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Below you will find a list of common terms and definitions used within the filtration industry.

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Migration of foreign material which enters the fluid stream from system components that wear from close tolerances, vibration or shock.


A term used to describe or define a degree of filtration. There are various methods used in the filtration industry to determine absolute ratings, which are not necessarily interchangeable. Absolute rated filters always state a removal efficiency at the micron rating, generally between 98 – 99.98%.

Absolute Filtration Rating

The diameter of the largest hard spherical particle that will pass through a filter under specified test conditions. This is an indication of the largest opening in a filter element.


To take up by cohesive, chemical or molecular action.


A filter medium that is similar to a sponge, drawing fluid and retaining it within its structure. In this sense it can act as a filter to remove (adsorb) and retain fluid.


The attraction and/or the retention of particles by molecular attraction or electrostatic forces present between the particles and a filter medium. Also, the attraction of gasses, liquids or solids to surface areas of textile fibers, yarns, fabrics, or any similar type of material.


A group of two or more particles combined, joined or clustered, by any means.


A relatively stable assembly of dry particles formed under the influence of physical forces.


Surrounding. For example, the ambient operating temperature of a vessel is temperature that is essentially the same as that surrounding the vessel.


The American Society of Mechanical Engineers.

Atmospheric Pressure

The force exerted on a unit area by the weight of the atmosphere.


Back Pressure

In filter use, resistance offered by the filter, usually measured in PSI.

Back Wash

To clean a filter element by reversing the direction of flow through it.

Basket Strainer

A vessel for the removal of coarse bulk solids from liquid, air or gas. The element is usually a steel perforated basket, or a mesh lined basket.

Beta Ratio (ß)

The Beta (ß) Ratio is a rating system introduced with the object of giving both filter manufacturer and user an accurate representative comparison among filter media. Also, an indication of how a filter performs throughout the life of the filter. The Beta Ratio is an average filtration rating (single pass and multi-pass).


A valve which diverts part of the fluid from the main flow of the system.

Bubble Point

The differential gas pressure at which the first steady stream of gas bubbles is emitted from a wetted filter element under specified test conditions. A form of filter element fabrication integrity test.


A synthetic rubber gasket material, used for vessel closures, flanges and filter elements.


An outward structural failure of the filter element caused by excessive differential pressure.


A condition that occurs when:

a) a bag or cartridge is not seated or sealed properly in the filter housing; or

b) the filter media is violated and permits unfiltered fluid to pass through.

Bypass Valve (Relief Valve)

A mechanism to assure system fluid flow when a pre-selected differential pressure across the filter is exceeded; the valve allows all or part of the fluid flow to bypass the filter element.



Solids deposited on the filter medium during filtration in sufficient thickness to be removed in sheets of sizeable pieces. In many cases, cake may provide its own filter media by adding to the surface of the media.


The volume of product which a vessel will accommodate, expressed in gallons or similar units. Also, an amount which will filter at a given efficiency and flow rate, expressed in gallons per minute or similar units.


A filter for the clarification of process liquids containing small amounts of solids. Made of a porous medium, it is used in a vessel, which performs the actual filtration process.

Center Rod/Post

The component of a vessel used for mounting the cartridge in the vessel, usually made of a round bar material. A center pipe can also be used for the same purpose, but is made instead with perforated effect and directs flow through the cartridge.


One one-hundredth of a poise. A poise is the unit of viscosity expressed as one dyne per second per square centimeter.


One one-hundredth of a stoke. A stoke is equal to the viscosity in poises times the density of the fluid in grams per cubic centimeter.


A filter element which, when loaded with contaminant, can be cleaned by a suitable process and returned to service with an acceptable percentage of its original dirt holding capacity.

Clear Water Pressure Drop

Differential pressure across the filter as measured using clean water at a particular flow rate.


That which produces agglomeration of suspended solids.


The action of uniting of small droplets of one liquid preparatory to its being separated from another liquid.


Any undesirable particle or impurity in a stream.


An inner material used for the center of an element as support, which may also be called a center tube when used with string-wound filters.


The conversion of metals into oxides, hydrated oxides, carbonates, or other compounds due to the action of air, water or both. Salts and sulphur are also important sources of corrosion. Removal of solids and water reduces the effect or speed of corrosion in many cases, and in other cases, corrosion inhibitors are used to reduce the effect of corrosion.



