections below. Note that all of these effects can be accumulative, and can cause the calibration to vary by as much as 20%, unless special precautions are taken.
MODEL 4900 VIBRATING WIRE LOAD CELL | INTRODUCTION | 3
4 | INTRODUCTION | GEOKON
1.2.1 FRICTION BETWEEN THE BEARING PLATE AND LOAD CELL
Friction between the bearing plate and the load cell can radically affect the performance of a load cell. Interposing deformable plates or lubricant between the bearing plates and the load cell in the field will cause the load cell to overregister, perhaps by as much as 10%. Again, for best results, it is important to calibrate the load cell in the laboratory under the same loading conditions as will be used in the field.
End effects of this nature can be reduced somewhat by using tall load cells. A rough rule of thumb for good load cell design calls for a load cell height at least four times the wall thickness of the loaded annulus. On some jobs where there are space restrictions calling for a pancake style load cell, friction between bearing plates and load cell can give rise to large hysteresis effects between loading and unloading cycles.
1.2.2 WARPING OF THE BEARING PLATES AND BEARING PLATE DESIGN
Warping of the bearing plates is caused primarily by a size mismatch between the hydraulic jack and the load cell. A jack larger than the load cell tends to wrap the intervening bearing plate around the load cell, causing the center of the load cell to hourglass or pinch inwards causing the load cell to under-register.
Conversely, a hydraulic jack smaller than the load cell will try to punch the intervening bearing plate through the center of the load cell, making the center of the load cell barrel outwards causing the load cell to over-register. Both effects are exacerbated by bearing plates that are too thin. For further details on this topic, see Appendix D.
Note: To protect the lead wires routed in a groove in one face of the load cell
GEOKON does not allow the use of a washer made from lead, copper, rubber or other soft material bearing against this surface. If a soft washer is used be sure it is used only on the other face, i.e., the one that does not have the annular epoxy filled groove.
Minimum bearing plate thickness is 25 mm (1") where load cell size matches hydraulic jack size, i.e., the load bearing annulus of the load cell falls within the load bearing annulus of the hydraulic jack. For any other condition of size mismatch, the bearing plates should be at least two inches thick and even thicker where the size mismatch is extreme or the loads large.
Bearing plates should be flat and smooth. The normal rolled steel plate surface is adequate. It is not necessary to have machined or ground surfaces. Where plates are cut from larger plates, using cutting torches, the edges should be carefully cleaned to remove welding slag and solidified molten lumps.
Consideration should be given to calibrating the load cell using the same bearing plates as will be used in the field. In addition, it is possible to simulate the size of the hydraulic jack using a suitably sized metal donut between the upper platen of the testing machine and the upper bearing plate. Load cells calibrated in this way will be much more likely to agree with the hydraulic jack in the field.
1.2.3 ECCENTRIC LOADING
Eccentric loading of load cells is the rule rather than the exception. Rarely is the axis of the tieback, rockbolt, or strut at right angles to the surface on which the anchor plate or strut rests. With tiebacks using multiple tendons, it is quite common for loads in individual tendons to vary markedly, despite best efforts to avoid this happening. In addition, struts are rarely at right angles to the soldier piles they may be supporting.
These factors combine to produce conditions in which the load cell experiences loads that are higher on one side than on the other. This effect is compensated for by the individual electrical resistance strain gauges, cemented to the cell, being connected together in a full Wheatstone Bridge circuit. Thus, the higher strains on one side are balanced by lower strains on the other and the average strain is not affected. Thus, even gross amounts of load eccentricity cause only slight (< 5%) variations in the load cell output and calibration.
Eccentric loading can be minimized by using spherical bearing plates, but this is expensive and is rarely done. Spherical seats may be of some value during pile load testing where uniformity of the load on the top of the pile is highly desirable.
1.2.4 ELASTIC BEHAVIOR
It is important that a load cell behave elastically, i.e. that the no-load zero will not change with time. For this reason, use only the highest quality strain gauges and adhesives. GEOKON uses transducer-grade strain gauges, along with scrupulous observation of the best installation practices and adhesive post curing techniques.
GEOKON Model 4900 Load Cells are designed to keep the normal working stresses below 30% of the yield stress of the load cell material. Wherever possible load cells are cycled to 150% of the design load prior to calibration. As long as the load cell is never overloaded above this range, the no-load reading will not change. The normal over-range capacity of an aluminum load cell is
200% FSR and 300−400% FSR for a steel load cell before the load cell will begin to fail.
If a load cell is over-ranged and the no-load reading is shifted due to plastic yielding of the cell, then the cell should be returned to the factory for inspection and recalibration. Note, however, that while the no-load zero may shift, the calibration constant will probably not be affected.
1.2.5 TEMPERATURE EFFECTS
Temperature compensation is achieved by using strain gauges whose thermal coefficient is the same as that of the load cell material. Normally, the temperature coefficient of the load cell is insignificant. In special cases, if required, the coefficient can be measured at the factory. Note that temperature changes on the loaded rockbolt, tieback, or strut can produce real changes of load and these will be recorded by the load cell. See Section 4.2 for more information on correcting for temperature.
MODEL 4900 VIBRATING WIRE LOAD CELL | INTRODUCTION | 5
6 | INTRODUCTION | GEOKON
2. INSTALLATION
2.1 PRELIMINARY TESTS
Before installing the load cell, it should be checked by connecting it to the readout box and taking a no-load reading. This reading, when compared with that given in the calibration data provided with the load cell, will show if the cell is functioning properly. The two readings should agree within about 50 digits
(assuming that the same readout box is used for both readings).
