What does it involve?
Filter media suppliers use the terms nominal or absolute to characterize their products in terms of filtration efficiency. A filter’s efficiency is its ability to retain particles, colloids and/or microorganisms present in a fluid as it passes through a porous medium.
Depending on the level of filtration quality required, it is necessary to choose between nominal filtration and absolute filtration. Each delivers different filtration results, they may be complementary, and they have a direct impact on filter purchasing costs.
Regardless of whether nominal or absolute efficiency is selected, based on the desired filtrate quality, it is essential to rigorously and precisely define the type of fluid, flow rate, pressure, temperature, and all operating conditions required for proper filtration system sizing.
Discover an example of sterile gas filtration implementations:
How to choose the correct sizing for sterile gas filtration?
What is the difference?
Nominal filtration corresponds to an efficiency, at the stated rating, of around 90%. However, filter manufacturers may define nominal efficiency differently. Either the efficiency is based on a percentage of retention of particles equal to the filtration rating, or it is defined as a percentage of retention of particles equal to or larger than the filtration rating. This difference in measurement results in a higher value in the latter case. In any event, with nominal filtration, it is accepted that a certain percentage of particles may pass through the filter.
If the filtration system must guarantee total particle retention, then an absolute filter must be selected.
Absolute filtration corresponds to an efficiency, at the stated rating, greater than 99.98%, i.e. a beta ratio of 5000.
This β ratio is determined by dividing the number of particles of a given size at the filter inlet by the number of particles that have passed through the filter:
β = number of upstream particles / number of downstream particles
In practice, filter efficiency is more commonly expressed as a percentage.
Thus, efficiency is expressed as: E = ((β − 1) / β) × 100
An efficiency of 99.98% means that one particle out of 5,000 passes through the filter media.
Learn more about filter efficiency
In several industrial sectors, such as the pharmaceutical, cosmetics, and food & beverage industries, it is necessary to use filter media that provide absolute filtration. In other words, undesirable and/or toxic elements must be retained.
In particular, in pharmaceutical-grade water, endotoxins must be eliminated. These toxins can have dramatic consequences, as they are recognized and attacked by the human immune system.
Learn more about endotoxin filtration in pharmaceutical environments
Contact a filtration expert
We are at your service
Our experts place their knowledge and expertise at your disposal for liquid filtration, air and gas filtration, decontamination, and separation technologies.
