Measuring a Filter's Performance 
Measuring a Filter’s Performance
To help determine whether a filter will be suitable for your applications, manufacturers use various tests to rate the performance of the filter under certain conditions.
Biological Safety Test is a general term used to categorize tests performed to determine whether the filter’s materials of construction are capable of including measurable degrees of systemic toxicity, localized in ski irritation, sensitization reaction, or other biological responses. Either in vivo or in vitro test methods may be employed. Tests like the “United States Pharmacopoeia (USP) Biological Reactivity Test, In Vivo <88>” ensure that the filters can be exposed to the test solutions without causing an adverse reaction.
Pyrogenicity is the tendency of a substance to raise body temperature when injected into the body. Filtration materials that come in contact with injectible liquids must meet Pyrogenicity standards and be classified as non-pyrogenic. Pyrogenicity can be determined by such standard tests as the Limulus Amoebocyte Lysate (LAL) test.
Bubble Point is a measure of the air pressure required to force liquid from the largest wetted pore of a membrane. It serves as an indication of pore size and rates the filter’s ability to serve as a particle barrier. The bubble point is dependent on the liquid used to wet the membrane. For a given pore size, the bubble point will be higher in a liquid with a higher surface tension (such as water) than in a liquid with a lower surface tension (such as isopropyl alcohol). The bubble point rating is determined when the largest pore yields a bubble; the larger pore, the less pressure required to form a bubble. Expressed in units of pounds per square inch (psi) or bar for membranes.
Water Breakthrough is a measure of the amount of pressure required to transmit water through the largest pore of a dry hydrophobic filter. It serves as an indication of pore size for a hydrophobic membrane, and rates a filter’s ability to serve as an aqueous barrier. The larger the pore size, the less pressure is required to push water through the pore. Expressed in the filtration industry in units of pounds per square inch (psi) or bar.
DOP Test is a measure of the efficiency of a filter for the removal of particulate from air, based on retention of 0.3 µm Dioctyl Phthalate (DOP) aerosol droplets, usually expressed as a percentage. A High Efficiency Particulate Air (HEPA) filter must retain at least 99.97% of 0.3 µm DOP droplets. The 0.3 µm size was chosen because particles of this size are the most difficult to retain in many air filters.
Filter Efficiency measures the percentage of particles which are removed from the fluid by the filter. In filtration of liquids, filter efficiency is given on the basis of particles at or above a certain diameter in size. In gas filtration, efficiency is stated as including all particles, including those at the most penetrating particle size. See the DOP Test for a test of efficiency in air filtration. Some filter manufacturers will report efficiency in terms of the percentage removal of the particles by weight, which does not reveal the number of particles what may pass through the filter. This is a type of nominal filter rating. For high-efficiency filters, this is often replaced by a beta rating. Efficiency can be calculated from a beta value as follows:
% Efficiency = (B-1/B) x 100
Filters rated as one micron or finer are often rated using titer reduction values or log reduction values.
Filter Life is a measure of how long a filter will last before requiring replacement or cleaning. It can be stated either in terms of time (e.g., 30 days between changes) or volume of fluid filtered (e.g., 10,000 liters processed between filter changes). A filter’s actual life will depend on what particulates and conditions it is exposed to in actual usage, so filter life ratings from lab testing with standard contaminants can be used for comparison, but do not necessarily predict actual service life. To predict actual life, testing with the application fluid under actual operating conditions is required.
Typically, the useful life of a filter can be determined by a two-to-four fold increase of differential pressure in a constant flow system or a drop in flow rate of 50 to 80% in a constant pressure system.