How often does the Pile Integrity Tester (PIT) accelerometer and instrumented hammer need recalibrated?

Pile Dynamics (PDI) recommends the recalibration of the PIT accelerometer and the PIT instrumented hammers every two-years. Nevertheless, the calibration of the PIT accelerometer and hammer are only necessary when obtaining simultaneous force and velocity measurements. When an engineer performs tests with only the PIT accelerometer (velocity only) the data is normalized. As a result, the actual velocity amplitude becomes irrelevant. PDI’s policy is to recalibrate any accelerometer or instrumented hammer returned to its Maintenance Department for checkup or repair.