Filter Ratings: Nominal vs. Absolute

By Rudy Vadbunker

Filters are rated on their ability to remove particles of a certain size from a fluid. For example, a filter that is rated as a “10 micron” filter can capture particles as small as 10 micrometers. The only way to know exactly what this means, however, is if the filter testing methods and rating standards utilized are both known. The most commonly used filter ratings are nominal and absolute micron rating.

Absolute Rating

The absolute rating, or cut-off point, of a filter is the diameter of the largest spherical glass particle which will pass through the filter. These diameter dimensions are expressed in micrometers – or one/one millionth of a meter. The absolute rating reflects the pore opening size of the medium. Filter media with an exact and consistent pore size have an exact absolute rating. This absolute rating should not be confused with the largest particle passed by a filter: the absolute rating simply determines the size of the largest glass bead that will pass through the filter under very low pressure differentials and non-pulsating conditions.

Filter media with exactly consistent pore sizes do not typically exist in practice. 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 than 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 bed 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 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 a perfect retention of particles.

Nominal Rating

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. These particles of each specific contaminant are measured by weight. The nominal rating also represents an efficiency figure or degree of filtration. 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, the nominal rating method is generally discouraged. During testing, differing conditions like operating pressure and contaminant concentration vary enough that the rating provides an inconsistent result and a lack of uniform measurement.

Beta Ratio

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 (2) divided by the number of particles downstream (1). Based on this method, filters with a higher beta ratio retain more particles, have a higher efficiency, and therefore are more effective filters.