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Date: March 1994
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This document describes the procedure for calibration of falling weight deflectometers (FWDs) which was originally developed by the Strategic Highway Research Program (SHRP). This protocol is now administered by the Long-Term Pavement Performance (LTPP) Division in the Federal Highway Administration.
The procedure is written primarily for use with the Dynatest falling weight deflectometer, however it can also be used with the KUAB FWD. Due to differences in the design of the KUAB certain details are not applicable. Special procedures for the calibration of KUAB FWDs are included in Appendix B. It may be possible to use the procedure for other types of FWDs with minor modifications of the hardware and of the data acquisition software. The procedure is not applicable to the calibration of cyclic loading and other types of pavement deflection testing equipment.
In this procedure, the deflection and load transducers from the FWD are first calibrated individually against independently-calibrated reference devices. This is called "reference calibration," and it is performed at a LTPP Regional Calibration Center, or any other properly equipped location. The calibration of the FWD deflection sensors is further refined by comparing them to each other in a process referred to as "relative calibration." Relative calibration is done as a final step that accompanies reference calibration, and it can also be carried out alone, at any suitable location. There is no corresponding relative calibration procedure for the load measurement system.
The procedure results in calibration factors which are entered into the FWD software as multipliers. When the FWD measurements are multiplied by the calibration factors the result is a measurement which has been corrected to agree with the calibration instrumentation. It is necessary that there be a place in the FWD software to enter the calibration factors. That is the responsibility of the FWD manufacturer.
To use this procedure Dynatest FWDs must have Edition 10 or higher software. Earlier Editions do not have the pause feature and do not allow programming the required number of drops in the test sequence. Furthermore, it is not possible to leave the load plate down, as is called for in this procedure. Thus, Dynatest FWDs must be upgraded to Edition 10 or higher software before calibration.
FREQUENCY OF CALIBRATION
Reference calibration should be performed at least once per year, or as soon as possible after a sensor has been replaced on the FWD. Relative calibration should be performed in conjunction with reference calibration.
Relative calibration should be performed on the deflection sensors at least once per month. It should also be performed immediately after a deflection sensor is replaced.
FWD System Operator
Calibration System Operator
REFERENCE CALIBRATION PROCEDURE
The FWD should be in good operating condition prior to performing reference calibration. Particular attention should be paid to cleaning the magnetic deflection sensor bases to insure that they seat properly. Also verify that the FWD load plate is firmly attached to the load cell and that the load plate swivel operates freely. In the event that the load plate is loose, the lower bolts should be tightened to a torque of 10.2 N-m (7.5 lbf-ft) and set with Locktite before proceeding. (Note: This torque requirement is applicable to the Dynatest FWDs. For non-Dynatest FWDs consult the manufacturer.) All electrical connectors should be inspected and, if necessary, cleaned and firmly seated.
The FWD should be at room temperature. If the FWD has been outdoors at a very low or a very high temperature, sufficient time should be allowed for it to equilibrate to room temperature. It is recommended that a series of warm-up drops be performed immediately prior to beginning calibration, to assure that the rubber buffers have been thoroughly warmed up.
Set the FWD mass and drop heights to produce loads within ±10 percent of 27, 40, 53, and 71 kN (6, 9, 12, and 16 kips). For the Dynatest FWD, it is possible to be within this tolerance for the highest load, and yet to have the drop height set too high. Before placing the reference load cell under the load plate, and with the mass positioned at drop height four (the highest position), verify that there is at least a 102 cm (4 in) clearance between the highest point on the mass subassembly and the underside of the brace between the two columns that surround the cylinders that raise and lower the load plate. If the clearance is too small, reposition the target for the fourth drop height to achieve the required clearance. This should assure that there will be adequate clearance when the reference load cell is in position under the load plate.
Before beginning any calibration work, and throughout the entire calibration period, it is necessary that there be no data filters in operation in the FWD. Verify that the "peak smoothing" processor has been turned off. This feature is accessed from the Dynatest Main Menu by selecting "Road Options" (item #3, followed by item #12), where "Peak Readings" should show "direct" and not "smooth."
The FWD load cell should be calibrated at least twice. Multiple calibration tests are performed on the load cell, and the results are averaged. Acceptance criteria based upon the repeatability of the calibration factor are identified in the load cell calibration procedure. If the results persist in failing the acceptance criteria, then the cause of the erratic results should be identified and corrected.
