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.
Fowler Associates uses whichever test method is appropriate to truly determine the material's functionality in its intended application.
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 homogenous 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 3 dimemsional 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
When W = L (a square configuration), then ss = R. regardless of the size of the square Thus the term "ohms/square". Although dimentionaly the same, "ohms/square" differentiates surface resistivity measurements from simple resistance measurements ( R).