2.2 LOAD CELL INSTALLATION
2.2.1 TRANSPORTATION
When transporting load cells, do not pull on the cable and, in particular, do not carry the load cell by the cable. On the larger load cells, threaded holes are provided in the ends to allow eyebolts to be attached for lifting purposes.
2.2.2 INITIAL NO-LOAD READING
Before installing the load cell, be sure to take the no-load reading. This reading is very important, since it will be subtracted from all subsequent readings to calculate the load. Note that each load cell has a different no-load reading that is not zero. See Section 3 for operation of the readout boxes.
2.2.3 INSTALLATION ON TIE-BACKS AND ROCKBOLTS
Note: To protect the lead wires routed in a groove in one face of the load cell
GEOKON does not allow the use of a washer made from lead, copper, rubber or other soft material bearing against this surface. If a soft washer is used, be sure it is used only on the other face, i.e., the one that does not have the annular epoxy-filled groove
Load cells should be installed between flat steel bearing plates of sufficient thickness; one inch thick where load cell and jack are about the same size, and two to three inches thick where size mismatches are greater. The normal rolled finish on the plates is good. Plates may need to be machined flat if they are warped. Make sure that the bearing plates completely cover the load-bearing surface of the load cell. Centralize the rockbolt or tieback inside the load cell.
Where the load cell I.D. is much bigger than the rockbolt or tieback, a centralizer bushing can be used.
Where the anchor block of a multi-tendon tieback bears directly on the load cell, make sure that the load cell bearing surface is completely covered by the anchor block. If the load cell is not completely covered, then make sure that the calibration was performed using the anchor block . If the calibration was performed without the anchor block then for best results consideration should be given to recalibration with the anchor block. Shield the cable for possible damage from blasting or traffic. Protect the end of the cable or the cable connector from dirt by either using a cap on the connector or by storing the end of the cable and/or connector inside a small box. Section 1 shows a typical load cell system.
2.3 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 gauges to be terminated at one location with complete protection of the lead wires. The interior panel of the terminal box
MODEL 4900 VIBRATING WIRE LOAD CELL | INSTALLATION | 7
8 | INSTALLATION | GEOKON can have built-in jacks or a single connection with a rotary position selector switch. Contact GEOKON for specific application information.
Because the vibrating wire output signal is a frequency rather than a current or voltage, variations in cable resistance have little effect on gauge readings; therefore, splicing of cables has no ill effects, and in some cases may 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 employ 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 into a receptacle on a special patch cord.
2.4 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 alongside AC power lines; they will pick up the noise from the power cable, which will likely cause unstable readings. Contact the factory concerning filtering options available for use with the GEOKON dataloggers and readouts.
2.5 ENVIRONMENTAL FACTORS
Since the purpose of the load cell installation is to monitor site conditions, factors which may affect these conditions should 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 or reservoir levels, excavation and fill levels and sequences, traffic, temperature and barometric changes, changes in personnel, nearby construction activities, seasonal changes, etc.
2.6 LIGHTNING PROTECTION
Unlike other types of instrumentation available from GEOKON, load cells do not have any integral lightning protection components, such as transorbs 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 gauge and possibly destroy it.
Recommended lightning protection is as follows:
■
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 load cell.
■
Plasma surge arrestors can be epoxied into the instrument cable, close to the load cell. A ground strap then connects the surge arrestor to an earth ground, such as a grounding stake.
Consult the factory for additional information on available lightning protection.
MODEL 4900 VIBRATING WIRE LOAD CELL | INSTALLATION | 9
10 | INSTALLATION | GEOKON
3. TAKING READINGS
3.1 GK-404 VIBRATING WIRE READOUT
The Model GK-404 VW Readout is a portable, low-power, hand-held 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 instruments, and is capable of displaying the reading in digits, frequency (Hz), period ( μ s), or microstrain (
µ
ε
). The GK-404 also displays the temperature of the transducer
(embedded thermistor) with a resolution of 0.1 °C.
6:
FIGURE 7:
Lemo connector to GK-404
FIGURE 6:
GK-404 Readout
3.1.1 OPERATING THE GK-404
1.
Attach the flying leads by aligning the red circle on the silver Lemo connector with the red line on the top of the GK-404 (see Figure 7). Insert the Lemo connector into the GK-404 until it locks into place.
2.
Connect each of the clips on the leads to the matching colors of the sensor conductors, with blue representing the shield (bare).
3.
To turn on the GK-404, press the On/Off button on the front panel of the unit. The initial startup screen will display.
4.
After a delay, 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 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 instrument in degrees Celsius.
Use the Pos and Mode buttons to select the correct position and display units for the model of equipment purchased.
The GK-404 will continue to take measurements and display readings until the unit is turned off, either manually or by the Auto-Off timer (if enabled).
For more information, consult the GK-404 manual.
MODEL 4900 VIBRATING WIRE LOAD CELL | TAKING READINGS | 11
FIGURE 8:
GK-406 Readout
3.2 GK-406 VIBRATING WIRE READOUT
The GK-406 VW Readout is a handheld unit ready to quickly measure a sensor, save the data, and communicate the results with custom PDF reports and spreadsheet output. GK-406 measurements are geo-located with an integrated
GPS, allowing the device to verify locations and direct the user to each sensor.
The GK-406 uses Campbell Scientific patented spectral-analysis technology
(VSPECT®) to provide the best vibrating-wire measurement possible while filtering out environmental and electrical noise. The large color display offers an easy-to-view graphical presentation of the data.
