1、Standard Practice for Calibrating the Reference Load Cell Used for Reference Calibrations for a Falling Weight Deflectometer AASHTO Designation: R 33-11 (2015) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-5a R 3
2、3-1 AASHTO Standard Practice for Calibrating the Reference Load Cell Used for Reference Calibrations for a Falling Weight Deflectometer AASHTO Designation: R 33-11 (2015) 1. SCOPE 1.1. This practice covers the calibration of the reference load cell that is used for calibration of a Falling Weight De
3、flectometer (FWD) in R 32. It is used to establish calibration coefficients for the reference load cell. 1.2. This procedure has been automated in a software package named RefLCCal. It is required that the computer program be used to carry out the procedure. 1.3. Reference load cell calibration is p
4、erformed at least once per year, or immediately after the reference load cell fails to pass certain quality assurance checks in the R 32 software (WinFWDCal). 1.4. The reference load cell, the cables, the associated signal conditioner, and the data acquisition system are considered a system of instr
5、uments that shall be calibrated together and used together. 1.5. This procedure results in load cell calibration coefficients that are entered into the R 32 software. These factors are used in WinFWDCal to convert analog voltage readings from the reference load cell to engineering units. 1.6. The va
6、lues stated in SI units are to be regarded as standard. U.S. Customary units given in parentheses are for information purposes only. 1.7. This standard practice may involve hazardous materials, operations, and equipment. It does not purport to address all of the safety concerns associated with its u
7、se. It is the responsibility of the user of this standard practice to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standard: R 32, Calibrating the Load Cell and Deflection Sen
8、sors for a Falling Weight Deflectometer 2.2. ASTM Standard: E74, Standard Practice of Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is
9、a violation of applicable law.TS-5a R 33-2 AASHTO 3. TERMINOLOGY 3.1. Description of Terms Specific to this Standard: 3.1.1. reference load cellcustom-built calibration device used to calibrate the FWD load cell. 3.1.2. reference load cell systemsystem composed of the reference load cell and the ass
10、ociated data acquisition system. This system is used in the FWD load cell calibrations in R 32. 3.1.3. data acquisition systemthe signal conditioner, data acquisition board, data acquisition software, computer, and cabling. The reference load cell is connected to the signal conditioner. 3.1.4. RefLC
11、Calthe software required for data acquisition and data analysis for reference load cell calibration.13.1.5. WinFWDCalthe software required for data acquisition and data analysis for FWD calibration in R 32. 3.1.6. FWD load celldevice located on the loading mechanism of an FWD that measures the load
12、response of the FWD system. 3.1.7. FWD load cell final gain factorcalibration factor for the load sensor on the FWD determined by the R 32 FWD calibration procedure. 4. SUMMARY OF METHOD 4.1. The reference load cell system shall be calibrated using a universal testing machine. The load indicated on
13、the universal testing machine shall be adjusted to the National Institute of Standards and Technology (NIST)traceable loads obtained during the calibration of the testing machine. The adjusted loads shall be used when calculating the calibration coefficients for the reference load cell. 4.2. The sam
14、e equipment set used in R 32 shall be used in this procedure. 4.3. For each trial run, the load shall be applied slowly and continuously to a maximum of 107 kN (24,000 lb). Readings shall be taken using the RefLCCal software during loading at each 4.45-kN (1000-lb) increment. 4.4. A minimum of three
15、 trials shall be carried out. The adjusted load data for three or more acceptable trials shall be combined using the RefLCCal software. 4.5. The RefLCCal software will provide the statistically significant coefficients for a second- to fifth-order polynomial regression equation. 4.6. The coefficient
16、s shall be evaluated in accordance with the acceptance criteria in Section 9.2 of this procedure. If they are found to be acceptable, a calibration certificate shall be issued for the load cell. 4.7. The coefficients from the certificate should be entered into the WinFWDCal data acquisition software
17、 used with the reference load cell system in R 32. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 33-3 AASHTO 5. SIGNIFICANCE AND USE 5.1. In order to be confident in the FWD load cell final ga
