Filtration Glossary

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 the filter cloth.

Absorption

Retention of liquids in the bulk of the fabric where the liquid is retained by filling up of void spaces, i.e. pores of the fiber, between fibers in a yarn and between yarns in the fabric.

Air flow/air permeability

Measure of the amount of air that flows through a filter–a variable of the degree of contamination, differential pressure, total porosity, and filter area. Expressed in either cubic feet/minute/square foot or liters/minute/square centimeter at a given pressure.

Antistatic

Material that minimizes static charge generation, provides ”controlled” static charge dissipation, or both.

Aperture size

See Mesh Opening.

Attenuation

Reduction of the signal power or field strength as a function of distance through a material. Also refers to shielding effectiveness.

Autoclave

Vessel for heating materials under high steam pressure. Used for sterilization and other applications.

Betamesh®

A type of dutch weave wire cloth.

Bolting cloth (silk)

Screens woven of twisted multifilament natural silk.

Bolting grade(wire cloth)

Uniformly woven of stainless steel to provide high strength and the largest possible openings.

BOPP-SI™

Precision woven wire screens woven by G. Bopp & Co., Switzerland.

Bubble point test

A test to determine the maximum pore size opening of a filter.

Bulked yarn

A yarn that has been geometrically changed to give it the appearance of having greater volume than a conventional yarn of the same linear density.

Cake

The solids discharged from a dewatering apparatus.

Calendering

A process by which fabric or wire is passed through a pair of heavy rollers to reduce thickness, to flatten the intersections of the threads/wires and to control air permeability. Rollers are heated when calendering synthetic materials.

Closed weave

Fabrics are constructed such that, when viewing the fabric perpendicular to its surface, you cannot see through the holes formed by the interwoven fibers. Closed weave fabrics tend to be thicker and stiffer than open weaves.

Decibel (dB)

A unit that expresses the relative difference in power, usually between acoustic or electromagnetic signals. One dB equals ten times the common logarithm of the ratio of incident and transmitted power; or twenty times the common logarithm of the ratio of incident and transmitted field strength.

Decitex (dtex)

The mass in grams of 10,000 meters of fiber or yarn.

A direct yarn numbering system used to define size of fiber or yarn. The higher the number, the coarser (larger) the yarn.

Deionized water

Water that goes through an ion exchange process in which all positive and negative ions are removed.

Denier

The mass in grams of 9000 meters of fiber or yarn.

A direct yarn numbering system used to define size of fiber or yarn. The higher the number, the coarser (larger) the yarn.

Depth filter

A filter medium consisting of randomly distributed particles or fibers resulting in openings with a non-uniform and tortuous path.

Differential pressure

The difference in pressure between two points of a system, such as between two sides of an orifice

Double layer fabric

By joining a fine filter layer with an open, coarser layer of mesh, these fabrics combine high flow capacity and fine particle capture efficiency with the extremely durable construction needed for large-scale process filtration.

Downstream side

The side of a product stream that has already passed through a given filter system; portion located after the filtration unit.

Dual chamber test method

Measures near-field shielding

effectiveness by indicating the signal attenuation caused by passage through a test material.

Dutch weave

Warp and weft wire diameters are different in size and the weft wires are closer together to provide excellent strength and high density.

Dyeing

The process of adding color to textiles in either fiber, yarn or fabric form.

Effective area

The total area of the porous medium exposed to flow in a filter element.

Efficiency

The ability, expressed as a percent, of a filter to remove specified artificial contaminant at a given contaminant concentration under specified test conditions.

E-Field (Electric field)

The dominant component of a high impedance electromagnetic field produced by a near-field source such as a short diapole, or the electric component of a far-field plane wave. Expressed in V/m.

Electromagnetic Interference (EMI)

Electromagnetic energy that causes interference in the operation of electronic equipment. Can be conducted, coupled or radiated. Can be natural or man-made.

Electromagnetic Capability (EMC)

The capability of electronic equipment on :systems to be operated in the intended operational electromagnetic environment at designed levels of efficiency.

Endotoxin

A toxin produced by bacteria. The toxin is present in the :environment only after the death of the bacteria.

Extractables

Substances that can be leached from a filter during the filtration process or under other specified conditions.

Faraday cage

A cage made of conductive material. Static fields and discharges do not pass through it. Electromagnetic energy passing through the skin or shield is attenuated to varying degrees.

Feed

The material entering a filter processing unit for treatment.

Filter life

Measure of the duration of a filter’s useful service. This is based on the amount of standard contaminant required to cause differential pressure to increase to an unacceptable level-typically 2-4 times the initial differential pressure, a 50-80% drop in initial flow, or an unacceptable downstream measure of particulate.

Filter media

A porous material for separating suspended particulate matter from fluid.

Filter medium

The permeable portion of a filtration system that provides the liquid/solid separation, such as screens, papers, non-wovens, granular beds and other porous media.

Filtrate

The discharge liquor in filtration.

Filtration

A process of separating particulate matter from a fluid by passing it through a permeable material.

Flow rate

Measure of the amount of fluid passing through the filter. This is always a variable of filter area, porosity, contamination and differential pressure.

Frazier test

Measures the amount of air transmitted through a filter under selected differential pressures. Historically used for textile products. See Air flow.

Frequency

Number of complete cycles of current per second, expressed in Hertz (Hz).

G.M.P.

Good Manufacturing Practices. Food and Drug Administration regulations governing the manufacture of drugs and medical devices (Ref. Code of Federal Regulations 21CFR).

Hydrophilic

Having an affinity for water and aqueous solutions.