The GK-406 converts measurements to engineering units, generates a printable
PDF report, and saves a CSV summary file. The graphical display allows confirmation of sensor output and operation. VSPECT® technology eliminates disruptive noise interference and provides sensor diagnostics for the best measurement possible. VSPECT® noise immunity allows gages that are otherwise unreadable to be evaluated with confidence.
A Project File maintains Site/Sensor information for 40 unique sites with 22 sensors per site. Site/Sensor locations are geolocated, allowing the internal GPS to guide a user directly to a sensor location. Site/Sensor and user information can be created or edited on the device or with a computer using the free
VwProjects software.
3.2.1 CONNECTING THE GK-406
Connect the load cell to the Model GK-406-MUX Load Cell Multiplexer using the cable with a 10-pin connector. Then connect the Model GK-406-MUX to the GK-
406 using the cable with the 6-pin connector by aligning the pins, pushing the connector into place, and twisting the outer ring of the male connector until it locks into place.
3.2.2 OPERATING THE GK-406
1.
Press the power button just under the left side of the readout screen on the
VW Analyzer.
2.
Select or add a user and the home screen is displayed.
3.
Select Read & Record and then Site/Sensor to get a load cell reading.
4.
Select the load cell location or add a new location. There is an option add a default sensor to the new location.
5.
Select Next and then select the specific load cell model to be used or add the information as a new sensor.
6.
Select Read and the readout screen is displayed.
7.
Select Details in the readout screen to show more data. Select Gauge from the details screen to toggle between the average of all the vibrating wire sensors measurements in the load cell and the individual measurements of each sensor. Spectrum and Time graphs are also available from the Details screen.
Note: The power button is generally used as a back button and will lead back to the home screen when pressed enough.
12 | TAKING READINGS | GEOKON
3.2.3 ESTABLISHING A BASELINE AND SETTING A GAUGE FACTOR
1.
In the Select Sensor screen, select Edit Sensor Parameters .
2.
Select Sensor Units and then Next .
3.
If you select Baseline (digits) and Read Baseline , the GK-406 will automatically take a baseline measurement. This will be the measurement that is used as R o
for the load calculations. You can also manually input a baseline instead of measuring one. Save the baseline to return to the previous screen.
4.
Select Gage Factor (G) and input the gage factor found in the calibration sheet provided with the 4900-Series load cell. Pay attention to the sign of the gage factor.
5.
Select Save until the Select Sensor screen and then select Read to get a current measurement.
3.2.4 READOUT AND DATA FILES
1.
To view the data, view a graph of the data, or export the CSV file, select
Data from the home screen.
2.
Select Site/Sensor and choose from which site you want the data.
3.
Select Next and choose the sensor from which you want the data.
4.
Select Next and there will be the options to View Data , View Graph , or to
Export CSV . The GK-406 must be connected to a computer for the Export
Data function to work.
Note: It takes 20-30 seconds for the reports to generate after taking a measurement. If you connect the GK-406 to a PC during these 20-30 seconds, it may not complete the reports.
3.3 MEASURING TEMPERATURES
All GEOKON vibrating wire instruments are equipped with a thermistor for reading temperature. The thermistor 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-404 and GK-405 readouts will read the thermistor and display the temperature in degrees Celsius.
TO READ TEMPERATURES USING AN OHMMETER:
1.
Connect an ohmmeter to the green and white thermistor leads coming from the instrument. 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 48.5Ω per km (14.7Ω per
1000') 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.
MODEL 4900 VIBRATING WIRE LOAD CELL | TAKING READINGS | 13
14 | DATA REDUCTION | GEOKON
4. DATA REDUCTION
4.1 LOAD CALCULATION
The basic units utilized by GEOKON for measurement and reduction of data from vibrating wire load cells are digits. Calculation of digits is based on the following equation:
Digits =
1
)
2
× 10
– 3 or
Digits =
Hz 2
1000
EQUATION 1: Digits Calculation
To convert the digits readings to load, the gauge readings for each cell must be averaged, and then the change in reading average multiplied by the gauge factor supplied with the load cell.
L = (R
1
– R
0
) × G × K
EQUATION 2:
Load Calculation Using Linear Regression
Where:
L is the load in lb, kg, etc.
R
0
is the regression no-load reading in digits (average of all gauges).
R
1
is the current reading in digits (average of all gauges).
G is the gauge factor as supplied on the calibration sheet. As the load increases, the reading decreases, this gives G a negative sign when entered into the equation (see the example below).
K is the conversion factor (optional) as listed in the table below.
1:
From Lb Kg Kips Tons Metric Tons
To
Lb
Kg
Kips
Tons
Metric Tons
1
0.4535
0.001
0.0005
0.0004535
2.205
1
0.002205
0.0011025
0.001
1000
453.5
1
2.0
0.4535
2000
907.0
2.0
1
0.907
2205
1000
2.205
1.1025
1
TABLE 1:
Engineering Units Conversion Multipliers
For example:
Model 4900 VW Load Cells have a regression no-load reading ( R
0
) of 7309 and a current average reading ( R
1
) of 5497, and the calibration factor is –397 lbs. per digit.
Inputting the values into Equation 2:
L = (5497 – 7309) × – 397 = 719,400 lbs.
Note that the equations assume a linear relationship between load and gauge readings over the full load range, and the linear coefficient is obtained using regression techniques. Note that when using the calibration factor obtained from the regression formula it is necessary to use the regression zero. This may introduce substantial errors at very low loads. A measure of the amount of nonlinearity is shown on the calibration sheet in the column entitled Linearity.