Each deflection sensor shall be calibrated once. Spare deflection sensors do not have to be calibrated until they are in active use. After all load and deflection sensors have been calibrated, the interim calibration factors shall be entered into the FWD computer before proceeding with relative calibration.
A sample reference calibration setup screen for the Dynatest FWD with Edition 10 or Edition 20 software is given in Figure 1. The information in Figure 1 can also be used as the basis for setup of Dynatest FWDs running Edition 25 and higher software.
18. Load another TEST SETUP.
A complete summary of the data to be recorded is given in Table 1. Before beginning to perform the calibrations, FWD-specific information should be recorded via printouts from the FWD data acquisition program screens (e.g., showing the deflection sensor serial numbers and calibration factors, load cell serial number, calibration factor, and sensitivity, and voltage screens from the Dynatest software), which have been annotated with the date and FWD identification information (i.e., FWD model and serial number).
Locate the calibration data acquisition system as close as possible to the FWD computer so that the two systems operators will be able to converse easily. Load the reference calibration software FWDREFCL into the reference system computer. Directions for performing reference calibration using this software are provided in the FWDREFCL User's Guide.
Before doing any calibrations, verify that the computers for the FWD and the reference data acquisition system are registering the correct date and time. If either is set incorrectly, correct it before proceeding.
|Data Item||Mode of Entry||Source1|
|FWD Operator Name||Manual||Operator|
|Calibration System Operator Name||Manual||Operator|
|Date and Time of Calibration||Automatic||Computer Clock|
|FWD Serial/ID Number||Manual||Operator|
|FWD Load Cell Serial Number||Manual||Transducer Setup and Gain Printout|
|FWD Deflection Sensor Serial Numbers||Manual||Transducer Setup and Gain Printout|
|Reference Load Cell Serial Number||Automatic||Configuration File2|
|Reference LVDT Serial Number||Automatic||Configuration File2|
|FWD Calibration Center Location||Automatic||Configuration File2|
|Current Calibration Factor for FWD Load Cell||Manual||Transducer Setup and Gain Printout|
|Current Cal. Factors for FWD Deflection Sensors||Manual||Transducer Setup and Gain Printout|
|Ref. Load Cell Calibration Constants||Automatic||Configuration File2|
|Ref. Load Cell Calibration Date||Automatic||Configuration File2|
|Ref. LVDT Calibration Constants||Computed||FWDREFCL Software|
|Ref. LVDT Calibration Date||Automatic||FWDREFCL Software|
|FWD Load Cell Readings (20 total)||Manual||FWD Computer|
|Ref. Load Cell Readings (20 total)||Automatic||Calibration Data Acquisition System|
|FWD Deflection Readings (20 per sensor)||Manual||FWD Computer|
|Ref. LVDT Readings (20 per sensor)||Automatic||Calibration Data Acquisition System|
|Interim Cal. Factors from Reference Calibration||Computed||FWDREFCL Software|
|FWD Relative Calibration Data||Automatic||Relative Calibration Data Files|
|Calibration Factors from Relative Calibration||Computed||FWDCAL2 Software|
|Final Calibration Factors||Manual||Final Gain Worksheet|
1For SHRP FWDs. Source may be different for FWDs from other manufacturers.
2Reference calibration configuration file (FWDREFCL.CNF).
As described in Appendix A.
FWD Load Cell Calibration Procedure
Note: For accurate results it is critically important that the reference load cell be zeroed with the FWD load plate in the raised position. Also, the signal conditioner excitation and gain must be set exactly to the levels at which the reference load cell was calibrated.
As shown in Figure 1, it is useful to program six drops at each height, rather than five, so that one can be considered a "spare" in case a drop is missed by the reference system instrumentation. If the first five drops are successfully recorded, then the data for the sixth drop can be discarded.
The plate should not be raised at any time during the sequence. Data from both the FWD load cell and the reference system should be recorded for all drops except the three seating drops.
Load Cell Calibration Acceptance Criteria
Process the data sets according to the procedure in the section on Reference Calibration Data Analysis. The presence of any of the following conditions invalidates the load cell calibration test results.
Should any of these conditions occur, the load cell calibration test procedure must be repeated after identifying the source of the problem and correcting it.
FWD Deflection Sensor Calibration Procedure
The micrometer should be moved in 0.5 mm increments to a final reading of 3.0 mm, with the micrometer reading and LVDT voltage output recorded at each 0.5 mm step. Turn the barrel of the micrometer in one direction only, to avoid errors due to backlash.
