Pore Size – Nominal Rating

Nominal pore size describes the average opening width of a filter medium used to retain particles above a certain diameter with a defined probability. Unlike absolute pore size, which defines the maximum opening through which no larger particle can pass, nominal pore size is a statistical value: it indicates that a percentage of particles – typically between 60 and 90 percent – above the stated size will be reliably retained. This value depends on the fabric structure, the shape of the openings, and operating conditions.
In practice, nominal pore sizes are often specified for depth filters and woven media whose pore distribution naturally varies. The filter fabric retains the majority of particles, while a small portion of similarly sized particles may pass through. For plant engineers and operators, the nominal pore size provides guidance for selecting the appropriate medium: depending on the required filtration quality and particle distribution, a fabric with a suitable nominal rating can be chosen to clarify the liquid efficiently while maintaining economic throughput.
GKD manufactures fabrics with precisely controlled mesh sizes made from metal and plastic monofilaments. Through advanced weaving technologies, pore spaces with defined nominal widths are created, delivering reproducible separation performance. Nominal pore size is expressed in micrometers and determined using test methods such as the bubble point test. As loading increases, a filter cake forms that reduces the effective pore size and thereby increases the retention rate.
For demanding applications requiring the highest purity, fabrics with absolute pore size are also available, ensuring near-complete retention of particles above the threshold value.