See Appendix F for additional information.
For greater accuracy, the data given can be represented by a polynomial or can be treated as a series of segments over the entire load range. For instance, using
the example calibration sheet, the load between 0 and 180,000 lb could be represented by the equation:
L = ((7304
–
6860) × 405 = 179,820
tons.
The gauge factor –405 lb/digit is calculated from the slope of the line between a load of 0 and 180,000 lb, i.e., (0 – 180,000) / (7304 – 6860) = –405 lb/digit.
A polynomial expression to fit the data is shown in Equation 3.
L = AR
1
2
+ BR
1
+ C
EQUATION 3:
Load Calculation Using Polynomial
Where:
L is the load in lb, kg, etc.
R
1
is the current reading (average of all gauges).
A, B, and C are the coefficients derived from the calibration data.
First calculate C from the initial average field zero reading.
For example: if C = 7,305 then 0 = –0.00247 × 73,052 – 367 × 7,305 + C from which
C = +2,812,740 . Therefore, when the applied load is 360,000, R
1
= 6,409, and the calculated load = –0.00247 × 64,092 – 367 × 6,409 + 2,812,740 = 359,180 lbs.
4.2 TEMPERATURE CORRECTION FACTOR
A small correction can be made for change in temperature. As the temperature goes up the average reading of all the sensors will go down approximately one digit per °C. To calculate the load, corrected for temperature, use Equation 4.
L = G [(R
1
– R
0
) + (T
1
– T
0
)]
EQUATION 4:
Load, Corrected for Temperature
The temperature effect shown above is for a load cell that has not been installed yet and is very minor. There is no telling what the actual temperature effect will be on a load cell that is installed on a tensioned bar or cable. This depends on the length of the bar or cable and on the properties of the surrounding ground.
The actual temperature effect can only be arrived at empirically by simultaneous measurements of load and temperature over a short period of time.
MODEL 4900 VIBRATING WIRE LOAD CELL | DATA REDUCTION | 15
16 | DATA REDUCTION | GEOKON
FIGURE 9: Typical Model 4900 Calibration Sheet
9:
5. TROUBLESHOOTING
Problems with the load cell are usually associated with cable damage or moisture getting into the system. Both problems can be minimized by protecting the cable from damage, by visual inspection of the cable if problems arise, and by always keeping the plug clean and dry.
Warning!
Do not carry the load cell by the cable.
Consult the following list of problems and possible solutions should difficulties arise. Consult the factory for additional troubleshooting help.
SYMPTOM: LOAD CELL GAUGE READINGS ARE UNSTABLE
□
Is the readout box position set correctly? (See Section 3.)
□
If using a datalogger to record readings automatically, are the swept frequency excitation settings correct?
□
Does the readout or datalogger work with another load cell? If not, the readout/datalogger may have a low battery or be malfunctioning.
□
Is there a source of electrical noise nearby? Most probable sources of electrical noise are motors, generators, and antennas.
□
Make sure the shield drain wire is connected to ground. Connect the shield drain wire to the readout using the blue clip.
SYMPTOM: LOAD CELL GAUGE FAILS TO READ
□
Is the cable cut or crushed? This can be checked with an ohmmeter.
Nominal resistance between the two-gauge leads is 45 to 50 Ω (75, 90, or
180 Ω , ± 10 Ω on some older models).
□
Remember to add cable resistance when checking (22 AWG stranded copper leads are approximately 14.7
Ω /1000' or 48.5
Ω /km, multiply by two for both directions). If the resistance reads very high or infinite (megohms), a cut wire must be suspected. If the resistance reads very low (<20 Ω ), a short in the cable is likely.
□
Does the readout or datalogger work with another load cell? If not, the readout or datalogger may be malfunctioning.
SYMPTOM: THERMISTOR RESISTANCE IS TOO HIGH
□
Is there an open circuit? Check all connections, terminals, and plugs. If a cut is located in the cable, splice according to instructions above.
SYMPTOM: THERMISTOR RESISTANCE IS TOO LOW
□
Is there a short? Check all connections, terminals, and plugs. If a short is located in the cable, splice according to instructions above.
□
Water may have penetrated the interior of the load cell. There is no remedial action.
MODEL 4900 VIBRATING WIRE LOAD CELL | TROUBLESHOOTING | 17
18 | SPECIFICATIONS | GEOKON
APPENDIX A. SPECIFICATIONS
A.1 MODEL 4900 LOAD CELL SPECIFICATIONS
Rated Capacities:
1
Accuracy:
2
Resolution:
Repeatability:
Temperature Effect:
Temperature Range:
Frequency Range
Over range:
Coil Resistance:
Cable Type (Three Gauge):
3
Cable Type (Four Gauge):
3
Cable Type (Six Gauge):
3
100 to 10,000 kN
±0.5% F.S.
0.025% F.S.
0.1% F.S.
0.02% F.S./°C
-20 to +80 °C
1400-3500 Hz
150%
45 to 50 Ω (70, 90, or 180 Ω on some older models)
Four twisted pair (six conductor) 22 AWG, Purple jacket
Foil shield, PVC jacket, nominal OD=9.5 mm (0.375")
Four twisted pair (eight conductor) 22 AWG, Purple jacket
Foil shield, PVC jacket, nominal OD=9.5 mm (0.375")
Six twisted pair (12 conductor) 22 AWG, Orange jacket
Foil shield, PVC jacket, nominal OD=12.7 mm (0.5")
TABLE 2:
Model 4900 Load Cell Specifications
Notes:
1
Other capacities and diameters available on request. Calibrations that exceed
GEOKON ’s NIST traceable capacity of approximately 10,675 kN are subcontracted to an accredited testing laboratory.