Hydrophobic

Cannot be wetted by aqueous and other high surface tension fluids.

Knit fabric

A fabric structure made by interlooping yarns.

Loaded (plugged)

A filter element that has collected a sufficient quantity of insoluble contaminants such that it can no longer pass rated flow without excessive differential pressure.

Mean filtration rating

Derived from Bubble Point test method. Data should be used as a guide only to compare overall retention capabilities between fabrics and should not be considered a guarantee of the particle size that the fabric will retain.

Membrane

A thin polymeric film with pores.

Mesh count

The number of threads in a linear centimeter or inch of fabric/wire cloth.

Mesh opening

Mesh opening is the difference between wires measured in the warp and weft direction in the projected plane of the wire mesh.

Monofilament

Single extruded filament.

Multifilament

Several monofilaments are bundled together to form a single textile yarn.

Non-woven

A porous web or sheet produced by mechanically, chemically or thermally bonding together polymers, fibers or filaments.

Open area

The proportion of total screen area that is open space. Expressed as a percentage.

Open weave

Fabrics are constructed such that, when viewing the fabric perpendicular to its surface, you can see through the holes formed by the interwoven fibers.

Particle

A relatively small subdivision of matter ranging in diameter from a few angstroms (as with gas molecules) to a few millimeters (as with large raindrops). A particle can have various shapes and dimensions.

Permeability

Ability of a membrane or other material to permit a substance to pass through it.

pH

Used to describe the hydrogen ion concentration of a solution. A pH of 7 is neutral. Below 7, acidity increases. Above 7 alkalinity increases.

Plain weave

Most basic weave- over and under pattern.

Plane wave

An electromagnetic wave with electric and magnetic components perpendicular to, and in phase with, each other.

Pore size

Filters are rated according to the size of particles they can remove.

Particles which are bigger than indicated pore size, are removed.
Smaller particles pass through the fabric The size of particles is
measured in micro-meters or "microns" one micron being equal
to one-millionth of a meter or one-thousandth of a millimeter, the
size of coal dust or baking flour. The smallest bacteria are about ½.

PRD

Plain Reverse Dutch weave. High thread count in the warp direction, low thread count in the weft direction.

Pyrogenic

A fever-producing substance. The presence of these substances is determined by the Limulus Amebocyte Lysate (LAL) test and measured in EU/ml (endotoxin units per milliliter).

Radio frequency interference (RFI)

EMI in electronic equipment caused by radio frequencies.

Residual shrinkage

The amount of shrinkage remaining in a fabric after it has undergone all fabric weaving, washing and heat setting steps.

Retentate

Substance retained in the upstream side of a filter.

RF (radio frequency) welding

Utilizes specific bands of radio frequency waves which are directed through specially constructed tooling to form localized melting/joining of certain dielectric thermoplastic materials. Can be used to form hermetic seals. Also known as high frequency or dielectric welding.

Selvage

A loom finished edge that prevents cloth unravelling.

Separation

This process divides or separates a mixture of particles or liquids into separate components.

Shielding effectiveness (SE)

Measure of a given material’s ability to block interference. Expressed in dB. See Decibel.

Sieve

A screen with apertures of uniform size used for sizing granular materials.

Solution dyeing

Method of adding color to the polymer melt or spinning solution before the fiber is extruded. Also known as Spun dyeing or Dope dyeing.

Spinning

The process of extruding polymers to form fibers.

Spun yarn

A yarn produced from short fibers.

Square weave

See Plain Weave.

Surface media

Captures particles on the upstream surface with efficiencies in excess of depth media, sometimes close to 100% with minimal or no off-loading. Commonly rated according to the smallest particle the media can repeatedly capture. Examples of surface media include ceramic media, microporous membranes, synthetic woven screening media and in certain cases, wire cloth. The media characteristically has a narrow pore size distribution.

Surface resistivity ( Ω/n)

Expressed in ohms/square. It is numerically equal to the resistance between two electrodes forming opposite sides of a square on the surface of a material. The size of the square is irrelevant. For conductive materials, surface resistivity is the ratio of the volume resistivity to the fabric thickness (r/t).

Taffeta weave

Plain weave.

Tangential crossflow filtration

Process where the feed stream ”sweeps” the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high and low molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes.

Textured yarn

A yarn that has been geometrically altered to impart bulk, increase moisture absorption, add resiliency, etc.

Thread count

See Mesh count.

Thread diameter

The cross-sectional measurement of an individual fabric thread/yarn or wire.

Throughput

The amount of solution which will pass through a filter prior to clogging.

Tricot

A type of knitted fabric.

Twill

weave: Formed by passing the warp or weft fiber over two or more fibers in the opposite direction.

Twist

Turns imparted to a length of yarn. Usually expressed in tpi

(turns per inch).

Ultrasonic (processes)

Process that utilizes specially-designed tooling usually vibrating at 15-80 KHz. Processes are designed to cause localized heating of thermoplastic materials which, in turn, will provide some type of welded or fused joint. Benefits are elimination of fillers and minimized heat stress on surrounding materials.

Upstream side

The feed side of the filter.

Useful life

Determined when contamination causes an adverse flow rate, low efficiency or high differential pressure.

Volume resistivity

Or specific resistivity of a material, expressed in W.cm. Resistance to electrical current flow through the bulk of an object.

Warp

Fibers or wire running the length of the cloth as woven.

Weft

Fibers or wires running across the width of the cloth as woven.

Wicking

The rapid movement of moisture along the fiber surface, usually by capillary action.

Wire diameter

See Thread diameter.