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Publication Number: FHWA-HRT-06-132
Date: December 2006

Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements

Chapter 8. Calibration and Verification

Highly accurate load, deflection, and associated data are necessary to meet the requirements of the LTPP program. This section includes several calibration and verification requirements to ensure the accuracy of FWD measurements. If the FWD cannot meet the requirements of any of these procedures, it may not be used to collect data for LTPP. In such a situation the operator should fill out and submit an FWD Problem Report (FWDPR) (see appendix B) to the FHWA LTPP FWD task leader with copy to the TSSC.

8.1 REFERENCE CALIBRATION

Every FWD performing data collection on behalf of LTPP should undergo yearly reference calibration. An exception is units testing in Hawaii, Alaska, and Puerto Rico. This calibration must be performed at one of the four calibration centers established by the Strategic Highway Research Program (SHRP), or at an equivalent center as determined by the FHWA LTPP FWD task leader. The nominal yearly interval should not exceed 400 days.

If the FWD load cell or signal processor is replaced, the FWD should undergo reference calibration before performing testing on behalf of LTPP regardless of the interval since the previous reference calibration.

If a major component such as a deflection sensor, multisignal cable, or trailer printed circuit board (PCB) board is replaced, the FWD should undergo reference calibration as soon as practical, but in the meantime it can be used to collect data on behalf of LTPP.

The most recent version of the FHWA LTPP FWD Reference and Relative Calibration Manual should be followed during reference calibration.

8.2 RELATIVE CALIBRATION

Data should not be collected by an FWD on behalf of LTPP unless the FWD has undergone relative calibration within the previous 42 days. If the FWD is in regular use, relative calibration should be performed on a nominal monthly interval. Relative calibration should not be performed during periods when the FWD is idle.

If a major component such as a deflection sensor, multisignal cable, or trailer PCB board is replaced, the FWD should undergo relative calibration before continuing to collect data on behalf of LTPP, regardless of the interval since the previous relative calibration.

The most recent version of the FHWA LTPP FWD Reference and Relative Calibration Manual should be followed during relative calibration.

8.3 DMI CALIBRATION

Every FWD performing data collection on behalf of LTPP should undergo DMI calibration monthly while in service. Data should not be collected unless the FWD has undergone DMI calibration within the previous 42 days.

If the tow vehicle undergoes maintenance, including replacement of tires, the DMI should be recalibrated before continuing to collect data on behalf of LTPP, regardless of the interval since the previous DMI calibration.

The FWD DMI is calibrated by driving the vehicle over a known distance. The FWD data collection software can then calculate an appropriate calibration factor.

The section used for DMI calibration must be straight, at least 150 m (492.13 ft) in length, and reasonably level. Because the FWD must be stopped at each end of the section, it cannot be performed on an active highway without traffic control. The section must be surveyed before the DMI calibration. The section should be measured using a surveyor’s tape (a measuring wheel is not acceptable). Ensure that the proper tension and alignment is applied to the tape. The operator should fill out form F07 immediately after each DMI calibration.

Immediately before DMI calibration, the tire pressure for all of the tow vehicle tires should be set to the manufacturer’s specification, and then the operator should drive the vehicle at least 15 min at highway speeds. The LTPP FWD Data Collection Software Manual has instructions on how to operate the FWD data collection software during DMI calibration.

8.4 TEMPERATURE SENSOR VERIFICATION

Every temperature sensor collecting data associated with LTPP FWD measurements should have its accuracy verified monthly, and unless the temperature sensor has been verified during the previous 42 days, data should not be collected.

Following is a summary of a verification procedure; it should not be used as a calibration procedure. If a temperature sensor fails this procedure, it should be returned to the manufacturer for repair or recalibration, or it should be replaced with a new sensor.

8.4.1 The following equipment and supplies are required for this procedure:
  
  • National Institute of Standards and Technology (NIST) traceable mercury thermometer (reference thermometer).
  • 4-liter (L) (1-gallon (gal)) bucket.
  • Hot plate.
  • Large wooden spoon or paint stirrer.
  • Medium-sized cooking pot, approximately 125 mm (5 inches) in diameter.
  • Leather heat-resistant gloves.
  • Cooking oil (about 0.5 L (1 pint)).
  • Ice.
  • Water.
  • Copies of forms F08 and F09 (see appendix B).
8.4.2 Park the FWD and tow vehicle on a smooth surface in an area with good ventilation that is not exposed to direct sunlight.
   
8.4.3 Start the FWD data collection software and enter a screen from which the air and IR surface temperature measurements can be read. Further instructions are given in the LTPP FWD Data Collection Software Manual.
   
8.4.4 Unclip the FWD air temperature mounted sensor so that it hangs freely.
   
8.4.5 Prepare an ice water bath. Place ice and water in the 4 liter (1 gal) bucket and stir with the wooden spoon or paint stirrer. Stir until the reference temperature records a temperature that is less than or equal to 2 °C (35.6 °F).
   
