A pendulum tester that is incorrectly calibrated produces incorrect PTV readings. The instrument's calibration regime — friction-disc verification, slider conditioning, sweep distance check, level setting — is therefore central to the validity of the test. Under UKAS ISO/IEC 17025, calibration is not optional; it is documented for every test.
BS EN 16165 Annex C specifies the calibration sequence the pendulum operator must complete before testing begins. The principal elements are:
The friction disc is a reference plate of known surface texture supplied with the instrument. Before the day's testing, the operator runs a sequence of swings on the friction disc and verifies the PTV reading falls within the specified range. If the reading is out of tolerance, the instrument is checked for level, slider condition and sweep distance until the disc reading falls within range.
The friction disc itself is periodically calibrated by the manufacturer or a calibration laboratory; this calibration is the chain of traceability back to national measurement standards.
The rubber slider's edge is the contact surface that strikes the floor. Its roughness affects the test result, so the edge is abraded to a standard roughness before each set of recorded swings. The conditioning is carried out by drawing the slider across a sheet of prescribed lapping paper for a specified number of strokes.
Insufficient or incorrect conditioning is one of the most common errors in non-accredited pendulum testing. The slider edge becomes glassy through repeated swings, and unless re-conditioned, subsequent swings produce artificially low PTV readings. UKAS-accredited operation includes documented conditioning between every test set.
The pendulum is positioned so that the slider sweeps a defined horizontal distance across the floor — typically 125 to 127 mm. If the instrument is set too high above the surface, the slider does not contact across this distance and the PTV reading is artificially low. If set too low, the slider drags excessively and the reading is artificially high.
Sweep distance is verified before testing each location by lowering the slider through the swing and measuring the contact arc. Modern pendulums include a slider-foot length gauge to make this verification fast.
The pendulum body has integrated spirit levels in two axes. Both must be central before testing. On uneven floors (which is most floors), this means levelling the instrument's tripod feet at every test point — a routine but non-trivial part of the procedure. Out-of-level testing produces results biased one way or the other depending on the direction of the unlevelness.
The pendulum arm releases from a fixed vertical position determined by the instrument's release mechanism. The release height is factory-set and verified during periodic calibration but should be checked before each day's testing using the integrated marker. Out-of-spec release height affects the swing's energy and biases all PTV readings on that day.
UKAS-accredited pendulum reports include the calibration record from the testing day:
This record is what makes the report defensible if the methodology is ever challenged.
The pendulum tester is sent to the manufacturer or a calibration laboratory annually for full mechanical calibration — bearing wear, mass-balance, slider-foot dimensions, friction-disc reference. UKAS-accredited laboratories maintain records of every instrument's calibration history and would not deploy an instrument outside its calibration validity period.
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