Filter Ratings: Nominal vs. Absolute filtration
Filters are rated on their ability to remove particles of a certain size from a fluid. But it’s the different ability in different filter media in the same micron since there are nominal and absolute micron rating.
The absolute rating of a filter refers to the diameter of the maximum particle which could pass through the filter. These diameter dimensions are expressed in micrometers (um) – or 1/1 millionth of a meter. The absolute rating reflects the hole opening size of the filter media. The filter media with an exact and consistent pore size have an exact absolute rating. For example the Micron Nylon Mesh. This absolute rating should not be confused with the largest particle passed by a filter.
Actually, filter media with exactly consistent pore sizes do not typically exist. Pore size is affected by the form of the filter element and is not necessarily consistent with the actual open areas. It is possible for the shape of the particle, say if it is cylindrical, to allow the particle to pass through a much smaller hole in the media that would have been expected, based on at least one of the particle’s dimensions. This type of passage hinges on the size and shape of the opening and on the fluid depth over which filtering is provided. A filter cake is typically created wherein particles collect on the media surface and result in an increased blocking action. This further decreases the permeability of the element. The blocking can increase so much, that the pressure drop across the filter becomes excessive and the flow rate through the system drops dramatically.
Some see the absolute rating as a non-realistic description. The term absolute means that no particle larger than that rating can pass through the filter, which limits the types of media to those with consistent pore size and ones that show perfect retention of particles.
A nominal rating indicates the filter’s ability to prevent the passage of a minimum percentage of solid particles greater than the nominal rating’s stated micron size. The nominal rating also represents an efficiency figure or degree of filtration. Featured product Micron Filter Felt. A nominal rating example is “95% of 10 micron” – where the filter prevents 95% of all 10 micron and larger particles from passing through. However, There is no industry standard has been jointly identified and complied with for nominal accuracy. In other words, company A may be set the nominal rating at 85-95%, while company B prefers to book 50-70%. It means the 25um from A may be equal to 5 micron from B, or finer. For this problem, experienced professional filter media suppliers would be helpful in choosing of precision, and testing is the basic solution.
Up until recently, a universally accepted test method to measure the media pore size has not existed. A newer test procedure called multi-pass testing or Beta ratio testing has changed this. This method yields readily comparable test results and was introduced to give the filter manufacturer and the end user an accurate comparison between filter media.
Multi-pass testing uses a specified contaminate, of known sizes, added regularly in measured quantities to the fluid which is being pumped through the filter. At timed intervals, samples of the fluid are simultaneously taken from both downstream and upstream of the filter. Using particle counters, particles in each sample are measured and counted. Based on the results of these measurements, a Beta ratio is determined by dividing the number of particles of a particular size in the upstream flow by the number of particles of the same size in the downstream flow. In essence, the Beta ratio is an indicator of how well a filter controls a specifically sized particulate. For example, if one out of every two particles in a fluid pass through the filter, the Beta ratio is 2/1 = 2. This shows the number of particles upstream divided by the number of particles downstream. Based on this method, filters with a higher Beta ratio retain more particles, have higher efficiency, and therefore are more effective filters.