The loss of desirable physical properties by a textile material as a result of some process or physical/chemical process. Also, the wearing down or reduction in the efficiency of a media.

Delta P

A symbol (P) designating pressure drop. The difference in pressure between two points, generally measured at the inlet and outlet point of a filter, separator/filter, etc. Normally measured in pounds per square inch (psi), inches of mercury (in. Hg.), or inches of water (in. H20). Also known as pressure drop.


The weight per unit volume of a substance (specific weight).


A filter medium which primarily retains contaminants with the tortuous passages within the thickness of the element wall. Depth-type filtration that is accomplished by flowing a fluid through a mass filter media, with a much longer and random path through the filter. The density of the structure can be density graded, which is of particular advantage where the particular sizes of the contaminant are widely distributed. Certain types of solids, or combinations of solids, do not work well with surface filtration, and depth filtration is found to be more suitable.

Differential Pressure

The difference in pressure across any two points of a system or component.


A flow condition where certain liquids will show an increase in viscosity as the rate of shear or flow is increased.


The flow rate through a filter.


A filter which is intended to be discarded and replaced at the end of its service life.


Effective Area

The area of a medium that is exposed to the flow, and is usable for its intended purpose: coalescing, filtering or separating. This is the opposite of blind spots or dead area.


The degree to which an element will perform in removing solids and/or liquids, usually expressed as output divided by input.


The medium used in a vessel to perform the function of filtration or separation. Also called the cartridge or filter. The porous device which performs the actual process of filtration.


A finely divided suspension of an oil in water or vice versa. Also, a dispersion of finely held particles in a stream which do not necessarily dissolve in each other, but are held in suspension.


Mist, fog or droplets of a liquid which are usually considered to be a contaminate when encountered in the filtration industry.



Liquid to be processed containing one or more liquid phases, such as an emulsion, and/or suspended solids, and/or insoluble solids.


A non-woven sheet of fibers, made by a combination of mechanical and chemical actions, including pressure, moisture and heat.


A flexible material with two relatively small dimensions and one long dimension.

Fiber Migration

Undesirable movement of filter material from the media into the feed stream.


A term generally applied to a device used to remove solid contaminants from a liquid or gas, or to separate one liquid from another liquid or gas. A filter, as referred to in the filtration industry, is a device which removes contaminants.

Filter Element Life

The span of operation from clean unit to a predetermined pressure drop build up, usually measured in elapsed time.

Filter Medium

The porous material mounted on a plate or frame which separates the solids from the liquids in filtering. Also referred to as filter cloth, filter plate or septum. The material that performs the actual process of filtration.


Filtered fluid which flows out of a filter.

Filtration Efficiency

Expressed as a percent of contaminant introduced to the system. It is the ability of a filter to remove specified contaminants at a given contaminant concentration under specified test conditions.

Filtration Rating

The diameter of the largest hard spherical particle that will pass through a filter under specified test conditions. This is an indication of the largest opening in the filter medium.


The physical or mechanical process of separating insoluble particulate matter from fluid, such as air or liquid, by passing the fluid through a filter medium that will not allow the particulate to pass through it.

Flow Characteristics

The nature of fluid movement as being turbulent, laminar, constant or of a variable rate, to various degrees.

Flow Rate

The rate at which a product is passed through a vessel or system, generally expressed as gallons per minute, cubic feet per minute, per hour, per day, etc.

Fluid Compatibility

The suitability of the filtration medium and seal materials for service with the fluid involved.


A liquid or gas which can be filtered by passing through a filter.


Gauge Pressure

All pressure greater than atmospheric pressure, as read on a pressure gauge.


A semi-solid that is susceptible to pressure deformation. Gels have the habit of sticking to other surfaces.

Glazed Finish

A finishing process that produces a smooth, highly polished surface using extreme temperature. Eliminates filter fiber migration.

Gradient Density

A media of different densities, with one media packed around the center tube and a media of less density around the outside. Both medias are tapered at opposite ends, which allows high flow through the less dense media, and tighter filtration through the dense media.



A ported enclosure which directs the flow of fluid through the filter element.


The study of fluids at rest or in motion.


Having a strong affinity for or the ability to absorb water.


Lacking affinity for or the ability to absorb water.

Hydrostatic Test

A test conducted with either air, water or other fluids at a given value over design pressure, to prove the structural integrity of a pressure vessel.