2
Established under laboratory conditions. System accuracy depends on end loading conditions.
3
Other cable types, e.g., armored, are available.
2:
A.2 THERMISTOR
Range: -80 to +150 °C
Accuracy: ±0.5 °C
MODEL 4900 VIBRATING WIRE LOAD CELL | SPECIFICATIONS | 19
APPENDIX B. THERMISTOR TEMPERATURE
DERIVATION
B.1 3KΩ THERMISTOR RESISTANCE
Thermistor Types:
■
YSI 44005, Dale #1C3001–B3, Alpha #13A3001–B3
■
Honeywell 192–302LET–A01
Resistance to Temperature Equation:
T =
1
A+B(LnR)+C(LnR 3 )
–
273.15
EQUATION 5:
3kΩ Thermistor Resistance
Where:
T = Temperature in °C
LnR = Natural Log of Thermistor Resistance
A = 1.4051 x 10
–3
B = 2.369 x 10
–4
C = 1.019 x 10
–7
Note: Coefficients calculated over the –50 to +150 ° C span.
11.44K
10.86K
10.31K
9796
9310
8851
8417
Ohms
15.72K
14.90K
14.12K
13.39K
12.70K
12.05K
8006
7618
7252
6905
6576
6265
5971
5692
5427
5177
3135
3000
2872
2750
2633
2523
2417
2317
4939
4714
4500
4297
4105
3922
3748
3583
3426
3277
-44
-43
-42
-41
-40
-39
-38
Temp
-50
-49
-48
-47
-46
-45
-33
-32
-31
-30
-29
-28
-37
-36
-35
-34
-17
-16
-15
-14
-13
-12
-11
-10
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
132.2K
123.5K
115.4K
107.9K
101.0K
94.48K
88.46K
Ohms
201.1K
187.3K
174.5K
162.7K
151.7K
141.6K
82.87K
77.66K
72.81K
68.30K
64.09K
60.17K
56.51K
53.10K
49.91K
46.94K
24.51K
23.16K
21.89K
20.70K
19.58K
18.52K
17.53K
16.60K
44.16K
41.56K
39.13K
36.86K
34.73K
32.74K
30.87K
29.13K
27.49K
25.95K
1733
1664
1598
1535
1475
1418
1363
Ohms
2221
2130
2042
1959
1880
1805
1310
1260
1212
1167
1123
1081
1040
1002
965.0
929.6
624.7
603.3
582.6
562.8
543.7
525.4
507.8
490.9
895.8
863.3
832.2
802.3
773.7
746.3
719.9
694.7
670.4
647.1
-3
-2
-1
0
1
2
3
-6
-5
-4
Temp
-9
-8
-7
11
12
13
8
9
10
6
7
4
5
24
25
26
27
28
29
30
31
17
18
19
20
14
15
16
21
22
23
389.3
376.9
364.9
353.4
342.2
331.5
321.2
Ohms
474.7
459.0
444.0
429.5
415.6
402.2
311.3
301.7
292.4
283.5
274.9
266.6
258.6
250.9
243.4
236.2
171.4
166.7
162.0
157.6
153.2
149.0
145.0
141.1
229.3
222.6
216.1
209.8
203.8
197.9
192.2
186.8
181.5
176.4
38
39
40
41
42
43
44
35
36
37
Temp
32
33
34
52
53
54
49
50
51
45
46
47
48
65
66
67
68
69
70
71
72
58
59
60
61
55
56
57
62
63
64
120
121
122
123
124
125
126
Temp
114
115
116
117
118
119
131
132
133
134
135
136
127
128
129
130
137
138
139
140
141
142
143
144
145
146
147
148
149
150
3:
116.8
113.8
110.8
107.9
105.2
102.5
99.9
Ohms
137.2
133.6
130.0
126.5
123.2
119.9
97.3
94.9
92.5
90.2
87.9
85.7
83.6
81.6
79.6
77.6
75.8
73.9
72.2
70.4
68.8
67.1
65.5
64.0
62.5
61.1
59.6
58.3
56.8
55.6
79
80
81
82
83
84
85
76
77
78
Temp
73
74
75
93
94
95
90
91
92
86
87
88
89
106
107
108
109
110
111
112
113
96
97
98
99
100
101
102
103
104
105
20 | THERMISTOR TEMPERATURE DERIVATION | GEOKON
TABLE 3:
3KΩ Thermistor Resistance
B.2 10KΩ THERMISTOR RESISTANCE
Thermistor Type: US Sensor 103JL1A
Resistance to Temperature Equation:
T =
1
A+B(LnR)+C(LnR) 3 +D(LnR) 5
–
273.15
EQUATION 6:
10KΩ Thermistor Resistance
Where:
T = Temperature in °C
LnR = Natural Log of Thermistor Resistance
A = 1.127670 x 10
–3
B = 2.344442 x 10
–4
C = 8.476921 x 10
–8
D = 1.175122 x 10
–11
Note: Coefficients optimized for a curve J Thermistor over the temperature range of 0 °C to +250 °C.
