These guidelines are provided to ensure that a trip to a Strategic Highway Research Program (SHRP) Falling Weight Deflectometer (FWD) calibration center is as productive and problem-free as possible. Your cooperation in following the guidelines is greatly appreciated by calibration center staff. If you have any questions about FWD calibration, contact a Regional Calibration Center Operator or the Federal Highway Administration (FHWA) Long-Term Pavement Performance (LTPP) Division Office (see Calibration Center Contacts). A copy of the complete calibration protocol can be obtained from the LTPP Division Office.

The SHRP FWD Calibration Centers were developed to calibrate SHRP's Dynatest FWDs. The procedures and equipment have also been used to calibrate KUAB FWDs, and additional adaptations to provide calibration of Phonix and JILS FWDs are under development. Should you wish to calibrate a Phonix or JILS FWD, please contact Mr. Eric Weaver at the FHWA LTPP Division Office.

WHAT TO EXPECT. . .

From the Calibration Process

The calibration process yields a set of multipliers (one for each sensor calibrated) that are used in the FWD software to adjust raw sensor readings (see figure 1). If you think of deflection measurements as shots at a target, the data from the uncalibrated sensors of an FWD could be represented by figure la. They may or may not fall within the specified accuracy of 2 percent, represented by the outermost ring on the target, and they may or may not be systematically biased (all to one side of the bull's-eye). After reference calibration (see figures 2 and 3), the readings will be closer to the bull's-eye and unbiased (randomly scattered around the bull's-eye), as shown in figure lb. Finally, after relative calibration, the readings will be randomly scattered and even closer to the true deflection (represented by the center of the bull's-eye), as shown in figure lc. Figure 2 illustrates the reference calibration of the load measurement system. Figure 3 illustrates the reference calibration of an FWD deflection sensor.

From the Calibration Center Staff

SPECIFIC FWD EQUIPMENT REQUIREMENTS FOR CALIBRATION

Before going to the calibration center, you should be prepared with the following items:

FWD software providing the following drop sequence, without raising the FWD load plate off the test pad at any time.

3 seating drops at height 3 followed by a pause,
5² drops at height 1 (about 6,000 lbs or lowest practical load), with a pause after each drop,
5² drops at height 2 (about 9,000 lbs), with a pause after each drop,
5² drops at height 3 (about 12,000 lbs), with a pause after each drop, and
5² drops at height 4 (about 16,000 lbs), with a pause after each drop


Dynatest FWDs require Version 10.0 or higher of the Dynatest field program. KUAB FWDs require operational program SFWD Version 4.0 or higher. For other FWDs, please consult your users guide, or check with the manufacturer. Figures 4 and 5 present sample reference and relative calibration test setups for LTPP Dynatest equipment.

An adequate supply of spare parts, including spare geophones or seismometers. The calibration centers do not stock spare parts.

A relative calibration stand, such as the one shown in figure 6, accommodates the maximum number of sensors used on your FWD. If you do not have one, consult the FWD manufacturer.

A procedure for entering and changing calibration results in the FWD software (for example, the Field Program). For Dynatest FWDs, the results are entered as relative gains. For KUAB FWDs, they are entered as gain factors. For other FWDs, consult the users manual or check with the vendor.

² It is desirable, but not essential, to program six drops at each drop height rather than five. This provides a margin for error in coordination between the FWD operator and calibration system operator.

GENERAL FWD EQUIPMENT REQUIREMENTS

Any damaged connections, fittings, or cables must be repaired or replaced. Damaged cables and bad connections may affect deflection data.

The load cell swivel must be properly lubricated, if applicable, and the bolts must be torqued to specification. Refer to your FWD owners manual for instructions. Also, you must assure that the 12-in (300-mm) diameter load plate is installed.

The rear extension bar must be removed on the Dynatest FWD, if it is currently installed.

The sensor bases must be cleaned. Fine grain emery cloth or Scotch-Brite^TM pads are useful for cleaning magnetic sensor bases of Dynatest geophones.

The printer must be in working order with adequate supply of printer paper, ink cartridges, etc.

Provide a formatted 3-1/2-in diskette for the calibration data. If your FWD has been calibrated, bring the diskette that has the latest set of calibration results.

FWD Manuals and typical spare parts should be in the tow vehicle as you may need them in case of unforeseen problems.

All batteries should be tested with a hydrometer or load tester. Battery terminals should be checked for corrosion and cleaned if necessary.

A relative calibration stand that accommodates the maximum number of sensors used on your FWD.

The hydraulic system must be properly maintained and have no leaks or other problems.

Proper test setups must be programmed into the FWD software (reference and relative calibration).

Target loads of 6,000³, 9,000, 12,000, and 16,000 lbs (± 10%) must be attained at drop heights 1, 2, 3, and 4.

KUAB FWD sensors must be statically calibrated prior to the SHRP reference calibration. Dynamic calibration factors must be entered as 1.05 for all sensors and not changed.

Note: Complete the Pre-trip Checklist and present to the calibration center operator prior to calibration.

CALIBRATION CENTER PROCEDURES

Allow the FWD buffers to stabilize to the temperature of the calibration center prior to the calibration. If possible, the FWD should be left inside the calibration center overnight.

Set up the FWD mass and adjust drop heights to produce loads within ± 10 percent of 6,000³ 9,000, 12,000 and 16,000 lbs (27, 40, 51 and 71 kN). For KUAB and JILS FWDs, use Load Mode 3, and adjust the drop height end switches to produce required loads.

³ If a load of 6000 lbs (± 10%) at drop height 1 cannot be achieved with the same FWD weight packages used for the remaining target loads, use lowest practical drop height.