8.4.6 Place the bucket under the FWD air temperature mounted sensor. When the reading from the IR temperature stabilizes, record the temperature from both the IR sensor and the reference thermometer on form F08. Remove the reference thermometer, then stir the ice bath for 1 min, and record the measurements from both sensors again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for either data set, stir for another minute, and then record both temperatures again. If the IR temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the IR temperature sensor is unacceptable.
   
8.4.7 Repeat the procedure in 8.4.6 for the hand-held IR temperature sensor. Hold the hand-held IR temperature sensor at a height consistent with the FWD-mounted IR temperature sensor.
   
8.4.8 Stir the ice bath for another minute. Place the FWD air temperature mounted sensor and the reference thermometer in the bath. When the reading from the air temperature sensor stabilizes, record the readings of both sensors. Stir for another minute and record both temperatures again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for either data set, stir for another minute, and then record both temperatures again. If the air temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the air temperature sensor is unacceptable.
   
8.4.9 Repeat the procedure in 8.4.6 for the hand-held temperature probe.
   
8.4.10 Prepare the room temperature water bath. Empty the bucket and fill it with warm tap water. Allow it to sit for 10 min and then stir for 1 min.
   
8.4.11 Place the bucket under the FWD air temperature mounted sensor. When the reading from the IR temperature sensor stabilizes, record the temperature from both the IR sensor and the reference thermometer on form F08. Stir the water for 1 min and then record the measurements from both sensors again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for either data set, stir for another minute and then record both temperatures again. If the IR temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the IR temperature sensor is unacceptable.
   
8.4.12 Repeat the procedure in 8.4.11 for the hand-held IR temperature sensor. Hold the hand-held IR temperature sensor at a height consistent with the FWD-mounted IR temperature sensor.
   
8.4.13 Stir the water for another minute. Place the FWD air temperature mounted sensor and the reference thermometer in the water. When the reading from the air temperature sensor stabilizes, record the readings of both sensors. Stir for another minute and record both temperatures again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for either data set, stir for another minute and then record both temperatures again. If the air temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the air temperature sensor is unacceptable.
   
8.4.14 Repeat the procedure in 8.4.13 for the hand-held temperature probe.
   
8.4.15 If any IR temperature sensor was determined to be unacceptable in the low temperature or ambient temperature check, then it need not be checked at the high temperature. The high temperature check is optional if it is being performed in the field. The high temperature check is to be performed only for IR temperature sensors.
   
8.4.16 Prepare the high-temperature oil bath. Pour cooking oil into the cooking pot to a depth of approximately 50 mm. Place the cooking pot on the hot plate and under the FWD air temperature mounted sensor. Stir the oil while it is warming on the hot plate. The operator who is stirring the oil must wear gloves. Heat until the oil temperature stabilizes at 60 °C (140 °F) (± 5 °C) (±3.6 °F), as determined using the FWD IR temperature sensor.
   
8.4.17 Record the FWD IR temperature sensor and reference thermometer readings. Wait 5 min, and then record both sensors again. If the recorded temperatures vary by more than 2 °C (3.6 °F) for either data set, then wait 5 min and record both temperatures again. If the IR temperature sensor varies by more than 2 °C (3.6 °F) from the reference thermometer for two or more data sets, then the IR temperature sensor is unacceptable.
   
8.4.18 Repeat the procedure in 8.4.17 for the hand-held IR temperature sensor. Hold the hand-held IR temperature sensor at a height consistent with the FWD-mounted IR temperature sensor.

8.5 REPORTING REQUIREMENTS

8.5.1 Reference Calibration

For LTPP-operated FWD equipment, the operator should submit the reference calibration results  to the FHWA LTPP FWD task leader within 7 days after the calibration is completed. For non LTPP-operated FWD equipment collecting data on behalf of LTPP, the reference calibration results should be submitted to the FHWA LTPP FWD task leader within 30 days of the time it first collects data for LTPP with those calibration factors.

A copy of all reference calibration results should be kept in the RSC office. A paper copy of the most recent reference calibration results should be kept in the FWD tow vehicle.

8.5.2 Relative Calibration

A copy of all relative calibration results for LTPP-operated FWD equipment should be kept at the RSC office. A paper copy of the results of the most recent relative calibration should be kept in the FWD tow vehicle. It is not necessary to submit copies to the FHWA LTPP FWD task leader.

8.5.3 DMI Calibration

The operator should fill out form F07 each time the DMI is calibrated. The RSC office should keep a copy of all completed form F07s for LTPP-operated FWD equipment. A paper copy of the most recently completed form F07 should be kept in the FWD tow vehicle. It is not necessary to submit copies to the FHWA LTPP FWD task leader.

8.5.4 Temperature Sensor Verification

A copy of all completed forms F08 and F09 for LTPP-operated FWD equipment should be kept at the RSC office. A paper copy of the most recently completed forms F08 and F09 should be kept in the FWD tow vehicle. It is not necessary to submit copies to the FHWA LTPP FWD task leader.

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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