Charge Generation (Liberation)
Measurement of charge generation has been a subject of controversy for many years. It has been recognized, in at least some standards development groups, that there is no reliable or practical measurement method for determining an intrinsic charge generation property for a material. Charge generation is controlled by the properties of the two (or more) contacting and separating materials. Therefore, any test to determine the charging characteristics of one material would require contacting that material with samples of all other materials it may contact in the predicted environment. This of course is impractical. Some of the methods of measuring charge generation of materials that have been used in the industry are described in ESD ADV11.2. Measuring the charge accumulation on personnel while walking on a floor is described in ANSI/ESD STM97.2.
Decay Rate
Decay rate measurements are typically used to evaluate the performance of ionizer equipment. An instrument called a charged plate monitor (CPM) was designed for this purpose. The measurement techniques and instrumentation are described in ANSI/ESD STM3.1. Test methods also exist for measuring the decay of a fixed charge on materials but these have been shown to be valid only for homogeneous materials. In the case of laminated (multi-layered) materials, the most conductive layer in the structure controls the discharge rate in decay testing. Fundamentally, the measurement of decay rate is simply a matter of charging an item or material by direct current, triboelectric interaction, or by exposure to a corona charging system, then grounding the item and recording the time for the charge to reach some preset level (typically 10% of initial charging level). This type of evaluation is done frequently by practitioners in the static control field. However, the methods have been shown to be difficult to standardize due to poor repeatability and lack of lab-to-lab correlation and reproducibility.