18、in factor determined in the FWD load cell calibration procedure, the reference load cell must be calibrated against highly accurate, known loads. 5.2. When calibrated using at least three trials in accordance with this procedure, the accuracy of the reference load cell may be expected to be 0.3 perc
19、ent of full scale or better. 6. APPARATUS 6.1. Testing MachineA static, universal testing machine, hydraulic or screw powered, with a maximum load capacity of at least 530 kN (120,000 lb) shall be used. The high capacity of the testing machine assures that the test frame will be adequately rigid. Se
20、rvo-controlled, closed-loop testing systems of any manufacture shall not be used for calibrating the reference load cell. The testing machine shall have a spherically seated bearing block that is adjusted so that it can easily align with the upper loading block. The testing machine shall have severa
21、l load ranges. Use a range equal to or slightly more than the maximum load to which the reference load cell will be calibrated. The universal testing machine shall have been calibrated according to ASTM E74 within 12 months prior to conducting this procedure. The load measurement device(s) used to c
22、alibrate the universal testing machine shall be certified to be traceable to the National Institute of Standards and Technology (NIST). Data from the certificate of calibration for the testing machine shall be used to develop an equation that will adjust the indicated load on the dial of the univers
23、al testing machine as closely as possible to the NIST-traceable load. The equation is of the general form: 234512345adjusted load BX BX BX BX BX=+ (1) where: adjusted load = the corrected load dial reading in kilonewtons or pounds, X = the indicated load dial reading in kilonewtons or pounds, and Bi
24、= the regression coefficients. A spreadsheet shall be used to develop the equation, and all regression coefficients shall be statistically significant at the 0.05 level or better. The regression coefficients Bishall be entered in the RefLCCal software. 6.2. Signal ConditionerThe same signal conditio
25、ner that is used in the reference load cell system in R 32 shall be used during the reference load cell calibration. 6.3. Data Acquisition BoardThe same data acquisition board that is used in the reference load cell system in R 32 shall be used during the reference load cell calibration. 6.4. Trigge
26、ring SystemAn analog push-button system shall be used during loading to trigger the RefLCCal software data acquisition system. 6.5. Loading BlocksUpper and lower loading blocks, equal in diameter to the reference load cell, shall be used to distribute the load over the reference load cell. The upper
27、 loading block shall have a ribbed rubber sheet affixed to the underside (in contact with the reference load cell) to simulate the rubber sheet that is used on the FWD load plate. The lower loading block shall not have a rubber sheet. 2016 by the American Association of State Highway and Transportat
28、ion Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 33-4 AASHTO 6.5.1. The loading blocks shall be very rigid. Suitable blocks can be made using a cylinder of laminated hard maple 300 mm in diameter by 75 mm tall (12 in. in diameter by 3 in. tall) that has been f
29、aced on the top and bottom with a 13-mm (0.5-in.) plate of aluminum held in place with epoxy. The top and bottom aluminum surfaces shall be machined to be parallel planes. 7. PREPARATION OF APPARATUS 7.1. Inspect the reference load cell carefully before calibration. Verify that the cable and the con
30、nectors are in good repair and making proper contact. 7.1.1. Make a continuity check to verify that there are no breaks in the wires. Verify that the three guides on the reference load cell are in place and firmly attached. Verify that the Allen screws on the load cell are tight (top and bottom of l
31、oad cell). Note 1The Allen screws are typically torqued to a level suggested by the load cell manufacturer and set with Locktite during initial assembly. These screws shall not be loosened unless it is absolutely necessary. If any of the screws have been loosened, they shall be removed one at a time
32、 and their threads shall be thoroughly cleaned. Locktite shall be reapplied to their threads, and they shall be retorqued to the specified level. 7.2. Attach all cables and warm up the signal conditioner for at least 60 min before beginning the calibration procedure. 7.3. Warm up the universal testi
33、ng machine for at least 15 min before beginning the calibration procedure. 8. PROCEDURE 8.1. The following steps constitute one calibration trial: 8.1.1. Boot up the RefLCCal software and follow the onscreen instructions to collect the preliminary data before beginning the loading procedure. 8.1.1.1
