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 any pool surround test. Pool environments add particular calibration challenges because surrounds typically slope toward drains and the tester must be levelled at every test point.
BS EN 16165 Annex C specifies the calibration sequence the 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 begins, 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 pool surround. 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 pool 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.
For pool work, where Slider 55 is the principal slider, conditioning is particularly important because the softer rubber compound responds to surface contact differently to Slider 96. UKAS-accredited operators are trained on Slider 55 conditioning specifically.
The pendulum is positioned so that the slider sweeps a defined horizontal distance across the surround — 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. Pool surrounds frequently have falls toward drains (often 1:80 to 1:100), so the instrument's tripod feet need to be re-levelled at every test point. Out-of-level testing produces results biased one way or the other depending on the direction of the unlevelness.
A common error in non-accredited pool testing is treating a single calibration at the start of the day as sufficient and not re-levelling at each test point. Pendulum results from such testing are systematically unreliable on any sloping surround.
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 pool pendulum reports include the calibration record from the testing day:
This record is what makes the report defensible if the methodology is ever challenged.
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