College Football has its AP Poll, Coaches Poll and the BCS. College Basketball uses the RPI-Rating Percentage Index and Strength of Schedule. Soccer sees teams ranked by FIFA based on their success in their games over a four year period. Plus we use rankings to decide on what restaurant to go to, what beer or wine to drink, what movie to see or rent.. So where is the common standard for Filter Ratings? Without one standard rating system for filtration capabilities the user can easily get confused. Of course, testing is always key, but I can at least help you understand the four commonly used rating methods.
- Nominal Rating — an arbitrary micron value given to the filter by the manufacturer based upon removal of some percentage of all particles of a given size or larger. Since the value is rarely well defined and not reproducible, these ratings have little to no value.
- Absolute Rating — gives the size of the largest hard spherical particle that will pass through the filter or screen under specified test conditions. This commonly used rating is an improvement on nominal ratings.
- Beta Ratio — a simple rating system based upon the ratio between the per unit volume number of particles above a given size in the influent (upstream) of the media suspension to the same parameter in the effluent (downstream) of the filter media.
- Air Permeability — the flow rate of air per unit area at a given differential pressure. This is normally expressed as cfm/ft2 at 0.5 inches water gauge. Typical ratings can be from 2 – 2000. Keep in mind, construction factors and finishing techniques have an effect upon air permeability as they can change airflow paths.
If there were one Absolute System when it came to rating, things would be easier, but maybe more boring too. Test. Test. Test. That’s how you’ll really find the best option for your project. In the meantime, I’d argue we need to start a dialog about rating systems in the chemical, pharmaceutical, energy, oil and gas industries! What do you think? Comment below.