4:
15,712 15
14,999 16
14,323 17
13,681 18
13,072 19
12,493 20
11,942 21
11,419 22
10,922 23
10,450 24
10,000 25
9,572 26
9,165 27
8,777 28
8,408 29
8,057 30
7,722 31
Ohms Temp Ohms Temp Ohms Temp Ohms Temp Ohms Temp Ohms Temp Ohms Temp Ohms Temp
32,650 0 7,402 32 2,157 64 763.5 96 316.6 128 148.4 160 76.5
192 42.8
224
31,029 1
29,498 2
7,098 33
6,808 34
2,083 65
2,011 66
741.2 97
719.6 98
308.7 129
301.0 130
145.1 161
142.0 162
75.0
73.6
193
194
42.1
41.4
225
226
28,052 3
26,685 4
25,392 5
24,170 6
6,531 35
6,267 36
6,015 37
5,775 38
1,942 67
1,876 68
1,813 69
1,752 70
698.7 99
678.6 100
659.1 101
640.3 102
293.5 131
286.3 132
279.2 133
272.4 134
138.9 163
135.9 164
133.0 165
130.1 166
72.2
70.8
69.5
68.2
195
196
197
198
40.7
40.0
39.3
38.7
227
228
229
230
23,013 7
21,918 8
20,882 9
19,901 10
18,971 11
18,090 12
17,255 13
16,463 14
5,545 39
5,326 40
5,117 41
4,917 42
4,725 43
4,543 44
4,368 45
4,201 46
1,693 71
1,637 72
1,582 73
1,530 74
1,480 75
1,432 76
1,385 77
1,340 78
622.2 103
604.6 104
587.6 105
571.2 106
555.3 107
539.9 108
525.0 109
510.6 110
265.8 135
259.3 136
253.1 137
247.0 138
241.1 139
235.3 140
229.7 141
224.3 142
127.3 167
124.6 168
122.0 169
119.4 170
116.9 171
114.5 172
112.1 173
109.8 174
66.9
65.7
64.4
63.3
62.1
61.0
59.9
58.8
199
200
201
202
203
204
205
206
38.0
37.4
36.8
36.2
35.6
35.1
34.5
33.9
231
232
233
234
235
236
237
238
4,041 47
3,888 48
3,742 49
3,602 50
3,468 51
3,340 52
3,217 53
3,099 54
2,986 55
2,878 56
2,774 57
2,675 58
2,579 59
2,488 60
2,400 61
2,316 62
2,235 63
1,297 79
1,255 80
1,215 81
1,177 82
1,140 83
1,104 84
1,070 85
1,037 86
1,005 87
973.8 88
944.1 89
915.5 90
887.8 91
861.2 92
835.4 93
810.6 94
786.6 95
496.7 111
483.2 112
470.1 113
457.5 114
445.3 115
433.4 116
421.9 117
410.8 118
400.0 119
389.6 120
379.4 121
369.6 122
360.1 123
350.9 124
341.9 125
333.2 126
324.8 127
219.0 143
213.9 144
208.9 145
204.1 146
199.4 147
194.8 148
190.3 149
186.1 150
181.9 151
177.7 152
173.7 153
169.8 154
166.0 155
162.3 156
158.6 157
155.1 158
151.7 159
107.5 175
105.3 176
103.2 177
101.1 178
99.0
97.0
95.1
93.2
91.3
89.5
87.7
86.0
84.3
82.7
81.1
79.5
78.0
179
180
181
182
183
184
185
186
187
188
189
190
191
57.7
56.7
55.7
54.7
53.7
52.7
51.8
50.9
50.0
49.1
48.3
47.4
46.6
45.8
45.0
44.3
43.5
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
33.4
32.9
32.3
31.8
31.3
30.8
30.4
29.9
29.4
29.0
28.5
28.1
239
240
241
242
243
244
245
246
247
248
249
250
TABLE 4:
10KΩ Thermistor Resistance
MODEL 4900 VIBRATING WIRE LOAD CELL | THERMISTOR TEMPERATURE DERIVATION | 21
APPENDIX C. WIRING AND CONNECTOR PINOUTS
C.1 LOAD CELL CONNECTOR AND CABLE (STANDARD WIRING)
10 ‑ pin Bendix
PT06A ‑ 12 ‑ 10P
Function 3 Gauge VW Load Cell
GEOKON Purple Cable
4 Gauge VW Load Cell
GEOKON Purple Cable
5:
6 Gauge VW Load Cell
GEOKON Orange Cabl e
G
H
E
F
J
K
C
D
A
B
Gauge #1
Gauge #2
Gauge #3
Gauge #4
Gauge #5
Gauge #6
Shield
Common
Thermistor
Thermistor
Red
Red's Black
White
NC
NC
NC
All Shields
White's Black¹
Green¹
Green's Black
Red
Red's Black
White
White's Black
NC
NC
All Shields
Green
Blue
Blue's Black
Red
Red's Black
White
White's Black
Green
Green's Black
All Shields
Blue
Yellow
Yellow's Black
TABLE 5: Standard Load Cell Wiring
Notes:
¹ White's black and Green wires are switched on GEOKON three-gauge VW load cells prior to serial number 3313.