Incapable of being mixed; insoluble; the opposite of miscible.


The process of treating a coarse filter medium with resins.


Any undesirable material in the fluid.

Indicator, Differential Pressure

A device that signals the difference in pressure between two points, typically between the upstream and downstream sides of the filter element.

Initial Pressure Drop

A loss in pressure between the inlet and the outlet connections upon the start of flow through a vessel using new elements.


When inlet and outlet connections are positioned at the same height on the opposite sides of a vessel so that an imaginary straight line can be drawn connecting one to the other.


Incapable of being dissolved in a fluid; the opposite of soluble.






The structural support yarn or twine in wound elements, usually wound in a diamond pattern.

Maximum Operating Pressure

The highest pressure allowed in a system.

Mean Filtration Rating

A measurement of the average size of the pores of the filter media.

Migration/Media Migration

The carry-over of fibers from the filter, separator elements or other filter, into the effluent. The contaminant or media released to pass downstream from the filter element.


A porous or slotted mass in a filter element that separates solids from a fluid by a difference in the size of openings, and also through direct containment. A material of controlled pore size or mass through which a product is passed in order to remove foreign particles held in suspension, or to repel droplets of coalesced water; or a material without controlled pore size, such as glass fiber mats, which contribute to filtration, coalescence, or separation of two immiscible liquids.


In the filtration industry, the term is used to describe the media through which the liquid stream is to be passed or exchanged. Membranes are usually associated with ion exchanged media such as dialysis, osmosis, diffusion, etc., although filter paper itself could be classified as a membrane.


A unit of length. A micrometre is a millionth of a meter or 0.000039″ (29 millionths of an inch). 25 micrometres are approximately equal to a thousandth of an inch (.001″).


A short unit of length in the metric system, equal to one millionth of a meter, 10-4 centimeter, 10-3 millimeter, or 0.000039 of an inch. A micron is used as a criterion to evaluate the performance or efficiency of a filter media, or to describe the condition of either the influent or effluent. Usually stated in terms of being either absolute or nominal.


A filter element which has no separate housing of its own, but whose housing is incorporated into the equipment it services. It may also incorporate a suitable enclosure for the filter cavity.

Monofilament Mesh

A woven fabric with evenly-spaced holes. Each thread is a single filament. The mesh combines excellent strength with little or no fiber migration.

Multifilament Mesh

A type or woven fabric, where each thread consists of many smaller diameter threads twisted together.

Multi-Pass Test

The test used to determine the beta ratio of an element.



A liquid which does not change in viscosity when faced with a change in rate of shear, agitation or flow rate.

Nominal Filtration Rating

An arbitrary micrometer value indicated by a filter manufacturer. Due to lack of reproducibility, this rating is deprecated.


National Pipe Thread standard.


Operating Pressure

The normal pressure at which a system operates.

Open Area Ratio

The ratio of pore area of a filter medium, expressed as a percent of total area.


Particle Count

The practice of counting particles of solid matter in groups based on relative size. Frequently used in engineering, a filter to a specific task, or to evaluate the performance of a filter under specific operating conditions.

Particle Size Distribution

A tabulation resulting from a particle count of solids grouped by specified micron sizes to determine the condition of either the influent or effluent stream.


The relationship of flow per unit area to differential pressure across the filter medium.


The value indicating the acidity or alkalinity of a material. It is the negative logarithm of the effective hydrogen ion concentration. A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is considered a base.


A filter element whose medium consists of a series of uniform folds and has a geometric form of a cylinder, cone, disc, plate, etc. Synonymous with “convoluted” and “corrugated.”


The condition of a filter when it has collected its full capacity of contaminants and will not pass any more fluid.

Pore Size Distribution

The ratio of the number of holes of a given size to the total number of holes per unit area expressed as a percent and as a function of hole size.


A small channel or opening in a filter medium which allows fluid to pass.


The property of a solid which contains many minute channels or open spaces. The fraction is a percentage of the total volume occupied by these channels or spaces. Also describes a filter media which may have larger pores than other media.


The force exerted per unit area by a fluid, typically measured in pounds per square inch (psi).

Pressure Drop

The difference in pressure between two points, generally at the inlet and outlet of a filter or a separator/filter. Measured in pounds per square inch gauge, or inches of mercury.

Pressure, Absolute

Gauge pressure plus 14.7 psi.

