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C. The Measurement of Flooring Resistance Today this is far from the case with the use of different methodologies as well as considerable variations even when nominally using the same method. As will be shown later, considerable variations in resistance properties may not be of great functional significance, but specifications must be made and standards must be met. Too often a material hovers between acceptance and rejection based on who makes the test and when. Four methods are in common use today, NFPA 99, ASTM F-150, ESD 7.1 and IEC 1340-4-1. 6,10,11 All are quite similar and are based on NFPA 99, using identical 2 1/2 inch, 5 pound electrodes (except for the IEC method which uses a 2 inch, 5 kg electrode). Aside from the differences in specified voltages and environmental conditions (100 or 500 volts, 12 or 50% RH), which could easily be reconciled by discussion, their fundamental flaw is the electrodes. They simply do not simulate the feet. The light weight and relatively hard surface of the NFPA electrodes render the resistance readings significantly sensitive to surface unevenness and even minor surface contamination. Much higher variability accompanies the use of a 100 volt test potential as in ESD 7.1, although this does seem like a reasonable level considering the intended purpose. Our test results from the lab and in the field indicate that more relevant and more reproducible results can be obtained by the use of heel electrodes rather than the 5 pound weights. The type of heel electrode used, which is now commercially available, is shown in Figure 4. |
 Figure 4: Heel Electrode |
 Figure 5: Heel Electrode vs. S7.1 Electrode |
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 Figure 6: Heel Electrode vs. S7.1 Electrode |
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 Figure 7: Heel Electrodes vs. S7.1 Electrodes |
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The area of the conductive contact is the same as that of the conventional electrodes and is made from heel grounder material. The pressure of the contact from body weight and the relative softness of the electrode both tend to produce a good electrical contact, just as a heel grounder or ESD shoe sole would do.Another advantage of this method is the great speed with which readings can be taken, thus encouraging large samplings which can be legitimately quantified statistically. Another important advantage is the virtual elimination of bias in floor resistance testing. Most auditors will, when they find a bad spot, wiggle the electrode a little or move it to an adjacent spot to try to get a good reading. This is probably legitimate from a functional point of view, but does leave too much room for personal judgment. It should be noted that, as described in previous work (2), significant surface homogeneity of resistance demands a modified electrode material for proper evaluation. Figure 5 shows the surface to ground readings on a vinyl tile panel where 21 marked points were tested by the methodologies of ESD 7.1 and with heel electrodes, both at 100 volts. The difference is obvious. It is not clear how much this difference would matter functionally, but it would certainly be important if you had a 1 or 2 Megohm specification to meet. Figures 6 and 7 show comparative results on a much less conductive vinyl using both 100 and 500 volts with the ESD/NFPA electrodes and both surface-to-surface and surface-to-ground configurations. It can be stated that in general that the heel electrode methodology using 100 Volts yields resistance readings which are lower and less variable than S7.1 type tests at 100 Volts. On average, the values obtained at 100 Volts using the heel electrodes tend towards those of the 500 Volt, 5 lb. electrode tests, but with less variability. |
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