Analyze the resulting data using a linear regression to determine the coefficient m in the equation Y = m X + b, where Y is the position of the LVDT tip in microns, as measured by the micrometer, and X is the corresponding voltage output in bits, as read by the computer data acquisition board. (The FWDREFCL software provides prompts for this entire process, reads and records the requisite data, and performs the computations.)
The slope m will be close to -1.00 microns per bit. In general the slope should not change by more than ±0.5 percent from one calibration to the next. The standard error of the slope should be less than 0.0010. If a larger standard error is obtained, the LVDT calibration should be repeated.
As shown in Figure 1, it is useful to program six drops at each height, rather than five, so that one can be considered a "spare " in case a drop is missed by the reference system instrumentation. If the first five drops are successfully recorded, then the data for the sixth drop can be discarded.
The plate should not be raised at any time after the seating drops. One complete FWD time history plot should be studied for the fifth drop at each drop height, to verify that the calibration beam does not move prior to the recorded peak deflection.
Deflection Sensor Calibration Acceptance Criteria
Process the data sets according to the procedure in the section on Reference Calibration Data Analysis. The presence of any of the following conditions invalidates the deflection sensor test results.
Should any of these conditions occur, the calibration test for the deflection sensor must be repeated after identifying the source of the problem and correcting it.
Reference Calibration Data Analysis
A. Perform a least squares regression forced through zero for all of the data for each. measurement device (i.e., 20 pairs of data per test -- 5 replicates at each of 4 load levels). The result of this regression will be the coefficient for an equation of the form Y = m X, where Y represents the response of the reference system, X represents the response of the FWD measurement device, and m is the slope of the regression line. Both X and Y should be measured in the same system of units.
B. The coefficient, m, determined in step A, represents the adjustment factor for the calibration factor in the FWD Field Program. The new calibration factor is computed by multiplying the former calibration factor by the coefficient m from step A. This is listed as the new calibration factor on the FWDREFCL report.
RELATIVE CALIBRATION PROCEDURE
Relative calibration of the FWD deflection sensors is used to ensure that all sensors on a given FWD are in calibration with respect to each other. As such, it serves as the final step in the overall FWD calibration process, and as a quick means to periodically verify that the sensors are functioning properly and consistently.
Relative calibration uses the relative calibration stand supplied by the FWD manufacturer. The sensors are stacked vertically in the stand, one above another, so that all sensors are subjected to the same pavement deflection. Relative calibration assumes that the overall mean deflection, as determined from simultaneous measurements by the full set of deflection sensors, yields an accurate estimate of the true deflection. This assumption requires that the deflection sensors must have first been subjected to the reference calibration procedure.
Some FWDs have fewer than or more than seven active deflection sensors. If they do, these procedures should be modified to calibrate the actual number of active sensors in use on the FWD.
FWD relative calibration stand with as many positions as the number of active deflection sensors. For the purpose of illustration a seven-position stand is assumed herein.
FWD relative calibration software (FWDCAL3) and documentation.
The process involves rotation of each deflection sensor through every position in the calibration stand. Each combination of sensors and levels is considered a "set," and the number of sets of data will be equal to the number of sensors. The test point is "conditioned " before beginning the calibration procedure to reduce the possibility that set will be significant in the data analysis. The required order of movement of the sensors is shown in Table 2. In order for the data processing with FWDCAL3 to be done correctly it is very important that the sensor rotation from set to set be done correctly. Spare deflection sensors do not have to be calibrated until they are in active use.
|Set 1||Set 2||Set 3||Set 4||Set 5||Set 6||Set 7|
Note: The rotation must be done as prescribed above in order for the software (FWDCAL2) to work properly. For instance, for Set 2, move Sensor 2 to the position formerly occupied by Sensor 1, etc.
When done in conjunction with reference calibration, the relative calibration procedure shall be repeated twice: Acceptance criteria based upon the repeatability of the calibration factor are identified in the relative calibration procedure. If the results persist in failing the acceptance criteria, then the cause of the erratic results should be identified and corrected.
After the relative calibration is completed, the final calibration factors shall be entered into the FWD computer.
A sample relative calibration setup screen for the Dynatest FWD with Edition 10 or Edition 20 software is given in Figure 2. The information in Figure 2 can also be used as the basis for setup of Dynatest FWDs running Edition 25 and higher software.
