Resistance and Resistivity

Many materials – particularly those that are polymer based and used in the control of static electricity – have additives, coatings, or treatments on their surface. Measurement of the electrical characteristics of such materials usually requires placing an electrode of appropriate configuration on the surface and applying a measuring current at some given potential. Depending on the electrode configuration, the results may be interpreted as resistance or resistivity. For surface treated materials, the conventional way to describe the electrical property has been ohms/square. Homogeneous volume conductive materials may be measured for volume resistivity using test method ASTM D991. A volume resistance measurement is described in ANSI/ESD STM11.12. Care must be taken in selecting the appropriate resistance or resistivity test method for any given application or material/item. Choosing the correct test method also requires an understanding of the function of the material/item in an application. Many static control items are formed from laminations of dissimilar materials. Resistance or resistivity tests may not be completely appropriate for such items, since the actual function of the item may not be predicted. A simple surface resistance measurement may not be enough to understand a static control item’s performanceattributes. For example, installed static control items, such as worksurfaces and flooring materials, are properly evaluated for in-use performance by measuring resistance to ground from points on their surface. Measuring methods are described in ESDA standards and standard test methods.