COF vs. Pendulum Test Value (PTV to DCOF conversion table)
Pendulum slip (or skid) resistance data are usually expressed as PTV, Pendulum Test Value (or sometimes BPN, British Pendulum Number, or even SRV, Slip Resistance Value). The question has been asked, “What’s the coefficient of friction?” This number (dynamic COF – DCOF) was calculated decades ago at the U.S. National Bureau of Standards, and can be obtained from the PTV. The table COF x 100 to PTV conversion (click link to open the PTV to DCOF conversion table) shows the relationship, but the COF as shown must be divided by 100.
In the lower range the COF is 0.01 x PTV (or BPN): in other words, for PTV (BPN) = 31, COF = 0.31. In the higher range, for historical reasons the two diverge. At PTV = 50, COF = 0.54, and at PTV = 82, COF = 1.00.
Yes, COF can and in some cases does exceed 1.00, and the pendulum can measure PTV as high as 150 (not shown on the graph). However, a PTV of 55 measured with a hard rubber slider is safe even for steep (1 in 14) outdoor ramps. For many other situations, a PTV of 36 (DCOF = 0.37) is adequate for good traction. Note that this DCOF is much lower than the 0.60 static coefficient of friction that was used for years as a misleading safety criterion. It’s even lower than the 0.42 minimum dynamic COF used as a safety criterion in ANSI A137.1 – but that DCOF is measured using the BOT-3000E digital tribometer.
It’s very important to realize that different measurement methods yield different COF’s for the same surface. Thus the same floor could have a COF of 0.4, 0.5, 0.6, or 0.7 depending solely on the test method used. That is why it was nonsensical that for years the U.S. OSHA unfortunately prescribed a minimum COF of 0.50 without specifying what test method was to be used. Without specifying a test method, the 0.50 was meaningless.
The 0.37 minimum DCOF for the pendulum (PTV = 36) is usually (that is, for most flooring types) more stringent than the 0.60 minimum static COF that was used for too many years.