Validation of the XL VIT
THE OLD ARGUMENT
Adversaries of walkway safety and meaningful tribometry have for nearly 20 years exerted great effort and influence to discredit and disprove the XL VIT and similar instruments that are capable of accurately, and reliably measuring slip
resistance related to the risk for human slip and fall injury. The most recent product of those efforts has been the misleading misinterpretation of the status of the F1679 Standard Test Method for Using a Variable Incidence Tribometer (XL VIT).
According to the 2007 Annual Book of ASTM Standards Volume 15.07, the F1679 Standard Test Method was “withdrawn as an active standard” by ASTM on September 30, 2006, for a violation of form and style “for including reference to proprietary apparatus where alternatives exist,” and “for failure to include an approved precision statement.”
At the same time F1679 was withdrawn, other similar standards that referenced proprietary products were also withdrawn for the same procedural reasons, for a violation of form and style. Other agencies that had referenced the ASTM documents followed the same procedural cleansing at the same time.
To imply that the XL VIT was “disapproved,” “disavowed,” or “disallowed” by ASTM in any way is misleading and a
misinterpretation of what the F1679 Standard Test Method for Using a Variable Incidence Tribometer (VIT) is; F1679 is a test method. The ASTM definition for test method is “a definitive procedure that produces a test result” (emphasis added). F1679 is a test method, not an instrument certification or validation.
To imply that the XL VIT was “disapproved,” “disavowed,” or “disallowed” by ASTM in any way is misleading and a
misinterpretation of the action of ASTM by their withdrawal of the test method. The test method was withdrawn for procedural reasons, for a violation of form and style, in the content of the test method document, not for lack of
performance of the instrument.
To say that the F1679 Standard Test Method for Using a Variable Incidence Tribometer was withdrawn by the ASTM because the XL VIT was or is inaccurate or unreliable is also misleading and a misinterpretation. At the time of
withdrawal, ASTM had in its possession the results of a number of round robins, workshops, and laboratory studies, as well as a precision statement based on an inter-laboratory study of six (6) different independent testing laboratories, all organized by the efforts of James Flynn of J2 Engineering in Fresno, CA, as the moderator (now chairman of ASTM F13). David Underwood, PhD, produced a statistical evaluation of the data using the ASTM E691 procedures which demonstrated the English XL VIT achieved satisfactory precision according to the ASTM standard. The results of the ASTM E691 precision statement were published to the ASTM F13 committee and elsewhere. F1679 was withdrawn
for a violation of form and style “for including reference to proprietary apparatus where alternatives exist,” before the precision statement was incorporated.
The F1679 Standard Test Method for Using a Variable Incidence Tribometer is just that, a list of instructions for using the XL VIT. The F1679 Standard Test Method for Using a Variable Incidence Tribometer still exists and is available for
purchase from ASTM. However, the current manufacturer’s User Guide for the XL VIT, as the up to date test method,
replaces the function of F1679. There is no void and never was one as to the proper use of the XL VIT.
Examples of peer-reviewed articles that contemporaneously support the validity of the XL VIT as a meaningful tribometer include:
“Repeatability and bias of two walkway safety tribometers,” Powers, Kulig, Flynn, and Brault, Journal of Testing and Evaluation, JTEVA, Vol. 27, No. 6, November 1999, pp. 368–374, which stated among other things:
- The stated purpose of this study was to determine the repeatability and bias of the PIAST (Brungraber Mark II) and XL VIT tribometers under both dry and wet conditions.
- Both devices were tested on an AMTI force platform over a wide range of angles.
- Bias was assessed by comparing the tribometer slip resistance reading to the actual Fx/Fz ratio measured by the force plate, while reliability was established by evaluating the ability of the tribometers to reproduce Fx, Fz, and the Fx/Fz ratio.
- Both tribometers demonstrated high degrees of bias and repeatability under both wet and dry conditions.