1. Test UNITS...: lbf.mil.inch (kPa.mu.mm)
2. Temperature...: Fahrenheit (Centigrade)
3. Stn.Request...: OFF (ON)
4. Test Checks...: NONE (Decreasing defls, Roll-Off, RollOFF+Decr)
5. Reject prompt.: OFF (ON)
6. Stationing....: [Doesn't matter]
7. Temp.Request..: OFF (ON)
8. Cond.Request..: OFF (ON)
9. Variation: Load NOT Checked! Deflections NOT Checked!
10. Diameter of Plate: 11.8
11. Deflector distances: [doesn't matter - keep what you have]
1 2 3 4
12. Drop No.: 1234567P8901234P5678901P2345678
13. Heights*: CC44444PCC44444PCC44444PCC44444PCC
14. Test Plots: ...............................................................
15. Save Peaks: ...*****.*****.*****.*****.*****.*****.*****.*****.............
16. Load His: ................................................................
17. Whole His: ................................................................
18. Load another TEST SETUP.
19. Store the CURRENT TEST SETUP.
*Note: Drop height should be adjusted to attain deflections within the specified
Relative Calibration of the Deflection Sensors
Relative Calibration Data Analysis
A three-way analysis of variance should be used to evaluate the data. This will partition the variance into four sources: (1) that due to sensor number, (2) that due to position in the calibration stand, (3) that due to set, and (4) that due random error of measurement. This analysis is performed by the FWDCAL3 software. In this analysis, deflection is the dependent variable, and sensor number, position and set are the three main factors. The three hypotheses that may be tested are:
H0: Sensor number is a significant source of error.
H0: Data set number is a significant source of error.
H0: Position in the stand is a significant source of error.
Through the use of hypothesis testing it is possible to determine whether random error due to sensor number, due to position in the calibration stand, and due to set number are statistically significant. The only factor that should result in a change in the deflection sensor calibration factors is sensor number.
If the random error due to sensor number is found to be statistically significant, then the calculated adjustments in the calibration factors for each sensor should be made. If a change is made in the calibration factor for one sensor, then the calibration factors for all sensors should be changed in accordance with the calculations.
If position in the stand is statistically significant, it is likely that the stand was not held vertical throughout all of the sets during the test. Or a connection in the stand may have been loose. The problem should be corrected, and the test should be repeated.
If set is statistically significant, there may have been a systematic change in the properties of the pavement materials, for instance due to compaction or liquefaction. The test should be repeated after the testing site has been further "conditioned" according to the procedure. If the deflection readings do not become relatively constant during the conditioning, then another site should be selected for the testing.
The mere fact that either position or set, or both, are significant does not necessarily invalidate the relative calibration. Judgement must be used to assess whether or not these factors may be of sufficient physical significance (as opposed to statistical significance) to require that the relative calibration should be repeated or that a new test site should be selected.
The standard error of measurement (e.g., the square root of the mean square error due to error) should be on the order of 2 microns (0.08 mils) or less if the system is working properly and the calibration test was conducted carefully.
The analysis of the data obtained from the relative calibration procedure and the method used to determine revised calibration factors is as follows (calculations are done automatically within the FWDCAL3 software):
Relative Calibration Acceptance Criteria
When relative calibration is conducted in conjunction with reference calibration, the procedure is repeated two times. If the two sets of calibration factors agree within 0.003 for each deflection sensor, then the results of the two tests shall be averaged. If they are outside the limit, then a third relative calibration shall be performed. If the standard deviation of the three results (based on n - 1 degrees of freedom) is less than 0.0030, then the three results shall be averaged. If the standard deviation exceeds 0.0030, the relative calibration procedure should be repeated.
An example of the calculations following this procedure is shown in Appendix C. The average final calibration factors should be computed, and the factor for each deflection sensor should be entered into the FWD computer software (e.g., the "FWD Field Program").
When relative calibration is done alone, typically on a monthly basis, then adjustment of the calibration factors in the FWD Field Program should be made only when those changes are both significant, and verified to be necessary. The following guidelines are to be used to evaluate the need for adjustment to the calibration factors.
The full FWD calibration report shall consist of the following:
Each of the above printouts is to be annotated with the FWD unit identification (e.g., manufacturer's serial number or agency ID), and the calibration date.
Distribution of this report shall be as follows:
The diskettes on which the reference and relative calibration data are stored should be kept in the FWD. It is recommended that labeled backup copies be kept on file with the calibration report at the office out of which the FWD is operated. For the LTPP FWDs, additional backup copies of the calibration diskettes are to be kept on file at the LTPP Regional Office.
