The Perils of Nominally Rated Depth Filters

Problem Solving | Filter Ratings | Nominal vs Absolute Filters

While filter ratings are an important indication of how well a filter will remove particles of varying sizes from a fluid or gas, it is important to note that nominally rated filters from different manufacturers can have the same rating, but yield different results. The opposite is also true – nominally rated filters from different suppliers can have different ratings, yet yield the same result. Naturally, this can create confusion and impact your filter buying decisions.

When these situations arise, we recommend focusing on achieving the targeted performance of a filter over the rating. With that in mind, let’s start with differentiating between the two types of ratings that indicate pore size and efficiency in removing particulates -- Nominal and Absolute ratings.

Nominal pore size ratings refer to the size of the particles (in microns) that are prevented from passing through the filter and the minimum percentage of how many particulates are blocked, which is typically 60-99%. Since the size, shape and concentration of particles can vary, manufacturers are approximating the pore size or some performance characteristic such as retention, beta rating, flow rate, or average pore size to assign a rating. Without a universal standard it’s important to test and understand the performance your process will achieve with a particular filter.

Absolute pore size ratings refer to the size of contaminants a filter can block at almost full efficiency (>99.9%). An example of absolute rated filters are those typically used for sterilization applications. For example, 0.1, 0.2, and 0.4 absolute-rated filters are designed for complete retention of bacteria (A. laidlawii, B. diminuta, S. marcescens respectively). Substituting an absolute filter from one manufacturer to another is likely to yield acceptable performance, in that the relevant bacteria will be removed from the filter stream. However, the same cannot be said for nominal filters.

At Critical Process Filtration, we see nominal ratings of other manufacturers fall all over the map. In one particular case, a company was using a 0.2 µm nominally rated polypropylene depth filter for water filtration. For many a 0.2 µm rating carries a higher meaning. While this may be the case for absolute filters (bacterial retention) it is not the case for nominally rated filters. When we tested the customer’s 0.2 µm water filter, it had the same retention, flow rate, and other filtration characteristics as our 1.0 µm polypropylene depth filter.





Test Results



1.0 µm polypropylene depth



Water filtration

Same retention, flow rate, pressure drop and throughput

Higher performance filter @ lower cost

Another Vendor

0.2 µm polypropylene depth



Water Filtration


By looking at the results and specifying the filter’s performance, the customer now had the opportunity to benefit from our 1.0 µm polypropylene depth filter which offered superior performance at a reduced cost.

When writing filter specifications, it is critical to define the purpose and required performance characteristics of the filter. This is important even for a nominally rated filter, as performance can differ from vendor to vendor. And as always, testing before implementing leads you to a best-fit solution.

While you concentrate on reaching your operational goals every day, we focus on testing, analyzing and providing the types of filtration solutions that make our customers successful. 

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