C.2 GK-403 TO MODULE CONNECTOR
Module 10 ‑ pin Bendix
Plug (PT06F ‑ 12 ‑ 10P)
A
B
E
F
C
D
J
K
G
H
TABLE 6:
Module Wiring
Interconnect Wire
Color (Six Pair)
Brown
Brown's Black
Red
Red's Black
Yellow
Yellow's Black
Green
Green's Black
Blue
Blue's Black
Interconnect Wire
Color (Belden)
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Black
Description
VW Gauge
VW Gauge Ground
Thermistor
Thermistor Ground
Shield
+12 VDC
Ground
Mux Sense
Mux Clock
Mux Type
Module Board
Connection
6:
JP1-2
JP1-1
JP1-3
JP1-1
JP1-1
JP1-4
JP1-9
JP1-9
JP1-8
JP1-9
22 | WIRING AND CONNECTOR PINOUTS | GEOKON
APPENDIX D. LOAD CELL GAUGE FACTOR
RECALCULATION
D.1 OVERVIEW
This appendix describes how to recalculate the gauge factor for a load cell and then approximate the load where one or more strain gauges in the cell have failed after installation. This is not a foolproof method. For example, if the load distribution changes during monitoring, the calculations based on the method described above will be in error.
D.2 PROCEDURE
If the load is applied uniformly to the load cell then, as the load changes the change in reading on each gauge will be the same and, should one gauge fail, the gauge factor given on the calibration sheet can be applied to the average change of the remaining gauges.
Note the following example, where gauge number three in a six-gauge load cell has failed. The load cell gauge factor for the six gauges is 0.2439 tons/digit. If the load is uniformly applied to the load cell, then to calculate the load this gauge factor would be applied to the average reading change of the remaining five active gauges. In the example below the load on 7/1/02 would be calculated to be 0.2439(7298-6139) = 282.7 tons. However, in the field it rare to have the cell uniformly stressed, therefore, it may be more accurate to calculate a new gauge factor using only the active gauges.
In cases where the load is eccentric (in the present example the reading change on gauge number was higher than the other five gauges), the new gauge factor can be calculated for the remaining five active gauges as follows:
7:
Date Gauge #1 Gauge #2 Gauge #3
Initial 7318
6/1/02 6485
7/1/02 6202
7363
6363
6034
7247
6220
No Reading
Gauge #4
7448
6618
6324
Gauge #5 Gauge #6 Avg Load
7222
6362
6075
7191
6331
6058
7298
6396
6139
0
220.2 tons
293.8 tons
TABLE 7:
Gauge Factor for Remaining Gauges
1.
Calculate a new zero load average using only the initial readings of the five remaining active gauges = 7308
2.
Using only the readings of the active gauges: #1, #2, #4, #5, and #6 from the time of the last reading when all six gauges were active (6/1/02), calculate the average reading = 6432 .
3.
Calculate the new gauge factor for the remaining five active gauges by dividing the calculated load at the last time when all gauges were active, (6/
1/02), by the change in the five gauge average readings calculated in steps one and two, = 220.2 /(7308-6432) = 0.2514
. This is the new gauge factor to be applied to all subsequent changes of the remaining five active gauges.
4.
Using the averages of the current and initial five-gauge readings, calculate the load on 7/1/02 by using the new gauge factor. Thus on 7/1/02: ( 7308 –
6139 ) x 0.2514
= 293.9 tons. This gives a better result than applying the old gauge factor for the six gauges to the average reading of the five active gauges. (The applied load was 291 tons).
5.
Repeat step four for subsequent readings or repeat all steps if more gauges in the load cell fail.
MODEL 4900 VIBRATING WIRE LOAD CELL | LOAD CELL GAUGE FACTOR RECALCULATION | 23
APPENDIX E. LOAD CELL CALIBRATIONS - EFFECTS
OF BEARING PLATE WARPING
E.1 INTRODUCTION
Load cells used to measure loads during testing of tiebacks, driven piles, and drilled shafts, give calculated loads that are frequently in disagreement with loads calculated based on hydraulic jack pressure and piston area. Because of this, there is a general lack of confidence in load cell data and the fault is often ascribed to manufacturing defects, or to improper, inaccurate calibration procedures. Nevertheless, it is also well known that the effects of eccentric loading and uneven and/or warped bearing plates do have a profound effect on load cell readings. The purpose of this technical note is to provide some insight into these effects.
E.2 LOAD CELL CALIBRATION PROCEDURES
The usual calibration procedure is to use a testing machine to apply a load to a load cell. The measured load cell output is then correlated against the known applied load as measured by the testing machine. Usually, the testing machine has a hydraulic pressure applied to a piston of known cross section area. The testing machine is checked out periodically by running tests on a load cell traceable to NIST and there is generally little doubt about the accuracy of the testing machine. Accuracy's of ¼ % FS ½ % FS or 1% FS are normal.
Usually, the calibration tests are performed between large, flat parallel platens in the testing machine so that there is no bending of the platens, only the elastic compression in the zone immediately bearing against the load cell.
E.3 FIELD ARRANGEMENT
Such a state of affairs may not exist on the job site since the bearing surfaces next to the load cell are usually much less rigid, and liable to bending.
This bending is particularly apparent if there is a mismatch in size between the load cell and the hydraulic jack. If the hydraulic jack is larger than the load cell there is a tendency for it to try to wrap the intervening bearing plate around the load cell. If the hydraulic jack is smaller than the load cell it will try to push the intervening bearing plate through the hole in the load cell.
Thicker bearing plates will bend less, but the effect will never be entirely eliminated. The consequence of this bending can be quite large since the effect on the load cell is to cause it to either barrel out at its midsection if the jack is too small, or pinch in at its midsection if the jack is too big. For vibrating wire load cells, the gauges are usually located in the center of the cell wall, on the neutral axis, thereby minimizing these effects.
E.4 EFFECTS OF JACK SIZE ON LOAD CELL READING
A series of tests were conducted in a testing machine to investigate the magnitude of the effect of jack size on load cell readings.
