Current Slip Resistance
Revised September 29, 2006 by William English
There are a number of standards for measuring walkway surface traction, some of which have some validity, and some have been demonstrated to be invalid. Almost any slipmeter having a rubberoid friction pad will find that clean dry surfaces are not slippery. That is, they will not find any surface that is much below .50 under dry conditions. But since nobody is slipping and falling while walking on clean dry surfaces with rubber shoes on, and since nearly all slip/fall accidents arise from some contaminant on the surface, any tribometry method that cannot meter the floor in the contaminated condition is uninteresting to the safety practitioner. It is important to be able to meter the traction properties of the floor under actual use conditions.
What Slip Resistance Testing Standards Are in Wide Use Today?
For Wet Testing or Dry
ASTM F1677 is for the Brungraber Mk II (a/k/a Portable Inclinable Articulated Strut Tribometer or PIAST) and is used for both dry and wet testing. [This standard has been officially withdrawn by ASTM, but it is still available for sale.]
ASTM F1679 applies to the wonderful English XL Variable Incidence Tribometer (VIT), which was designed primarily for wet testing. The VIT has produced the highest precision of any tribometer for which there are published statistics today. It has recently been revised and reissued in 2004 to incorporate operational techniques derived from the precision and bias workshops. [This standard has been officially withdrawn by ASTM, but it is still available for sale.]
ANSI A1264.2 is for Provision of Slip Resistance in the Workplace. It is different from the above standards in that it originated with ANSI and it is primarily oriented to workplaces rather than public walkway areas. It is also unusual in that it specifies a threshold of safety of .50. It references several of the ASTM F13 standards (including F1679 for the VIT) as means of measuring performance and suggests some means of mitigating slippery conditions.
NFPA 1901 Standard for Automotive Apparatus, 13-7.3 recognizes only the VIT and the PIAST for testing wet surfaces.
For Dry Testing Only
ASTM F609 for the Horizontal Pull Slipmeter (HPS), which was an early electrically-operated dragsled meter, does not permit wet testing. The HPS is no longer commercially available, so this is not a particularly relevant standard today. It is important to note that F609 applies only to the HPS and is not applicable to the Model 80, ASM 725 or any other device.
ASTM F1678 has to do with the Portable Articulated Strut Tribometer (PAST) which is also known as the Brungraber, Mk I. Because of its sticktion problem, it is not useful for wet testing. [This standard has been officially withdrawn by ASTM, but it is still available for sale.]
ASTM C1028 for the Horizontal Pull Dynamometer is for factory quality assurance testing of ceramic tile. It is a 50-pound drag weight that is pulled by a hand-held force meter, and the COF number is calculated using the H/V formula. As with all dragsleds, this 50-pound monster is not capable of valid wet testing, nor has a satisfactory precision and bias study ever been completed.
ASTM D2047 is the basis of the testing of floor polishes for slip resistance under laboratory conditions. It involves the venerable James Machine, a leather friction pad, and specifies that all testing must be performed dry. It cannot be used on a floor either, since it is not portable. There are four iterations of the James Machine, and although its proponents in D21 allege that their results are interchangeable, it remains to be seen whether a precision and bias study can substantiate that.
UL410 is an Underwriters Laboratories standard for rating of various materials and surfaces as "slip resistant." Any material or coating can be listed by UL as slip resistant if it achieves an index on .50 or higher on a James Machine with a leather pad. This is the original slip resistance standard and little has been done to upgrade it over the years. Its limitations are similar to D2047.
Other Tribometers (used
mostly outside the US) Based on the Pendulum and
The Tortus has been resurrected in Europe and there are at least three reincarnations having digital output; but the dynamics are the same as for the original Tortus, and they are invalid for the same reasons (in Chapter 3 on "Validation of Slipmeters" in Pedestrian Slip Resistance.) There is no current ISO standard for its use, but in Australia and New Zealand it has been standardized for dry testing. Certain purveyors of these invalid junkscience testers in the US are touting them as "new" and the "latest from Europe." What they are not telling you is that the technology is so old that the Tortus patent has expired, and it has never been accepted in the EU nor does it have prospects for becoming the basis for a testing standard there now. The Tortus and its derivatives are cleverly disguised dragsleds that have difficulty distinguishing between performance of dry surfaces and wet.
The Sigler Pendulum Tester was designated as the "NBS Standard Dynamic Coefficient of Friction Tester" at one time, but there is no US standard for its use. Its dynamics do not mimic human ambulation and it is very difficult to use and to interpret. Virtually all former US users have converted to the VIT.
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A slight refinement of the Sigler was produced in England and it has been known there at the TRRL tester (currently as the TRL) and in the US as the BPST. ASTM E303-03 tells how to use it for measuring the skid-resistance of tires on pavements. Some specifications refer to BPN performance numbers, which is a reference to the BPST rating of a surface.
What does all of this mean for you?
Analysis of the above information reveals that you have a choice of two slipmeters for real-world traction testing on walking surfaces: (1) the Brungraber Mk II PIAST (according to F1677) and (2) the English XL VIT (according to F1679). Of course, no one who has used both testers prefers the PIAST, but this is still the Land of the Free to that extent.
You are also free to use any of the dragsleds (HPS, ASM 725, Model 80, C1028) and the home-cooked contraptions. You just aren't going to get the right answers on wet surfaces, nor will you win in court when opposed by an expert. And remember that an ASTM standard for one tribometer is not applicable to another, even when it is listed in its manufacturer's literature as a "reference standard."
Some of the most popular tribometers, most notably the low-cost Technical Products Model 80 and the American Slip Meter ASM 725 are not recognized by any US consensus or regulatory standard.
Other Related Standards
A number of codes and standards mention that surfaces must be "slip resistant," without defining the term. As can be seen from a perusal of http://www.englishxl.com/point5.html, slip resistant has been widely held to mean any surface having available traction of .50 or higher under rubber shoe bottoms. Some codes and standards incorporating the "slip resistant" terminology include ASTM F1637 Practice for Safe Walking and the Uniform Federal Accessibility Standards and Americans with Disability Act Accessibility Guidelines (ADAAG) as well as the regional building codes (BOCA, UBC and Southern, and now the Universal Building code).
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© William English 2006. All Rights Reserved Worldwide. You may print off any information on this Web Site for your personal use, but no part of these documents may be otherwise reproduced in part or in full in other publications without the express, written approval of William English. The English XL Variable Incidence Tribometer is patented.