When relative calibration is done alone (e.g., as a monthly calibration check), the relative calibration report will consist of all printouts from the FWDCAL3 software, annotated as necessary to explain any problems which might have been encountered.
Indoor space with:
Note: Calculations indicate that an acceptable fatigue life can be achieved with a 5-inch-thick portland cement concrete slab resting on an 8-inch open-graded crushed stone base. A layer of filter fabric should be placed below the base to protect it from intrusion of subgrade fines. To achieve adequate deflections, the subgrade modulus should be less than 12,000 psi (80 MPa) with bedrock deeper than 25-30 feet. Where bedrock exists at depths of 15 to 25 feet, a subgrade modulus of 7,500 psi (50 MPa) or less will be needed. Test pads located where bedrock is less than 15 feet deep are likely to be very sensitive to minor variations in subgrade moisture, and hence are not advisable.
Note: Drawings of each of the special items of equipment, and cabling diagrams, are available from the Long-Term Pavement Performance (LTPP) Division at the Federal Highway Administration, Turner-Fairbank Highway Research Center, McLean, Virginia.
IBM PC-XT or PC-AT, or compatible, computer recommended; IBM PSI2 computer acceptable. Configuration:
(Where both "recommended" and "acceptable" options are given in the above specifications, an effort has been made in the software development to accommodate both alternatives. However, since most of the testing has been done on computer hardware meeting the "recommended" specifications, installation of the calibration station will go more smoothly if those specifications are met. A demonstration version of the FWDREFCL software is available from the LTPP Division in the Federal Highway Administration (located at the Turner-Fairbank Highway Research Center, McLean, VA) which can be used to determine if the computer and peripherals will work satisfactorily with the program.).
Reference calibration of the KUAB FWD can be carried out in a manner very similar to the procedure outlined for the Dynatest FWD. However, because the KUAB has its load plate forward of the deflection sensor beam (i.e., toward the towing vehicle), it will be necessary to place the trailer on an angle with respect to the test pad, so that the load plate can be positioned as close as possible to the LVDT and the deflection sensor holder. The end of the aluminum beam holding the LVDT should be just behind the trailer wheels, near the place where the "foot" of the KUAB A-frame rests on the floor.
KUAB FWDs must have operational program SFWD version 4.0 or higher to perform reference calibrations. This version can be obtained from the manufacturer.
Before the reference calibration procedure is performed, the FWD Operator should first conduct a static calibration of the deflection sensors. The KUAB software will automatically file the static calibration factors. The manufacturer recommends that the dynamic calibration factors be entered as 1.05 for all sensors. These values should not be changed during or after the reference calibration.
Due to the larger distance between the center of the load plate and the seismometer holder it may not be possible to achieve the specified deflection of 400 microns (16 mils) at 16,000 pounds. The deflections should be as large as possible.
To achieve the specified load levels refer to the information provided by the manufacturer. The exact combination of weights and buffers will vary between the different KUABs. Adjust the drop height endswitches as necessary to be within the specified load tolerance.
In general the KUAB will be tested with the 17-millisecond rubber buffers installed. The reference data acquisition system and the FWDREFCL software allow for calibration using the 25-millisecond buffers, but the movement of the aluminum beam should be checked carefully to assure that there is no motion before the ground deflection peaked out.
The FWDREFCL software contains an number of special features to accommodate the KUAB, and thus in initializing the software, the FWD type should be set for "KUAB." The deflection sensor that is mounted through the load plate (i.e., the center sensor) is called sensor number zero on the KUAB, and it is in position number 0 as far as FWDREFCL is concerned.
KUAB FWDs with version 4.0 software are able to pause during the drop sequence, prior to releasing the mass. This is achieved by entering the letter "P" after the drop height position code during programming of the drop sequence. For example, the required reference calibration drop sequence would be entered as follows (drop height, number of drops):
The pause occurs with the mass elevated, ready to drop. The mass will not be released until the FWD operator strikes a key.
The S programmed at the end of the drop sequence will stop the testing sequence with the load plate remaining on the test pad after the final drop. To repeat the drop sequence without raising the load plate initiate the drop sequence in the normal manner and override the ensuing error statements regarding transport position, etc., by choosing to ignore.
