Different Testing Methods

Absolute filter rating means no particles greater than a given size are permitted to pass through the filter element. Depending on the industry that you are in or the type of filter media employed, many different tests will be used to obtain an absolute rating of the filter element. For fixed pore media such as sintered wire cloth or sintered powdered metal, absolute rating is defined as the diameter of the largest hard spherical particle that will pass through the filter under specified test conditions. A glass bead challenge test is often used to determine the absolute rating of a filter element. Glass beads of varying sizes are introduced into a fluid upstream of the filter, and those glass beads that pass through the filter element are collected on an analysis membrane. Those glass beads are examined microscopically to determine the largest spherical particle passed. An absolute filter test is destructive. After testing, the filter element must be discarded.

Elements in the pharmaceutical and biological industries are absolute rated based upon a bacterial challenge test. This destructive test utilizes industry standard microorganisms to validate 0.2 ?m sterilizing grade filters. In this test, Brevundimonas diminuta ATCC 19146 (formerly known as Pseudomonas diminuta ATCC 19146) are introduced upstream of the filter element. A culture is made from the fluid taken downstream of the element to detect the presence of the microorganism. Similarly, pseudomonas sepatia is used to challenge filters rated at 0.32 ?m.

Bubble Point
Another method used to determine the largest single pore size in a rigid media is the bubble point test. A fluid of know surface tension, usually isopropyl alcohol if using ARP 901 as the bubble point test standard, is used to totally wet the element filter media. Air is introduced into the filter element while holding the element surface 0.5" below the surface of the alcohol. The pressure is measured with a manometer or other sensitive instrument. The bubble point pressure is noted at the pressure that the first bubble appears in the filter media. Using a constant for the surface tension of the fluid, a maximum pore size of the element is determined. The advantage of this test is that it is non-destructive.

Beta Rating
Hydraulic, lubricating and electronics filter elements are rated according to a Beta (?m) rating system. This test utilizes contaminant of a known particle size distribution and concentration in a known fluid. Contaminant is injected into the fluid stream upstream of the filter element to be tested. Automatic particle counters are used to determine the size and quantity of particles upstream of the test element. Additional particle counters measure the size and quantity of particles downstream of the filter element. The beta ratio is simply a ratio of the number of particles of a given micron size and larger upstream of the filter divided by the number of particles of the same micron size and larger downstream of the filter element. From the beta rating, percentage removal efficiency can be calculated directly according to the equation:

% removal efficiency = ß-1 / ß x 100

The relationship between beta value and removal efficiency is shown below

Nominally Rated Elements
According to the National Fluid Power Association (NFPA), nominal rating is defined as: "an arbitrary micron value assigned by the filter manufacturer, based upon removal of some percentage of all particle of a given size or larger. It is rarely well defined and not reproducible."

Nominally rated filter elements are not easily correlated between different manufacturers due to the lack of a governing specification.

The quantity of solids retained by a filter element before blinding or reaching a terminal pressure drop is known as the solids or dirt holding capacity of the filter.

Most manufacturers challenge filter elements with standardized test dust to determine the element's average dirt holding capacity, although manufacturer's test conditions will vary. Changing test conditions can significantly change the dirt holding capacity of a filter. Additionally, sometimes a quick comparison of dirt holding capacity between a filter that has a coarse media and a filter than has a finer media can be misleading. It is best to consider other variables before making a decision including micron rating or Beta rating.

Dirt holding capacity is critical in determining the total operating cost of the filter system because it has a direct effect on:

• Replacement element cost
• Labor cost
• Disposal of used element cost
• Downtime costs

It is important to recognize that the contaminant being removed in each system will load differently than test dust, which will affect the anticipated life of the filter element.

Most filter companies today utilize standard acceptable tests for rating performance of their filter elements. For example, in the testing of hydraulic and lubrication cartridges, most companies recognize the ISO 4572 multipass testing standard as the standard for rating element performance. If a company does not have in-house testing capabilities, or wants to show their objectivity in the rating of element performance, an outside testing laboratory is used. If filter data does not contain a reference to an accepted standard, it should be questioned.

Absolute micron ratings of filter elements should include the efficiency and a description of the testing methodology.