“Prediction of Slips: an evaluation of utilized coefficient of friction and available slip resistance,” by Burnfield et al, Journal of Forensic Sciences, Volume 47, Issue 6, November 2002, and Ergonomics Vol. 49, No. 10, 15 August 2006, 982–995, which stated among other things:
- The purpose of this study was to investigate the relationship between slip resistance as measured using a XL VIT, and peak utilized coefficient of friction (COFu) determined from force plate recordings, on the probability of a slip occurring during level walking.
- During wet surface conditions, the slip index reading of the XL VIT has been shown to demonstrate a good level of agreement with COF values recorded from the force plate, and agreement between repeated measures. Additionally, an excellent level of agreement has been documented between successive force plate recordings of COF and the XL VIT’s slip index values.
- The results from this study indicate that the available slip resistance, as measured by the XL VIT, can accurately predict slip events. Knowledge of the available slip resistance, in combination with an individual’s COFu allowed for
greater accuracy in classifying slip outcome.
- Collectively, the findings of Hanson et al. (1999) and the current study suggest that both testers of available slip resistance (Brungraber Mark II and the XL VIT) provide information that is useful for identifying human and environmental combinations that may create a greater risk for slip onset. Care must be exercised when generalizing these data to other types of tribometers, however, as different styles of tribometers yield different measurements that do not necessarily correlate (Powers et al. 1999-above; Grieser, et al. 2002, Slip resistance: Field measurements using two modern slipmeters, Professional Safety, June, 43–48; Gronqvist, et al. 2003, The validity and reliability of a portable slip meter for determining floor slipperiness during simulated heel strike. Accident Analysis and Prevention, 35, 211–225).
- This study demonstrated that knowledge of a person’s COFu and the available slip resistance, as measured by the XL VIT, can be used to predict the probability of a slip event during level walking in young adults. As measures of available slip resistance vary across the type of tribometer used, the relationships reported in the current study will likely apply to the XL VIT only.
ASTM F2508 VALIDATION
Currently, all of the foregoing is moot. The ASTM F2508 Standard Practice for Validation and Calibration of Walkway Tribometers Using Reference Surfaces was approved March 15, 2011, and then immediately published. The F2508 Standard Practice clearly establishes a methodology to assure the precision and repeatability of walkway tribometers.
The XL VIT base-model and the XL VIT with sequencer-model both conform to all requirements of ASTM F2508-11 Standard Practice for Validation and Calibration of Walkway Tribometers Using Reference Surfaces, which conformance validates the XL VIT base-model and the XL VIT with sequencer-model both properly and adequately correlate the slip characteristics of a surface or contaminant, or both, to the actual propensity for human slips against a human gait-based reference system and standard practice with relevance to human ambulation.
EXCEL TRIBOMETERS, LLC, as the patent owner, manufacturer and sole authorized supplier of the English XL Variable Incidence/Articulated Strut Tribometer (both the XL VIT base-model and the XL VIT with sequencer-model) performed and completed all requisite testing that validates both models of the XL VIT as in full compliance with the requirements of F2508. The details of the validation are contained in the Report of ASTM F2508 Validation of the
XL VIT available on the Excel Tribometers LLC website.
As with any scientific measuring instrument, the user must follow the manufacturer’s instructions to get reliable, consistent, accurate results. All tests and studies, past and present, have shown that if the XL VIT is current in its instrument calibration, if the test foot is properly calibrated and prepared for each test, and if the current test method (the User Guide) is followed, the XL VIT instrument is and always has been proper, accurate, and reliable for measuring slip resistance related to the risk for human slip and fall injury.
PRECISION BEFORE F2508
The English XL VIT has demonstrated its reliability, accuracy and precision in numerous well documented studies throughout its history, some of which are described below. Contemporary users of the English XL VIT can be assured of even more reliability and accuracy than ever with recent expanded procedures for:
- test foot calibration before metering using the certified or standard TCNA XL calibration tile,
- clearly defined test foot preparation techniques,
- all as detailed in the current XL VIT User Guide.
