COF can be measured in two different ways and can cause confusion amongst those unfamiliar with the science of slip resistance. When the COF is measured from a resting position, it is called the "Static COF." When it is measured when the surfaces are in relative motion, it is called the "Dynamic COF." The dynamic COF is very difficult to measure and almost all portable and laboratory meters measure only the static COF. It is important to know this difference because you may see both measurements referred to in different literature. Most measuring devices (slip meters) will refer to the static COF. The measurements you will find in most literature and those discussed here will be the static COF. A COF of 0.5 is considered to be a slip-resistance surface. The higher the COF, the less slippery the surface. It is possible to have too high a COF. In other words, the surface can be too slip-resistant and an individual would find it difficult to walk on.
How To Measure Slip Resistance
There are basically two types of machines that can measure static COF: permanent laboratory models and portable field models, commonly refered to as pull meters. The most popular and widely accepted laboratory slip meter is the James Machine. The James Machine uses an 80-lb. weight that is applied through an arm to a leather shoe placed on a panel. The panel and the leather shoe are moved horizontally. The distance the panel moves before slipping is measured and recorded on a chart and is the coefficient of friction.
The James Machine was invented in 1940 and was the machine that established the 0.5 COF as the minimum for slip resistance. This standard was accepted in 1953 by the Federal Trade Commission. Many still consider the James Machine as the only true slip tester.
There are also many portable slip meters on the market that claim to measure the static COF. The American Society for Testing and Materials (ASTM) recognizes several portable meters. If purchasing these meters, make sure they comply with ASTM C-1028 which is the recognized slip test for tile flooring.
For further information on slip meters contact the ASTM at 1916 Race St., Philadelphia, PA 19103-1187.
About the author:
Frederick M. Hueston runs the National Training Center for Stone & Masonry Trades, Asheville, NC 28806; www.ntc-stone.com; and can be reached at Fhueston@aol.com.