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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

Surface Resistance

By William Klein, K & S Labs


Why electrode configuration is important to the electrical testing of materials

 

Concentric electrodes may not be used for determining resistances of materials which are not homogeneous and for those with directionality in their surface conductivity

 

ASTM D257, AATCC 76 and ESD S11.11 may be used to characterize materials if the technologist understands the material being tested and its intended application

 


     False assumptions are implicit in most simple meters and test methods purporting to measure surface resistivity directly.  They employ electrode configurations valid only for special cases.  Surface resistivity measurements assume a desire to know the electrical resistance from point to point on the surface.  This can be measured directly by the use of a pair of electrodes, either circular or rectangular, on the surface without regard to the path of flow.  This is the basis of NFPA-type resistance measurements for floors adopted by both ASTM and UL, and for both working surfaces and walking surfaces by the ESD Association.  It is necessary to specify electrode size, spacing, and voltage.

     The disadvantage of this method is that it does not measure an inherent property of the material, but only of the particular test specimen in the configuration employed.  The advantage is that it usually yields desired results without ascribing false properties which may lead to serious design errors.

     The use of only concentric electrodes does not allow valid characterization of materials if the material has any non-uniformity in its conductivity either in surface or volume directionality.  These electrodes may mask serious continuity problems.

 


ASTM D257:  Most cited test method for surface or volume resistivity, is more of a tutorial than a method, is specified for insulating materials only, has broad applicability and gives wide options on methodology both in electrode configuration and applied voltage, based on intended use.

 

AATCC 76:  Used by the textile industry as a simple, direct method to measure the surface resistive properties of fabrics.  The results are meaningful for thin, flexible, relatively homogeneous materials with conductive ability on or near the surface; and for such materials having directional differences in surface conductance, such as with textile materials.  The method is a special case of a surface resistivity measurement from D257 in which parallel electrodes or concentric rings are used, with sizes and spacing appropriate to the application, and no guard electrode employed.

 

ESD S11.11:  Used for electrostatic control applications of planar materials.  The method is a specific case of ASTM D257 with the electrode configuration, weight and applied voltage fixed in an attempt to reduce lab-to-lab variations.  The electrodes are an inner disk surrounded by a concentric ring in a fixed three-dimensional relationship.  The applied voltage is 100 volts for materials in the dissipative range.

 

ASTM D257 users usually do not read it fully before they use it or cite it. It is particularly important to note that "surface resistivity" is often not definable or measurable, even in insulative materials of the sort specifically dealt with in D257.   Surface resistivity does not exist as a basic material property in most of the materials of interest in ESD protection.  This is because surfaces do not usually have electrical properties which are distinct from bulk properties and the flow of current from point to point over a surface cannot be described as a surface phenomenon only.

 

 

Figure 1 Parallel Electrodes

 

of the square, thus the term "ohms/square".  Although dimensionally the same, "ohms/square" differentiates surface resistivity measurements from simple resistance measurements (R).

Figure 2 Concentric Ring Electrodes



    The concept of surface resistivity is unnecessary and often erroneous in materials used for ESD control.  If the material is thin, homogeneous and volume conductive as shown in Fig.1, then the apparent surface resistivity is equal to the true volume resistivity divided by the thickness.

 

Figure 3 Conductive Backing

 

A is the contact area of each electrode.  If L is much greater than t and the

circuit can be approximately shown as in Figure 3b.  It is actually volume resistivity which is being measured and surface resistivity, in fact a nonexistent material property, acquires its apparent value depending on volume resistivity and specimen geometry.  Note that R is independent of electrode spacing.

 

     In materials which are surface treated to be conductive and the great bulk of which is much more resistive, the concept of surface resistivity makes sense, although it is usually unnecessary.  This is indicated in Figure 4.


Figure 4 Conductive Surface Layer

 

     If a material with directional differences in conductivity, such as woven fabric or some fiber-loaded materials, is being measured, then the property of resistivity may not be defined or measured.  The concentric electrode option is not valid if there is any appreciable axial or directional non-uniformity.  As shown below, concentric electrodes mask serious continuity problems while appropriate use of parallel electrodes allows material characterization.

Figure 5 Directional Conductivity

 

Figure 6 Random Fibers

 


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