34、. Carefully zero the testing machine and the signal conditioner as instructed by the software. Make no further adjustments in these parameters until after conclusion of the calibration procedure. 8.1.1.2. Special attention should be paid to setting up the signal conditioner gain, filter, and excitat
35、ion voltage precisely. 8.1.2. Center the reference load cell between the upper and lower loading blocks, and carefully align and center the system under the spherical bearing block of the universal testing machine. (See also Section 6.4.) 8.1.3. Preload the reference load cell to 107 kN (24,000 lb)
36、three times, unloading after each repetition. Apply the load at approximately 50 to 100 kN (10,000 to 20,000 lb) per minute. Note 2The signal conditioner output voltage will not return to zero due to the weight of the upper loading block. This is compensated in the software. The testing machine dial
37、s indicated load should return to zero. 8.1.4. Apply load at a rate no faster than 4.45 kN (1000 lb) per minute. Use RefLCCal to record the voltage output from the reference load cell. Take readings at 4.45-kN (1000-lb) intervals up to a maximum load of 107 kN (24,000 lb). While releasing the load,
38、record a reading at 53 kN (12,000 lb) and at zero load. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 33-5 AASHTO Note 3Avoid overloading the reference load cell during this procedure as it ma
39、y be permanently damaged, and it may become unusable. 8.1.5. Follow the onscreen instructions to collect the postloading data. 8.1.6. The RefLCCal software will provide the coefficients for a second- to fifth-order polynomial regression equation for the data. The equation is of the form: 234512345lo
40、ad AV AV AV AV AV=+ (2) where: load = the load reading in kilonewtons or pounds, V = the analog voltage reading in volts, and Ai= the regression coefficients. A step-up regression procedure shall be used. 8.2. Data Acceptance Criteria (Single Trial)The RefLCCal software will compute the following st
41、atistics for the trial. If all three criteria are met, the trial should be accepted. Otherwise, the data should be discarded and the trial repeated. 8.2.1. The average rate of loading shall be no more than 4.45 kN (1000 lb) per minute. 8.2.2. All of the polynomial regression coefficients shall be st
42、atistically significant at the 95th percentile level. The software will flag the coefficients that are not significant. 8.2.3. The standard error of the Y-estimate shall be no more than 0.045 kN (10 lb). 9. ACCEPTANCE AND CERTIFICATION 9.1. Repeat the calibration process in Section 8 to achieve a to
43、tal of at least three acceptable trials. Note 4More than three trials are encouraged and will strengthen the final calibration coefficients. 9.1.1. The adjusted load data for all acceptable trials shall be combined using the RefLCCal software. 9.1.2. The RefLCCal software will provide the coefficien
44、ts for a second- to fifth-order polynomial regression equation for the combined data (Equation 2). 9.2. Data Acceptance Criteria (Multiple Trials)The RefLCCal software will compute the following statistics for the trial. If all three criteria are met, the calibration should be accepted. If one or mo
45、re trials fail to satisfy Section 9.2.1, they should be dropped. If the data for fewer than three trials remain, additional trials should be made. 9.2.1. At each load level, the range of the voltages (minimum to maximum reading) shall not exceed 0.020 V. 9.2.2. All of the polynomial regression coeff
46、icients shall be statistically significant at the 95th percentile level. The software will flag the coefficients that are not significant. 9.2.3. The standard error of the Y-estimate shall be no more than 0.090 kN (20 lb). 9.3. Certificate of CalibrationIf the final calibration results satisfy the a
47、cceptance criteria, a certificate of calibration listing the calibration coefficients (Ai) for the load cell shall be issued. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 33-6 AASHTO 9.3.1. T
48、he calibration coefficients shall be entered into the WinFWDCal software. 10. REPORT 10.1. Report the following information: 10.1.1. Date of the calibration, and 10.1.2. The final calibration coefficients from Section 9.3. 11. FREQUENCY OF CALIBRATION 11.1. Calibration of the reference load cell sha
49、ll be performed at least once per year. 11.1.1. In the event that the reference load cell fails to pass the load cell quality assurance checks in the R 32 software (WinFWDCal), and if the cause for such failure cannot be identified and corrected, then the reference load cell should immediately be recalibrated. 12. PRECISION AND ACCURACY 12.1. No precision or accuracy statements are available at this time. 13. KEYWORDS 13.1. Calibration; falling weight deflectometer; FWD; load cell; load cell calibration; pavement testing; sensors. 1The RefLCCal