Round robins were conducted with the English XL VIT, beginning with the F13 slipmeter workshop conducted at ASTM Headquarters at the June 1997 meeting of ASTM F13. This first run at determining precision was an attempt to work out operating procedures and statistical protocol in compliance with the ASTM E691 standard on precision (guidelines for statistical validation of ASTM test procedures.)
In this initial all-day testing workshop, both the Brungraber Mark II, (PIAST) and the English XL VIT were operated by a number of volunteer participants. There were three kinds of test surfaces. The English XL VIT was tested using the traditional 400 grit sandpaper for conditioning of the test feet and again using 180 grit. The 180 grit sandpaper was determined to produce a test foot surface texture that was more in line with actual heel wear. As far as the English XL VIT is concerned, the conclusions of this experiment were that (1) the 180 grit paper would become standard for the English XL VIT test foot preparation, and (2) that more uniform test surfaces were needed. There was wide variability on the textured vinyl surface, and there were two significant outliers on all of the three different test surfaces.
A more limited workshop for the English XL VIT was conducted at the January 1998 ASTM F13 meeting in San Diego where the same prior test surfaces were metered. Every participant observed and critiqued everybody else’s operating technique. Progress was made and the previous outliers were eliminated as far as test results are concerned. Jim Flynn of J2 Engineering in Fresno, CA is the moderator of all of the experiments.
In the same era as the workshops at the ASTM F13 meetings, two round robins were conducted in which the same three test surfaces were circulated to seven different independent testing labs, where everybody used the same test foot and test procedure. The purpose was to establish that the different slip meters produced repeatable results in different laboratories.
A third similar round robin was performed where the same three test surfaces were circulated to six (6) different independent testing laboratories, each using their own English XL VIT. David Underwood, PhD, produced a statistical evaluation using the ASTM E691 procedures which demonstrated the English XL VIT achieved satisfactory precision. The results were published to the ASTM F13 committee and elsewhere.
The listing below defines the historic precision statement for the English XL VIT as well as for other slipmeters for which precision figures are published in their respective ASTM standards.
English XL VIT
Dry reproducibility: std. deviation 0.02, wet reproducibility: std. deviation 0.03.
C1028 Horizontal Pull Dynamometer
Dry reproducibility: std. deviation 0.07, wet reproducibility: std. deviation 0.05 (testing performed on 1 surface using 6 testers)
D2047 James Machine
Dry reproducibility: std. deviation 0.03 (The procedure for establishing this figure is not clear—consult original ASTM research report for more detail), should not be used for wet testing because of sticktion. (testing was performed with a leather foot using 3 testers)
F609 Horizontal Pull Slipmeter
Dry reproducibility: std. deviation 0.07 (The procedure for establishing this figure is not clear—consult original ASTM research report for more detail), should not used for wet testing because of sticktion, (testing performed on 2 surfaces, 17 kinds of feet, using 4 testers)
On the evening of the ASTM F13 meeting of June 8, 1998, another workshop was conducted using the English XL VIT to determine the repeatability of Neolite® as a friction pad material. Results of this study were published at the January 1999 ASTM F13 meeting in Memphis, Tn. The results were very good. Using ten different test feet of different ages and from different production batches, the workshop conducted by Jim Flynn obtained repeatability virtually as good as was obtained in the round robins where the same test foot was used on all machines. The workshop not only confirmed the precision of the English XL VIT, Neolite® was confirmed as consistent for test foot friction pad material.
An additional supporting precision study was conducted in conjunction with the January 2000 ASTM F13 meeting in New Orleans, La. Eleven operators using eleven different English XL VIT slipmeters, each with a different testfoot, produced approximately the same precision as the single foot on six machines. In addition to the slipmeter output numbers, Shore A and Shore D hardness values of the testfeet were measured. No clear link between Neolite® hardness and slipmeter output was observed, and the repeatability was as good as in the prior studies. The statistical analyses of the January 2000 New Orleans precision studies are reproduced in the Second Edition of Pedestrian Slip Resistance.
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