The pendulum and the tribometer are both portable instruments measuring floor friction. Both are recognised under BS EN 16165 — pendulum as Annex C, tribometer as Annex D. But UK practice strongly favours the pendulum for in-situ slip-risk assessment, for reasons grounded in four decades of forensic experience.
Pendulum: a weighted arm releases from a fixed height, swinging through an arc. The slider strikes the floor at a known velocity (heel-strike simulation) and friction reduces the residual swing. PTV is read off the calibrated scale.
Tribometer: a self-contained portable instrument drags a rubber slider across the surface at a controlled speed. A load cell records the horizontal force needed to maintain motion. The output is the dynamic coefficient of friction (typically 0.05 to 0.80).
The UK National Foreword to BS EN 16165:2021 explicitly states that the pendulum (Annex C) is considered the only one of the four methods that should be relied on to correctly assess pedestrian slip risk in wet conditions. This position is unusual in international standardisation work — it is a published preference embedded in the standard itself.
The reasoning is grounded in:
A pedestrian slips at heel-strike — the moment when the heel makes initial contact with the floor at a sharp angle and significant vertical velocity. The pendulum's arc swing reproduces this geometry: the slider strikes the surface at heel-strike angle, with comparable instantaneous velocity. The dynamic friction at that moment is what determines whether the foot will slip.
The tribometer's continuous drag at low speed produces a different kind of friction measurement — closer to mid-stance friction than to heel-strike friction. For some surfaces the two correlate well; for others they diverge.
Tribometer testing is most useful as a supplement to pendulum data in specific contexts:
There is no general formula for converting tribometer COF to pendulum PTV. The relationship is surface-dependent and contamination-dependent. For specific surface types and contamination conditions, empirical correlations exist, but they are not transferable outside their tested range.
If tribometer data needs to be expressed against UK PTV thresholds, the safest approach is to take both measurements on the same surface and report each on its own scale. Any narrative correlation should be qualified.
The North American standard ANSI A326.3 specifies tribometer testing using the BOT-3000 instrument, producing the Dynamic Coefficient of Friction (DCOF). North American thresholds are typically DCOF 0.42 wet for satisfactory performance. UK readers encountering ANSI DCOF data should not assume direct comparability with UK PTV. A floor with DCOF 0.42 may achieve PTV anywhere from 25 to 50, depending on the surface.
UK courts and insurers have a strong working familiarity with pendulum data. Loss adjusters, defendant solicitors, claimant solicitors, and judges are all routinely engaged with PTV results and the HSE bands. Tribometer data, by contrast, is typically the subject of additional explanation and methodology discussion that pendulum data does not require. For UK litigation and insurance work, the pendulum is the path of least resistance evidentially as well as technically.
For complex forensic work, both methods can be used together. The pendulum produces the headline UK-aligned PTV; the tribometer provides supplementary continuous-friction data where it adds investigative depth. Reports that combine both methods are not common but are highly defensible where the additional data is genuinely informative rather than decorative.
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