Air-gap and liquid displacement techniques: Dielectric measurements without attached electrodes

Anyone engaged seriously in the business of performing measurements of dielectric constant and dissipation factor on the large variety of solid electrical insulating materials in use today sooner or later encounters materials which present real problems with respect to the proper choice of electrodes to use. Sommerman(1) has stated that "it is not an exaggeration to say that electrode problems are encountered in at least half of the dielectric investigations undertaken." In many cases troublesome electrode problems can be eliminated by abandoning the use of electrodes of any kind attached directly to the test specimens and resorting instead to a series fluid (air or liquid) gap technique employing a prefabricated electrode system, or cell, consisting of two or more parallel metal plates completely immersed in the fluid medium. Electrical measurements are performed on the cell with and without the flat sheet specimens inserted loosely between the plates. This seemingly unorthodox approach has been in successful and extensive use now for about ten years, and one development from it, viz., the liquid displacement procedure, has provided us with the most accurate, reproducible and rapid method available for the determination of the dielectric constants of materials such as polyethylene, polypropylene, polytetrafluoroethylene, etc. It is not intended to imply that the air-gap and liquid displacement techniques are panaceas for all electrode ills. However, their use may extend dielectric measurements to materials that cannot be tested conveniently, if at all, by other methods. These techniques will eliminate the tedious and time-consuming operation of applying electrodes and minimize or eliminate edge-capacitance, stray-capacitance, and lead-capacitance corrections from dielectric constant calculations. In the air-gap method the physical dimensions of the test specimens and electrode system must be accurately known. In the liquid displacement method, particularly when the standard immersion liquid and the material under test have approximately the same dielectric constants, the exact physical dimensions are much less critical ; and when two different liquids having only slightly different dielectric constants are used consecutively in the same test cell the dielectric constant of a given test specimen may be determined without any knowledge of specimen and cell dimensions. However, the dielectric constants of standard liquids used, as well as their temperature dependence, must be precisely known.