In filter media, one of the terms that you will often see is “nominal”. Nominal size rating means that the filter in use is going to be able to retain the vast majority – but not all – of the particulates. It will be able to retain between 60% and 98% of them, which is actually quite good. One of the things that one will have to consider when talking about nominal size efficiency is going to be the concentration of the particulates. Quite a few different things are going to determine just how efficient these are.
Another of the terms that you will find is “absolute”. The absolute rating is the pore size in the filtration media that will be able to retain 100% of the particulates. In the majority of applications, you will find nominal size ratings filter and media in use. The absolute micro filters are usually useful in final filtration circumstances. In most cases, these are going to be more expensive.