Pressure, Atmospheric

The force exerted by the atmosphere at sea level, which is equivalent to 14.7 psi.


Pounds per square inch.


Pounds per square inch absolute.


Pound per square inch differential.


Pounds per square inch gauge.



Rated Flow

The optimum flow rate for which a filter is designed.



The Society of Automotive Engineers.

SAE Number

A classification of lubricating oils for either crankcases or transmissions, in terms of viscosity, as standardized by the Society of Automotive Engineers.

Saybold Seconds Universal (SSU)

Units of viscosity as measured by observing the time in seconds required for 60 ml. of a fluid to drain through a tubular orifice 0.483 inches long by 0.0695 inches in diameter at stated conditions of temperature and pressure.


Standard cubic feet per day.


Standard cubic feet per hour.


Standard cubic feet per minute.


The action of separating solids or liquids from fluids. May be accomplished by impingement, filtration or by coalescing.


A vessel which removes solids and entrained liquids from another liquid or gas, using some combination of a baffle and/or coalescer, filter or separator element. A vessel may be single stage, two stage, or single or two stage with prefilter section for gross solids removal. The usual application is the removal of water from gas or another immiscible liquid. General reference to term applies the equipment capable of both separation and filtration to specific degrees of efficiencies.

Service Life

The length of time an element operates before reaching the maximum allowable pressure drop.


The outer wall of a vessel, usually referred to as the body.

Singed Finish

The process of removing fibers from a cartridge or fabric by passing over a flame or other heat source. The process creates a smooth finish that inhibits fiber migration.


The process of removing fibers from a cartridge or fabric by passing over a flame or other heat source. The process creates a smooth finish that inhibits fiber migration.

Skid Mounted

When one or more vessels with pumps and motors are mounted on a portable platform.


Dirt, carbon, water and chemical compounds found in oils.


A mass or matter contained in a stream which is considered undesirable and should be removed.


A single phase combination of liquid and non-liquid substances, or two or more liquids.

Specific Gravity

The ratio of a substance’s weight to that of some standard substance (water for liquids and solids, air or hydrogen for gases). This is by definition a unitless value.

Surface Area

The total area of an element that is exposed to an approaching flow.

Suspended Solids

Non-settled particles in a fluid.


Solids or liquids that are held in other liquids.

System Silting

The agglomeration and settling of ultra-fine particles in a fluid system.


Tensile Strength

The maximum stress a material that is subjected to a stretching load can withstand, without tearing.

Thermal Lockout

A device that prevents the actuation of the signal button in a differential pressure indicator below a preset temperature. Used to prevent false indicator actuations during periods of high fluid viscosity such as experienced in starting a cold system.


A liquid which shows a marked reduction in viscosity as the rate of shear, agitation or flow rate is increased.


The ratio of the average effective flow path length to the minimum theoretical flow path length (thickness) of a filter medium.

T-Type Filter

A filter in which the inlet and outlet ports are located at one end of the filter with the major axes of its ports in a straight line and with the filter element axis perpendicular to this line.


A cloudy or hazy appearance in a naturally clear liquid, caused by the suspension of colloidal liquid droplets or fine solids.

Turn Over

The number of times the contents of the system pass through a filter per unit of time.


Ultra Filter

A type of membrane used to remove very fine suspended sub-micronic particles as well as some dissolved solids.


A type of membrane used to remove very fine suspended sub-micronic particles as well as some dissolved solids.



A reference to a pressure that measures below atmospheric pressure.


A container in which the filtration process occurs, through a filter media such as cartridges or bags that are installed inside.


The degree of fluidity; also, the property of a fluid’s molecular structure by virtue of which they resist flow; the internal flow resistance of a fluid; or, the resistance of flow exhibited by a liquid resulting from the combined effects of cohesion and adhesion. The units of measurement are the poise and the stoke.

A liquid has the viscosity of one poise if a force of one dyne per square centimeter causes two parallel liquid surfaces one square centimeter in area and one centimeter apart to move past each other at a rate of one centimeter per second. There are a great many crude and empirical methods for measuring viscosity, which generally involve measurements for the time of flow or movement of a ball, ring or other object in a specially shaped or sized apparatus.



A filter medium comprised of two or more layers of helical wraps of a continuous strand or filament in a predetermined pattern.


A filter medium made from strands of fiber, thread or wire, interlaced into a cloth through the action of a loom.