Nominally Rated Depth Filters - not all are created equal

Problem Solving | Filter Ratings | Nominal vs Absolute Filters

Pore size ratings are an important indicator of how well a filter will remove particles of varying sizes from a liquid or gas. It is just as important to take into consideration whether the filter rating is Absolute or Nominal, and the removal efficiency stated for the rated pore size, as not all nominal rated filters are created equal.


Absolute-rated filters indicate the particle size that will be fully retained (>99.999%), and this definition is consistent for all manufacturers. If you are using a sterilizing grade absolute rated 0.22 µm filter from one manufacturer, likely, substituting an absolute filter of the same rating from a different manufacturer will also yield acceptable performance.

For nominal-rated filters, this might not be the case. Removal efficiencies for nominal-rated filters can vary from 60% to 99% depending on the manufacturer. This means that filters from different manufacturers with the same pore size rating can exhibit drastically different performances. If you are using a nominal rating of 0.2 µm with 99% efficiency, replacing it with a nominal rating of 0.2 µm with 60% efficiency will allow a much greater proportion of 0.2 µm particles to pass through the filter. 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 pore size rating. Since there is no universal standard for nominal ratings, we must first compare all performance characteristics of filters under consideration - pore size, efficiency, flow rate, throughput, and retention. Then, it is important to test the filters to verify they will work as required for your process.

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. They initially purchased an installed CPF GPD 0.2 µm polypropylene depth filter as a direct replacement, without doing any testing.

The CPF filter did not perform the same. They experienced lower flow rates and faster filter clogging. Examining the data sheet information from the customer’s filter showed the efficiency rating was only 70%, compared to >90% for the CPF filter. The CPF Applications team then tested the customer’s 0.2 µm water filter and found it had similar retention, flow rate, and other filtration characteristics as our 1.0 µm polypropylene depth filter.

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.

Comparing Nominal Filters Between Vendors

  Filter Rating Use Test Results Benefits
Critical Process Filtration 1.0 µm Polypropylene Depth Nominal >90% Water Filtration Better retention, same flow rate, pressure drop, and throughput Superior performance filter @ lower cost
Another Vendor 0.2 µm Polypropylene Depth Nominal 70% Water Filtration  


When writing filter specifications, it is critical to define the purpose (particle removal, clarification, prefiltration) and required performance characteristics (pore size rating, filter efficiency, flow rate, throughput) of the filter, and most importantly the desired filtration result (particle reduction, turbidity reduction, batch size, processing time). This is especially important 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.

Criteria Necessary to Choose the Right Filter

Filter Purpose Performance Characteristics Desired Result
  • Particle removal
  • Clarification
  • Prefiltration
  • Pore size
  • Filter efficiency
  • Flow rate
  • Throughput
  • Particle Reduction
  • Turbidity reduction
  • Batch size
  • Processing time

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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