Because the top of the reference load cell is 300 millimeters in diameter, it will only be possible to calibrate the small (300 mm) load plate on the KUAB. If the KUAB is outfitted with the large (450 mm) load plate, it should be replaced with the 300 millimeter load plate in order to attain accurate results.
A special holder is provided for mounting the KUAB seismometer under the LVDT. The Dynatest geophone holder should be removed and the KUAB holder bolted down in its place. The LVDT mounting plate that attaches to the end of the aluminum beam should be removed from its position under the beam and reinstalled on top of the beam. The KUAB deflection sensors will be slid upward off the two rods that hold them in position on the sensor beam in the trailer. Remove the tripod foot by loosening its holding screw, and then slip the deflection sensor over the peg on the holder under the LVDT. Tighten the holding screw firmly.
Conducting load plate calibration is particularly difficult with the KUAB, because it is hard to detect when the FWD mass has been released. To make this easier, a double layer (or thicker) of "duct tape" should be wrapped around the guide post (down which the runners under the falling mass roll), located an inch or two above the bottom of the stroke. The proper position for the tape can be found when the mass is at its lowest drop height. Adjust the KUAB load sensitivity in the reference system computer to a value of 5 to 10 bits.
The presence of the tape during load plate calibration may generate warning messages about "excess noise in the average zero." The "bump" that is generated as the mass runs past the (tape triggers the start of load data acquisition, but the vibration that it causes may not yet be damped out when the mass strikes the plate. Check the time history traces carefully to be sure that the vibration had no effect on the accuracy of the peak load that was registered. If there is a problem, try moving the tape to a position higher on the guide post.
Remove the tape after completion of the load plate calibration.
Enter the new calibration factors for the deflection sensors as the "SHRP Calibration Factors" under the Calibrate menu in the KUAB operational program. The calibration factor for the 300 mm load plate is entered in the same manner. The calibration factor for the large (450 mm) load plate should remain unchanged.
Most KUAB FWDs do not have a calibration stand for performing relative calibration. Thus it may not be possible to perform the relative calibration procedure as described herein.
Limited experience in the calibration of KUAB FWDs has shown that the combination of static calibration and dynamic calibration may be adequate to yield a satisfactory calibration and accurate final calibration factors. However, relative calibration further refines the reference calibration factors, and it allows a monthly check of the accuracy of the deflection sensors. Thus it is highly recommended that a means of performing relative calibration with the KUAB FWD be obtained.
The reference load cell is a precision instrument, capable of measuring loads within ±0.3 percent or better. Such a high degree of precision can be attained, however, only if this calibration procedure is followed exactly. It is essential that the reference load cell be calibrated using a universal testing machine that is properly maintained and accurately calibrated.
FREQUENCY OF CALIBRATION
Calibration of the reference load cell should be performed at least once per year. It should also be performed immediately after any of the six Allen head screws that attach the load measurement links to the upper or lower plates of the reference load cell are loosened. Calibration would also be necessary if the load cell fails to pass the unbalanced zero test (within ±5 percent) as detected by the FWDREFCL program.
Note: Do not use a servo-controlled, closed-loop testing system such as a MTS machine for this purpose. In general such equipment does not provide the high degree of accuracy that is required for this calibration.
The reference load cell and its cable, and the associated signal conditioner, data acquisition board and computer should be considered a system of instruments, which should be calibrated together and used together.
CALIBRATION OF EQUIPMENT
The universal testing machine should be calibrated according to ASTM procedure E-74 within twelve months prior to conducting this procedure. The device(s) used to calibrate the universal testing machine should be certified to be traceable to the National Institute for Science and Technology (NIST - formerly the National Bureau of Standards) calibration(s). The certificate of calibration provided for the universal testing machine should be used to develop an adjustment algorithm which will correct the indicated load on the universal testing machine to the NIST load. It is highly recommended that the reference load cell be calibrated soon after the universal testing machine is calibrated.
The MetraByte board should be calibrated according to the procedure described in the manufacturer's instruction manual. Its accuracy should be verified using a reference voltage source such as a 1.350 volt mercury cell (e.g., camera battery in new condition).
The 2310 signal conditioner amplifier should be balanced according to the procedure described in the manufacturer's instruction manual. With the signal input terminals shorted together, at gain 100 the ac noise on the ±10 volt output terminals should be 1 millivolt or less.
Inspect the reference load cell carefully before calibration. Verify that the cable and the Amphenol connectors are making proper contact in their sockets (e.g., fitting and locking tightly). Make a continuity check to verify that there are no breaks in the wires. Verify that the Allen screws on the load cell are tight.