A load cell with a bearing surface of 4" ID, 5¾" OD was used.
Simulated jack A had a bearing surface of 2" ID, 4" OD.
Simulated jack B had a bearing surface of 4" ID, 5¾" OD.
Simulated jack C had a bearing surface of 6" ID, 8" OD.
The maximum applied load was 150 tons.
24 | LOAD CELL CALIBRATIONS - EFFECTS OF BEARING PLATE WARPING | GEOKON
8:
Jack
Load Cell response to applied load (100%)
1" thick plate 2" thick plate
J
A
(smaller)
108% 102%
LC
J
B
(same size)
100% 100%
LC
J
C
(bigger)
96% 98%
LC
TABLE 8: Effects of Jack Sizing on Readings
From the results, it can be seen that if the jack is smaller than the load cell, the load cell will over-register, while a jack bigger than the load cell will cause the load cell to under-register. The effect is bigger if the bearing plate between jack and load cell is thinner.
The correct bearing plate thickness will of course depend on the extent of the mismatch between jack and load cell. However, as a rough rule of thumb the following thickness should be required:
100-200 kip load: 25 mm (1") thick
Up to 400 kip load: 37 mm (1.5") thick
Up to 1000 kip load: 50 mm (2") thick
Up to 2000 kip load: 75 mm (3") thick
E.5 CONCLUSION
The consequences of all this would seem to indicate that, for best results, the load cell calibration should be performed with the actual hydraulic jack that will be used, both being placed in the testing machine at the same time. If that is not possible, the load cell should be loaded through a ring, having the same dimensions as the hydraulic jack bearing surface, positioned on the other side of a bearing plate of the correct thickness. In this way, one of the variables affecting the agreement between load cell readings and hydraulic jack readings can be removed and the agreement should be that much closer.
This technical note has addressed only the subject of the size mismatch between load cells and hydraulic jacks. Other factors affecting the agreement between load cell readings and hydraulic jack load are important, thus frictional losses within the hydraulic jack can cause under-registering of jack load indications by as much as 15%. (Dunnicliff 1988' Section 13.2.6)
Also, annular style load cells are susceptible to end effects and eccentrically applied loads. The height of the load cell should exceed four times the wall thickness of the annulus, and at least three strain gauges should be used, increasing in number as the size of the load cell increases, up to six total.
REFERENCES:
J. Dunnicliff. 1988. Geotechnical Instrumentation for Monitoring Field
Performance, John Wiley & Sons, New York, NY: 577pp.
MODEL 4900 VIBRATING WIRE LOAD CELL | LOAD CELL CALIBRATIONS - EFFECTS OF BEARING PLATE WARPING | 25
APPENDIX F. USE OF THE REGRESSION ZERO WHEN
USING THE LINEAR GAUGE FACTOR
It is normal for load cells, having an annular design and for solid load cells that do not have ‘button heads’ or spherical seated bearing plates, to be susceptible to irregular load distributions at low loads. This is because there is a 'bedding in' process that takes place while the surfaces at both ends of the load cell conform to the surfaces they bear against causing the load cell to deform in an unpredictable way giving rise to strange strain patterns and faulty readings at low loads.
This irregularity of load disappears once the load cell surfaces have bedded in and from that point on the load cell behaves in the a more linear fashion such that there is a constant relationship between the applied load and the observed change in readout as quantified by the linear gauge factor shown on the calibration sheet.
Because of this the linear gauge factor shown on the calibration sheet has been calculated after excluding the often anomalous zero reading from the data points. And this gauge factor best describes the performance of the load cell at moderate to higher loads
This linear gauge factor describes the slope of the best fit line drawn through the calibration data points and the reading where the line intersects the zero load point on the load axis is called the 'Regression Zero' shown on the calibration sheet.
It is important when using the linear gauge factor to calculate loads that the value of R
0
in the linear equation be equal to the regression zero.
For greater accuracy, a second order polynomial can be used to map the data points. In this case, the regression zero is replaced by the factor C shown on the calibration sheet.
It may be, for a variety of reasons (for example if the load cell is used repeatedly on a number of jobs), that the no load zero reading might change significantly.
Again, for greater accuracy, the value of the Regression Zero can be adjusted by an amount equal to the observed change in the no load zero from that shown on the calibration sheet. Similarly, the C factor of the polynomial can be adjusted by the amount of the zero load change multiplied by the linear gauge factor to convert it into the corresponding load change.
26 | USE OF THE REGRESSION ZERO WHEN USING THE LINEAR GAUGE FACTOR | GEOKON
APPENDIX G. MODEL 8032-27 AND LOAD CELL
WIRING
Connect the load cell common VW– conductor to the jumper as follows:
1.
Lift up on the orange tab located on the opposite side of the six black conductors.
2.
Fully insert the common conductor into the 8032-27 Jumper Wire
Assembly.
3.
Push down on the orange tab until it snaps into place.
Refer to Appendix C to identify which conductor of the load cell carries the
common VW– signal.
10:
Insert common VW– conductor here
FIGURE 10:
Model 8032-27 Jumper Wire Assembly
After attaching the common conductor from the load cell to the 8032-27, the black conductors supplied with the 8032-27 may be wired into a GEOKON terminal or multiplexer board where the VW– signal would normally go. Wire in one black conductor from the 8032-27 for each gauge of the load cell.
MODEL 4900 VIBRATING WIRE LOAD CELL | MODEL 8032-27 AND LOAD CELL WIRING | 27
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