Note: The six Allen screws on the top and the bottom of the load cell were torqued to 100 lb.-in. and set with Locktite during assembly. These screws should not be loosened unless it is absolutely necessary. If any of the screws are loosened, they should be removed one at a time and their threads cleaned. Locktite should be reapplied to their threads, and they should be torqued to precisely 100 lb.-in.
Verify that the three steel pads on the bottom of the reference load cell are in good condition. Verify that one of the wood/aluminum bearing blocks has a ribbed rubber pad cemented to it. If the edges of the rubber pad are loose, use rubber cement to reattach it.
Install a spherically-seated bearing block in the cross head of the universal testing machine.
Make the following settings on the front panel of the 2310 signal conditioner:
Verify that the Tape Playback switch on the rear panel of the signal conditioner is OFF. Position the signal conditioner and the computer several feet apart near the testing machine and attach them to ac line power.
Use the same computer system for reference load cell calibration that is used for FWD calibration. A graphics printer must be available.
Load the software LDCELCAL into the reference system computer. This program should be located in the same subdirectory with FWDREFCL.EXE and FWDREFCL.CNF. A disk with the files REFLCCAL.WK1 and REFLCCAL.FMT on it should be inserted in drive A. The computer must be running under DOS and not under WINDOWS during the calibration.
The computer program LDCELCAL is designed to interact with a Lotus 1-2-3, version 2.3, spreadsheet to accomplish the data analysis. The subdirectory containing the 1-2-3 program must be on the PATH in order for the two programs to work together successfully. The WYSIWYG add-in utility should be installed according to the Lotus directions. Defaults in Lotus 1-2-3 should be set as follows.
See the Lotus User's Manual for instructions regarding setting the defaults. If the program is correctly installed and set up, the data analysis will be run, a listing of the data will be produced, and graphical output will be printed automatically. A demonstration version of LDCELCAL is available to use with Lotus 1-2-3 to verify that your computer system can interact properly with the program.
Note: When the load is released the MetraByte board will not read exactly zero because it was zeroed without the upper bearing block in place. Do not rezero the signal conditioner at this point.
Using a spreadsheet utility program such as Lotus 1-2-3, enter the results of the calibration. In column A enter the nominal loads registered by the universal testing machine (i.e., 0, 1000, 2000, etc.). In column B correct these loads to the NIST traceable loads, based on the certificate of calibration for the testing machine. In column C subtract the tare weight of the upper bearing block from the loads in column B. In column D enter the MetraByte board readings in bits. Note that the readings are negative. In columns E, F, G and H calculate V2, V3, V4, and V5, respectively (where V represents the readings in column D).
Use the spreadsheet regression utility to calculate a linear regression of corrected load (as the Y-variable) versus bits (as the X-variable). The regression should be forced through zero, yielding an equation of the form Y = mV, where Y is the corrected load (column C), V is the voltage (column D), and m is the slope of the line of best fit. The coefficient m should be approximately -10 pounds per bit.
Use the regression utility to calculate a fifth degree polynomial regression of the form:
where the coefficients Ai are determined by the regression. Evaluate the polynomial solution according to the following criteria.
If the standard error of any of the coefficients is too large (e.g., not significant), repeat the, regression using a fourth degree polynomial of the form:
Again evaluate the polynomial according to the criteria in 1 and 2 above. When the evaluation criteria are satisfied, and all of the coefficients are significant (usually this will happen with either a fourth degree, third degree, or second degree polynomial), record the regression coefficients.
Note: The LOTUS 1-2-3 spreadsheet template that accompanies LDCELCAL will perform the step-down egression automatically, and it will choose the correct polynomial.
ENTER THE REGRESSION COEFFICIENTS IN FWDREFCL
The regression coefficients should be entered in the data acquisition program FWDREFCL. Instructions for doing this can be found in the Load Cell Setup section of the FWDREFCL User's Guide. All of the unused higher order terms should have their coefficients entered as 0.0.
When the regression coefficients are entered in FWDREFCL, the unbalanced zero, the +B and -B calibration factors, the load cell signal conditioner gain factor, and the date of calibration should also be entered.
Topics: research, infrastructure, pavements and materials
Keywords: research, infrastructure, pavements and materials
TRT Terms: research, facilities, transportation, highway facilities, roads, parts of roads, pavements